Coder using forward aliasing cancellation.

(54) Title: ENCODER USING CANCELLATION OF THE ALIASING EFFECT FORWARD. (54) Title: CODER USING FORWARD ALIASING CANCELLATION.

(57) Summary

A codec that supports switching between aliasing effect cancellation in time domain transform encoding mode and time domain encoding mode becomes less prone to frame loss by adding a portion of syntax additional to the tables, depending on which the decoder parser can choose a first action that consists of estimating that the current table includes, and therefore reading, the cancellation data of the aliasing effect forward of the current frame and a second option that consists of estimating that the current frame does not understand, and therefore, not reading the cancellation data of the aliasing effect forward of the current frame. In other words, although a bit of coding efficiency is lost due to the inclusion of a new syntax portion, it is precisely the new syntax portion that confers the ability to use the codec in the case of a communication channel with loss of frames. Without the new syntax chunk, the decoder would not have the ability to decode any chunk of the data stream after a loss and would fail when trying to resume parsing. Consequently, in an error-prone environment, the disappearance of coding efficiency is prevented by introducing the new syntax portion.

(57) Abstract

A codee supporting switching between time-domain aliasing cancellation transform coding mode and time-domain coding mode is made less liable to trame loss by adding a further syntax portion to the trames, depending on which the parser of the decoder may select between af ​​irst action of expecting the current trame to comprise, and thus reading forward aliasing cancellation data from the current frame and a second action of not-expecting the current frame to comprise, and thus not reading forward aliasing cancellation data from the current frame. In other words, while a bit of coding efficiency is lost due to the provision of the new syntax portion, it is merely the new syntax portion which provides for the ability to use the codee in case of a communication channel with frame loss. Without the new syntax portion, the decoder would not be capable of decoding any data stream portion after a loss and will crash in trying to resume parsing. Thus, in an error prone environment, the coding efficiency is prevented from vanishing by the introduction of the new syntax portion.

ENCODER USING CANCELLATION OF THE ALIASING EFFECT TOWARDS

IN FRONT OF

Description

The present invention relates to a codec that supports the time-domain aliasing effect cancellation transform encoding mode and a time domain encoding mode, as well as forward aliasing effect cancellation to switch between both modes.

It is favorable to mix different encoding modes to encode general audio signals representing a mixture of audio signals of different types such as voice, music or the like. The individual encoding modes can be tailored to specific audio topos and consequently the multimodal audio encoder can take advantage of changes in encoding modes over time, corresponding to the change of the type of audio content. In other words, the multimodal audio encoder can decide, for example, to encode portions of the audio signal; with speech content, using an encoding mode dedicated especially to speech encoding and to use another encoding mode to encode different portions audio content that represents non-voice content, such as music. Time domain encoding modes such as codebook excited linear prediction encoding modes tend to be more suitable for encoding speech content, whereas transform encoding modes tend to perform better than modes. encoding in the time domain when it comes to music encoding, for example.

Solutions have already been devised to address the problem of coping with the coexistence of different types of audio within an audio signal. The current emerging USAC, for example, suggests switching between one mode of a time domain coding mode that largely complies with the AAC standard and two additional linear prediction modes similar to the subframe modes of the standard. AMR-WB plus, ie the MDCT (Modified Discrete Cosine Transform) -based variant of the TCX mode (TCX = transformed coded excitation) and an ACELP mode (code excited linear prediction of an adaptive code). To be more precise, in the AMR-WB + standard, the TCX is based on a DFT transform, although in USAC the TCX has an MDCT transform base. A certain framing structure is used to switch between the AAC-like FD coding domain and the AMR-WB + -like linear prediction domain. The AMR-WB + standard itself uses its own framing structure that forms a subframe structure with respect to the standard. The AMR-WB + standard results in a certain subdivision configuration that subdivides AMR-WB + frames into smaller TCX and / or ACELP frames. Similarly, the AAC standard uses a base framing structure, although it results in the use of different window lengths to transform encode and transform box content. For example, you can use a long window and a long associated transform length, or eight short windows with shorter associated length transformations.

The MDCT causes an aliasing effect. This is true, therefore, within the limits of the TXC and FD tables. In other words, as in the case of any encoder in the frequency domain that MDCT uses, aliasing occurs in the overlapping rulers of the windows, which is canceled with the help of adjacent frames. In other words, in the case of transitions between two FD frames or between two TCX frames (MDCT) or the transition between FD and TCX or from TCX to FD, there is an Implicit cancellation of the aliasing effect by the overlap and sum procedure within of the decoder side reconstruction. After that there is no more aliasing effect after overlap and sum.

However, in the case of transitions with ACELP, there is no inherent cancellation of the aliasing effect. Therefore, a new tool must be presented that can be called FAC (cancellation of the forward aliasing effect). The FAC serves to cancel the aliasing effect coming from the adjacent boxes, if they are different from ACELP.

In other words, aliasing effect cancellation problems occur when transitions occur between the transform encoding mode and the time domain encoding mode. In order to execute the transformation from the time domain to the spectral domain in the most efficient way possible, we use the transformed coding with cancellation of the effect in the time domain, such as MDCT, that is, a coding mode that uses an overlapping transform where poisoned portions are transformed into an overlapping form of a signal using a transform according to which the number of transformation coefficients per portion is less than the number of samples per portion, so that the aliasing effect occurs only with regard to individual portions, where this aliasing effect is canceled thanks to the cancellation of the aliasing effect in the time domain, that is, adding the overlapping aliasing portions of the portions of the adjacent retransformed signal. MDCT is that type of aliasing effect cancellation transform in the time domain. Unfortunately, TDAC (the cancellation of the aliasing effect in the time domain) is not possible in the transitions between the TC coding mode and the coding mode in the time domain.

To solve this problem, forward aliasing effect cancellation (FAC) can be used whereby the encoder signals within the stream of additional FAC data within a current frame whenever a change in encoding mode occurs. transform coding to time domain coding. However, this makes it essential to compare the encoding modes of consecutive frames in order to determine whether the current decoded frame comprises FAC data within its syntax or not. This, in turn, means that there are tables for which the decoder may not be sure whether to read or analyze the FAC data for the current table or not. In other words, in the event that one or more frames are lost during transmission, the decoder does not know whether or not a mode change has occurred in successive (received) frames, and whether the data bit stream encoded in the current box contains FAC data or not. Consequently, the decoder has to drop the current frame and wait for the next frame. On the other hand, the decoder can carry out the syntactic analysis of the current table by executing two decoding tests, one assuming the presence of FAC data and the other assuming the absence of FAC data, to subsequently decide if any of these alternatives fails.

The decoding process will most likely cause the decoder to fail in one of two conditions. In other words, this last possibility is not really a feasible approach. The decoder must know at all times how to interpret the data and not rely on his own speculation on how to handle the data.

Accordingly, an object of the present invention is to provide a codec that is stronger against errors or stronger against frame loss, and yet supports switching between aliasing effect cancellation in transform encoding mode. time domain and coding mode in the time domain.

This objective is achieved by the subject matter set forth in any of the appended claims herein.

The present invention is based on the finding that a more robust codec against errors or more robust against loss of frames can be obtained that supports switching between the cancellation of the aliasing effect in the time-domain transform coding mode and the time domain coding mode if another portion of syntax is added to the tables on the basis of which the decoder parser can select from a first estimate action the current frame understand, and thus read the cancellation data of the aliasing effect forward from the current frame and a second action that does not expect the current frame to understand, and therefore does not read the cancellation data of the aliasing effect forward of the current frame. In other words, although a bit of coding efficiency is lost due to the inclusion of the second syntax portion, it is precisely the second syntax portion that confers the ability to use the codec in the case of a communication channel with loss of frames. Without the second syntax chunk, the decoder would not be able to decode any chunk of the data stream after a loss and would fail when trying to resume parsing. Therefore, in an error-prone environment, coding efficiency is prevented from disappearing by introducing the second syntax portion.

Other preferred embodiments of the present invention form the subject of the dependent claims. Furthermore, the preferred embodiments of the present invention are described in more detail below in connection with the figures. In particular

Figure 1 illustrates a schematic decoder diagram according to one embodiment;

Figure 2 illustrates a schematic block diagram of an encoder according to one embodiment;

Figure 3 illustrates a block diagram of a possible implementation of the reconstructor of Figure 2;

blocks a

Figure 4 illustrates a block diagram of a possible implementation of the FD decoder module of Figure 3;

Figure 5 illustrates a block diagram of possible implementations of the LPD decoding modules of Figure 3;

Figure 6 illustrates a schematic diagram describing the coding procedure in order to generate FAC data according to one embodiment:

Figure 7 illustrates a schematic diagram of the possible retransformed TDAC transform according to one embodiment;

Figures 8, 9 illustrate block diagrams for describing the path routing of the FAC data in the encoder of other processing performed in the encoder to evaluate the change of encoding mode in the direction of optimization;

Figures 10, 11 illustrate block diagrams of decoder handling in order to derive the data in Figures 8 and 9 from the data stream;

I

Figure 12 illustrates a schematic diagram of the FAC-based reconstruction of the decoder side across the frame boundaries of different encoding modes;

Figures 13, 14 schematically illustrate the processing performed on the manipulator of Figure 3 to carry out the reconstruction of Figure ¹²: ;

Figures 15 to 19B illustrate portions of a syntax structure according to one embodiment and

Figures 20 A through 22 illustrate portions of a syntax structure according to another embodiment.

The Figure illustrates a decoder 10 in accordance with an embodiment of the present invention. Decoder 10 is for decoding a data stream comprising a sequence of frames 14a, 14b and 14c in which time slots 16a-c of an information signal 18 are encoded, respectively. As illustrated in Figure 1, the time segments 16a to 16c are non-overlapping segments that simply splice together in time and are sequentially ordered in time. As illustrated in Figure 1, time slots 16a through 16c can be of equal size, although other embodiments are also feasible. Each of the time slots 16a to 16c is encoded in one of the respective boxes 14a to 14c. In other words, each time segment 16a to 16c is exclusively associated with one of the tables 14a to 14c, which, in turn, also have a defined order between them, which follows the order of the segments 16a to 16c encoded in the Tables 14a to 14c, respectively. Although Figure 1 suggests that each frame 14a to 14c is of equal length, measured for example in coded bits, this is of course not exclusive. On the contrary, the length of the frames 14a to 14c can vary according to the complexity of the time segment 16a to

16c with which the respective table 14a to 14c is associated.

To facilitate the explanation of the embodiments outlined below, the information signal 18 is presumed to be an audio signal. However, it should be noted that the information signal could also be any other signal, such as for example a signal emitted by a physical sensor or the like, such as an optical sensor or the like. In particular, signal 18 can be sampled at a certain sampling rate, and time segments 16a through 16c can cover immediately consecutive portions of this signal 18 equal in time and number of samples, respectively. A number of samples per time segment 16a to 16c can be, for example, 1024 samples.

Decoder 10 comprises a parser 20 and a rebuilder 22. Parser 20 is configured to parse data stream 12 and, by parsing data stream 12, read a first portion of syntax.

24 and a second syntax portion 26 of a current frame 14b, ie a frame to be decoded at the moment. In Figure 1, it is presumed, by way of example, that frame 14b is the frame to be decoded at the moment, while frame 14a is the frame that has just been decoded. Each table 14a to 14c consists of a first syntax portion and a second syntax portion which is incorporated therein and has a significance or meaning outlined below. In Figure 1, the first syntax portion within Tables 14a to 14c is indicated with a box containing a "1" and the second syntax portion is indicated with a box titled "2".

Naturally, each frame 14a to 14c further contains other information which is to represent the associated time segment 16a to 16c in a manner set forth in more detail below. This information is indicated in Figure 1 by a grid block in which reference number 28 is used for the additional information of the current table 14b. The parser 20 is configured to read the information 28 in the current table 14b when analyzing the data stream 12.

Reconstructor 22 is configured to reconstruct current time slot 16b of information signal 18 associated with current frame 14b based on additional information 28 using one of a time domain transform decoding mode with effect cancellation aliasing and a decoding mode in the time domain. Selection depends on first syntax element 24. Both decoding modes differ from each other by the presence or absence of some transition from the spectral domain4 back to the time domain using a retransform. The retransformation (along with its corresponding transformation) introduces the aliasing effect for the individual time slots, aliasing which can, however, be offset by a cancellation of the aliasing effect in the time domain as regards transitions at the boundaries between consecutive frames encoded in transform encoding mode in * »the time domain with cancellation of the aliasing effect. The decoding mode in the time domain does not require any transformation. Rather, decoding remains in the time domain. Therefore, in general terms, the time domain transform decoding mode with cancellation of the aliasing effect of the reconstructor 22 entails the execution of a retransformation by the reconstructor 22. This retransformed maps a first number of transform coefficients obtained from the information 28 of the current table 14b (which is in a TDAC transform decoding mode) on a retransformed signal segment with a sampling length of a second number of samples, which is greater than the first number, thus causing the aliasing effect. The decoding mode in the time domain, in turn, may entail a linear prediction decoding mode according to which the excitation and linear prediction coefficients are reconstructed from Information 28 of the current table which, in that case, it is the time domain encoding mode.

Therefore, as evidenced in the discussion above, in the time domain transform decoding mode with cancellation of the aliasing effect, the reconstructor 22 obtains from the information 28, a signal segment to reconstruct the signal of information in the respective time segment 16b by a retransformed. The retransformed signal segment is actually longer than current time segment 16b and participates in the reconstruction of information signal 18 within a time slice that includes and extends beyond time segment 16b. Figure 1 illustrates a transformation window 32 used in the transformation of the original signal or both transformation and retransformation. As can be seen, the window 32 can comprise the zero portion 32i at the beginning thereof and a zero portion 32₂ at the drag end of it, and aliasing effect portions 32₃ and 324 at a leading and trailing edge the current time slice 16b, where a portion with no aliasing effect 32₅ where window 32 is one, it can be located between both overlapping portions 32₃ and 32₄. Zero portions 32-i and 32₂ they are optional. It is also possible that only one of the zero portions 32i and 32 is present.₂. As illustrated in Fig. 1, the window function can be monotonically increasing / decreasing within the overlapping portions. The allasing effect takes place within the overlapping portions 32₃ and 32₄ where window 32 continuously advances from zero to one or vice versa. Overlap is not critical, provided that the previous and successive time slices are also encoded in. I the time domain transform encoding mode with cancellation of the allasing effect. This possibility is illustrated in figure 1 with respect to the time segment 16c. A dashed line illustrates a respective transformation window 32 'corresponding to time segment 16c whose overlapping portion coincides with overlapping portion 32₄ of the current time segment 16b. The sum of the retransformed segment signals of time segments 16b and 16c by reconstructor 22 cancels the overlap of both retransformed signal segments with each other. . .

However, in cases where the preceding or succeeding frame 14a or 14c is encoded in the time domain encoding mode, a transition between different encoding modes occurs at the leading or trailing edge of the current time segment. 16b and, in order to justify the respective allasing, data stream 12 comprises cancellation data of the forward allasing effect within the respective frame following Immediate to transition to enable decoder 10 to compensate for the allasing that takes place in this respective transition. For example, it may happen that the current frame 14b is in the time domain transform encoding mode with aliasing effect cancellation, although decoder 10 does not know if the previous frame 14a was in the time domain encoding mode. For example, frame 14a may have been lost during transmission, and therefore decoder 10 does not have access to it. However, depending on the encoding mode of frame 14a, current frame 14b comprises forward aliasing effect cancellation data in order to compensate for aliasing that occurs in overlapping portion 323 or not. Similarly, if the current frame 14b was from the time domain encoding mode, and the previous frame 14a has not yet been received by decoder 10, then the current frame 14b has built-in forward aliasing effect cancellation data to it or not, depending on the mode of the previous table 14a. In particular, if the above frame 14a was from the other encoding mode, i.e. the cancellation of the aliasing effect in the time domain transform encoding mode, then there would be forward cancellation data from the aliasing effect present in the current frame. 14b to cancel the aliasing effect that otherwise takes place at the boundary between. time segments 16a and 16b. However, if the previous frame 14a was the same coding mode, i.e. the coding mode in the time domain, then the parser 20 would not expect there to be any forward aliasing effect cancellation data present in the current frame 14b.

Accordingly, parser 20 takes advantage of a second syntax portion 26 to elucidate whether or not forward aliasing effect cancellation data 34 is present in current frame 14b or not. In performing the synthetic analysis of the data stream 12, the parser 20 can select one of a first action which is to estimate that the current frame 14b comprises, and therefore reads, cancellation data of the forward aliasing effect 34 of the current frame 14b and a second action that consists of not estimating that the current frame 14b understands, and therefore does not read, cancellation data of the forward aliasing effect 34 of the current frame 14b, where the selection depends on the second syntax portion 26. If present, the reconstructor 22 is configured to execute the cancellation of the forward aliasing effect on the boundary between the current time segment 16b and the previous time segment 16a of the Table 14a above using the cancellation data of the forward aliasing effect.

Accordingly, compared to the situation where the second syntax portion is not present, the decoder of figure 1 does not have to discard, nor unfavorably interrupt the synthetic analysis; of the current frame 14b even in the event that the coding mode of the previous frame 14a is unknown by the decoder 10 due to the loss of frames, for example. Rather, decoder 10 can take advantage of second syntax portion 26 to elucidate whether or not current frame 14b has effet cancellation data. forward aliasing 34. In other words, the second portion of syntax gives rise to the application of a clear criterion to determine if one of the alternatives, that is, if there is presence of FAC data corresponding to the limit with the preceding table or not, and guarantees that any decoder can work in the same way regardless of its implementation, even in case of frame loss. Accordingly, the above-outlined embodiment introduces mechanisms to overcome the problem of frame loss.

Before describing the more detailed embodiments set forth below, an encoder suitable for generating the data stream 12 of FIG. 1 with the respective FIG. 2 is described. The encoder of FIG. 2 is generally indicated with reference number 40 and is for encoding the information signal in data stream 12 such that data stream 12 comprises the sequence of frames in which they encode the time segments 16a to 16c of the information signal, respectively. Encoder 40 comprises a constructor 42 and an inserter 44. The constructor is configured to encode a current time slot 16b of the information signal into the current frame information 14b using a first mode selected from the cancellation of the aliasing effect in the time domain transform encoding mode and a mode coding in the time domain. Inserter 44 is configured to insert information 28 into current frame 14b along with a first syntax portion 24 and a second syntax portion 26, where the first syntax portion signals the first selection, i.e. the encoding mode selection . Builder 42, in turn, is configured to determine the forward aliasing effect cancellation data corresponding to the forward aliasing effect cancellation at a boundary between the current time segment 16b and a previous time segment 16a of a previous frame 14a and inserts the cancellation data of the forward aliasing effect 34 in the current frame 14b in case the current frame 14b and the previous frame 14a are encoded using different modes between cancellation of the aliasing effect in the time domain transform encoding mode and the time domain encoding mode, and refrains from inserting cancellation data of the aliasing effect forward into the current frame 14b in case the frame Current 14b and Table 14a above are encoded using the same mode, the time domain transform encoding with cancellation of the aliasing effect and the time domain encoding mode. In other words, whenever the constructor 42 of the encoder 40 decides that it is preferable, in the optimization sense, to switch from one of both encoding modes to the other, the constructor 42 and the inserter 44 are configured to determine and insert data from cancellation of the forward aliasing effect 34 in the current frame 14b, whereas, if the encoding mode is maintained between the frames 14a and 14b, no FAC 34 data is inserted in the current frame 14b. To allow the decoder to deduce from the current frame 14b, without knowledge of the contents of the previous frame 14a, whether there is FAC 34 data or not within the current frame 14b, the given portion of syntax 26 is set depending on whether the current frame 14b and Table 14a above is encoded using the same or different encoding modes, either the time domain transform encoding mode with aliasing effect cancellation or the time domain encoding mode. Examples of the embodiment of the second portion of syntax 26 are outlined below.

• An embodiment to which a codec, a decoder and an encoder according to the above-described embodiments belong is described below, which supports a special type of frame structure according to which the frames 14a to 14c themselves are subject to subframe and there are two different versions of time domain transform encoding mode with cancellation of aliasing effect. In particular, according to these embodiments which are described in detail below, the first portion of syntax 24 associates the respective frame from which it has been read with a first frame type, hereinafter referred to as encoding mode FD (frequency domain) or a second frame type, hereinafter referred to as the LPD encoding mode, and if the respective frame is of the second frame type, associates the subframes of a subdivision of the respective frame, made up of a number of subframes, with a respective subframe type, the first subframe type and the second subframe type. As more specifically detailed below, the first type of subframe may imply that the corresponding subframes are encoded by TCX, while the second type of subframe may entail that their respective subframes are encoded using ACELP, i.e. Adaptive Codebook Excitation Linear Prediction. However, any other type of codebook-excited linear prediction linear coding mode can also be used.

The reconstructor 22 of Figure 1 is configured to manipulate these different possible coding modes. For this purpose, the rebuilder 22 may be constructed in the manner indicated in Figure 3. In accordance with the embodiment of Figure 3, the rebuilder 22 comprises two switches 50 and 52 and three decoding modules 54, 56 and 58 each of which is configured to decode frames and subframes of a specific type, as further described below in more detail.

Switch 50 has an input into which information 28 from the frame being decoded at time 14b is input and a control input through which switch 50 can be controlled depending on the first syntax portion 25 of the current frame . The switch 50 has two outputs, one of which is connected to the input of the decoder module 54, which is responsible for the FD decoding (FD = frequency domain), and the other of which is connected to the input of the sub-switch. 52 which also consists of two outputs, one of which is connected to an input decoder module 56 responsible for decoding by linear prediction excited by codebook. All the encoder modules 54 to 58 emit signal segments that reconstruct the respective time segments associated with the respective frames and sub-frames from which these signal segments will derive by means of the respective decoding mode, and a 'transition manipulator 60 that receives the signal segments at the respective inputs thereof in order to execute the manipulation of the transitions and the cancellation of the aliasing effect described above and which is described in detail later on when it leaves the Reconstructed Information sign. The transition manipulator uses the forward aliasing effect cancellation data 34 as illustrated in Figure 3.

In accordance with the embodiment of Figure 3, the rebuilder 22 operates as follows. If the first syntax portion 24 associates the current frame with a first frame type, FD encoding mode, switch 50 forwards information 28 to FD Decoding Module 54 to use decoding in the frequency domain as the first version of the time domain transform decoding mode with aliasing effect cancellation to reconstruct time segment 16b associated with current frame 15b. Otherwise, that is, if the first syntax portion 24 associates the current frame 14b with the second frame type, the LPD encoding mode, switch 50 forwards information 28 to sub-switch 52 which, in turn, operates on the structure of the sub-frame of the current table 14. To be more precise, according to the LPD mode; A frame is divided into one or more subframes, where the subdivision corresponds to a subdivision of the corresponding time segment 16b into non-overlapping sub-portions of the current time segment 16b, as described in more detail below with respect to the following figures. Syntax portion 24 signals each of the sub-portions, whether they are associated with a first or second sub-frame type, respectively. If a respective subframe is of the first type of subframe, the sub-switch 52 forwards the respective information 28 belonging to that subframe to the TCX decoding module 56 in order to use linear prescription decoding excited by transform codes as_: second version ./ of the time domain transform decoding mode with cancellation of the aliasing effect to reconstruct the respective sub-portion of the current time segment 16b. If, on the other hand, the respective sub-frame is of the second type of sub-frame, the sub-switch 52 forwards the information 28 to the module 58 in order to execute the encoding by linear prediction excited by codebook as a decoding mode in the domain of time to reconstruct the respective sub-portion of the current time signal 16b.

The reconstructed signal segments emitted by the modules 54 to 58 are brought together by the transition manipulator 60 in the correct temporal order (display) with the execution of the respective manipulation and overlap and addition of the transition and the cancellation processing of the aliasing effect in the time domain described above and described below in more detail.

In particular, the FD Decoding Module 54 can be constructed in the manner outlined in Figure 4 and operates in the manner described below. According to FIG. 4, the FD decoding module 54 comprises a dequantizer 70 and a retransformer 72 connected in series with each other. As described above, if current frame 14b is an FD frame, it is sent to module 54 and quantizing device 70 performs a spectral variable dequantization of transform coefficient information 74 into information 28 of current frame 14b using the scale factor information 76 also included in information 28. Scale factors have been determined on the decoder side using, for example, psychoacoustic principles in order to keep the quantization noise below the masking threshold.

The retransformer 72 then performs a retransformation of the quartered transform coefficient information to obtain a retransformed signal segment 78 that extends, in time, over and beyond the time segment 16b associated with current frame 14b. As described in more detail below, the retransformation executed by retransformer 72 may be an IMDCT (Reverse Modified Discrete Cosine Transform) that involves a DCT IV followed by a deployment operation in which, once the poisoning has been executed using a retransform window that could be the same, or deviate, of the transformation window used to generate the transformation coefficient information 74 by executing the steps mentioned above in the Inverse order, that is, the poisoning followed by a folding operation followed by a DCT IV followed by the quantization that can be guided by psychoacoustic principles in order to keep quantization noise below the masking threshold.

It should be noted that the amount of Transformation Coefficients Information 28 is due to the TDAC nature of the retransformed of the retransformer 72, lower than the number of samples than the length of the reconstructed signal segment 78. In the case of the IMDCT, the number of transformation coefficients within Information 47 is practically equal to the number of samples in time segment 16b. In other words, the underlying transform can be called a critical sampling transform that requires cancellation of the aliasing effect in the time domain to cancel the aliasing effect that occurs due to the transformation at the boundaries, i.e. leading and trailing edges. drag the current time segment

16b.

As a minor note, it should be noted that as in the case of the subframe structure of LPD tables, FD tables could also be the subject of a subframe structure. For example, FD frames could be long window mode in which a single window is used to poison a portion of the signal that extends beyond the leading and trailing edges of the current time slot in order to encode the respective time segment, or a short window mode in which the respective signal portion extends beyond the edges of the current FD frame time segment is subdivided into smaller sub-clones, each of which undergoes a respective poisoning and transformation individually. In that case, the FD encoding module 54 would output a retransformed signal segment corresponding to the sub-portion of the current time segment 16b.

Having described a possible implementation of the coding module

FD 54 describes a possible implementation of the TCX LP decoding module and the LP decoding module excited by codebook 56 and 58, respectively, with respect to Figure 5. In other words, Figure 5 refers to the case in which the current frame is an LPD frame. In that case, the current frame 14b is structured into one or more subframes. In the present case, a structure is illustrated in three sub-frames 90a, 90b and 90c. It is possible that a structuring is limited, by default, to certain possibilities of substructuring. Each of the sub-portions is associated with a respective sub-portion 92a,

92b and 92c of the current time segment 16b. That is to say that said one or more sub-portions 92a to 92c cover, without gaps and without overlapping, the entire time segment 16b. In accordance with the order of sub-portions 92a to 92c within time segment 16b, a sequential order is defined between sub-frames 92a to 92c. As illustrated in Figure 5, the current table 14b is not fully subdivided into sub-tables 90a through 90c. In other words, some portions of current table 14b belong to all sub-boxes in common, such as the first and second syntax portions 24 and 26, FAC 34 data, and potentially other data such as LPC information, such as it is described in detail later, although LPC Information can also be substructured into individual sub-boxes. ¡

In order to manage the TCX subframes, the decoding module

TCX LP 56 comprises a spectral weighting shifter 94, a spectral weighting 96 and a retransformer 98. By way of illustration, it is; exposes the first sub-box 90a as the TCX sub-box, while the second sub-box 90b is presumed to be an ACELP sub-box.

To process the TCX subframe 90a, the wrapper 94 derives a spectral weight filter from the LPC information 104 within the information 28 of the current frame 14b, and the spectral weight 96 spectrally weights the transformation coefficient information within the respective subframe

90a using the spectral weight filter received from shunt 94 indicated by arrow 106.

Retransformer 98, in turn, retransforms the spectrally weighted transform coefficient Information to obtain a retransformed signal segment 108 that extends, at time t, over and beyond sub-portion 92a of the current time segment. The retransformed executed by retransformer 98 can be the same as the one executed by retransformer 72. In effect, the retransformer 72 and 98 may have common hardware, a software routine, or a portion of programmable hardware.

The LPC information 104 comprised of the information 28 of the current LPD table 16b can represent LPC coefficients of a single instance within time segment 16b or of multiple time instances within time segment 16b such as a series of coefficients of LPC for each sub portion 92a to 92c. The spectral weight filter wrapper 94 converts the LPC coefficients into spectral weight factors that spectrally weights the transformation coefficients within information 90a according to a transfer function that is derived from the LPC coefficients by the wrapper 94 of such that it substantially approximates the LPC synthesis filter or some modified version thereof. Any dequantization performed beyond the spectral weighting by the weighting

96, may be spectrally invariant. Therefore, unlike the FD decoding mode, the quantization noise according to the TCX encoding mode is spectrally formed using LPC analysis.

Due to the use of the retransformed, however, the retransformed signal segment 108 is affected by aliasing. Using the same retransformed, however, the aliasing effect of the retransformed signal segments 78 and 108 of the consecutive frames and subframes can be canceled by the transition manipulator 60 by merely adding the overlapping portions thereof.

In processing the subframes of (A) CELP 90b, the excitation signal wrapper 100 derives an excitation signal from the excitation update information within the respective subframe 90b and the LPC synthesis filter 102 performs the synthesis filtering LPC of the drive signal using LPC Information 104 in order to obtain a signal segment synthesized by LP 110 corresponding to sub-portion 92b of current time slot 16b.

Derivatives 94 and 100 may be configured to perform some interpolation to adapt LPC information 104 within current frame 16b to the variable position of the current subframe corresponding to the current subportion within current time slot 16b.

Collectively describing Figures 3 to 5, the various signal segments 108, 110, and 78 enter transition manipulator 60 which, in turn, gathers all signal segments in the correct order of time. In particular, transition manipulator 60 executes .time domain aliasing effect cancellation within temporarily overlapping window portions at the boundaries between immediately consecutive FD frame and TCX subframe time segments to reconstruct the information signal through these limits. Accordingly, there is no need for forward aliasing effect cancellation data for the boundaries between consecutive FD frames, the boundaries between FD frames followed by TCX frames, and TCX subframes followed by FD frames, respectively.

However, the situation changes whenever an FD box or TCX sub-box (which in both cases represents a variant of the transform encoding mode) comes from an ACELP sub-box (representing a form of encoding mode in the domain weather). In that case, transition manipulator 16 derives a forward aliasing effect cancel synthesis signal from the forward aliasing effect cancel data of the current frame and adds the first forward aliasing effect cancel synthesis signal to the segment signal retransformed 100 or 78 of the immediately preceding time slot to reconstruct the information signal across the respective boundary. IF the limit falls on the inner part of the current time slice 16b because a TCX sub-frame and an ACELP sub-box within the current frame define the boundary between the associated time segment sub-portions, the transition manipulator can determine the existence of the respective cancellation data of the forward aliasing effect corresponding to these transitions of the first syntax portion 24 and the subframe structure defined therein. Syntax portion 26 is not required. Table 14a above may or may not have been lost.

However, in the case of a boundary that coincides with the boundary between consecutive time slots 16a and 16b, parser 20 has to inspect the second syntax portion 26 within the current frame in order to determine if the current frame 14b has cancellation data of the forward aliasing effect 34, where the data of FAC 34 is to cancel the aliasing effect that occurs at the forward end of the current time slot 16b, since the previous box is an FD box or the last sub-box of the box

LPD above is a subframe of TCX. At the very least, the parser 20 must know the syntax portion 26 in case the content of the above frame has been lost.

Similar concepts apply to transitions in the other direction, that is, from ACELP subframes to FD frames or TCX frames. As long as the respective boundaries between the respective segments and segment sub-portions within the current time segment, the parser 20 has no problem determining the existence of forward aliasing effect cancellation data 34 corresponding to these frame transitions. actual 14b itself, ie from the first syntax portion 24. The second syntax portion is not necessary and is even irrelevant. However, if the boundary occurs at or coincides with the previous time segment 16a and the current time segment 16b, the parser 20 must inspect the second syntax portion 26 in order to determine if there is data cancellation of the forward allasing effect 34 present corresponding to the transition at the forward end of the current time segment 16b or not - at least in the case of not having access to the previous table.

In the case of ACELP to FD or TCX transitions, transition manipulator 60 derives a second forward allasing effect cancellation synthesis signal from the forward aliasing effect cancellation data 34 and adds the second synthesis synthesis signal. cancellation of the forward allasing effect to the retransformed signal segment within the current time segment in order to reconstruct the Information signal across the limit.

After describing the embodiments with respect to Figures 3 to 5, which generally referred to an embodiment according to which frames and sub-frames of different coding modes existed, a specific implementation of these embodiments is described below in more detail. detailed. The description of these embodiments Simultaneously includes possible measures to generate the respective data stream comprising said tables and sub-tables, respectively. This specific embodiment is described below as a unified voice and audio codec (USAC), although the principles outlined here apply to other signals as well.

Window switching in USAC has several purposes. It mixes the FD frames, that is, the encoded frames with frequency coding and LPD frames, which, in turn, are structured in sub) ACELP frames and (sub) TCX frames. The ACELP (time domain coding) boxes apply rectangular, non-overlapping poisoning to the input samples, while the TCX (frequency domain coding) boxes apply an overlapping non-rectangular poisoning to the samples. input and then encode the signal using a time domain aliasing effect cancellation transform (TDAC), i.e. the MDCT, for example.

To fully harmonize windows, TCX frames can use centered windows with homogeneous shapes and to handle transitions in ACELP frame boundaries, explicit information to cancel the aliasing effect in the time domain and transmit the effects of poisoning of the harmonized TCX windows. This additional information can be considered as cancellation of the forward aliasing effect (FAC). The FAC data is quantized in the following embodiment in the LPC weighted domain, so that the FAC quantization noises and the decoded MDCT are of the same nature.

Figure 6 illustrates the processing that takes place at the encoder in a transform encoding (TC) encoded frame 120 that is preceded and followed by a frame 122, 124 encoded by ACELP. In line with the foregoing description, the concept of TC includes long and short block MDCT using AAC, as well as MDCT-based TCX. In other words, frame 120 can be an FD frame or a (sub) TCX frame such as sub frame 20a 90a, 92a of Figure 5, for example. Figure 6 illustrates time domain markers and frame boundaries. The boundaries of the boxes or time segments are indicated by dashed lines, while the markers in the time domain are short vertical lines along horizontal axes.

It should be mentioned here that, in the following description, the terms "time segment" and "frame" are sometimes used synonymously due to the unique association between them.

Accordingly, the vertical dashed lines in FIG. 6 illustrate the beginning and end of frame 120 which may be a subframe / subpart of a time segment or a frame / time segment. LPC1 and LPC2 have to indicate the center of an analysis window corresponding to the LPC filter coefficients or LPC filters that are used next to, in order to execute the cancellation of the aliasing effect. These filter coefficients are derived in the decoder, for example, by the rebuilder 22 or by the tappers 90 and 100 by using interpolation using the information from LPC 104 (see Figure 5). The LPC filters comprise: LPC1 corresponding to a calculation thereof at the beginning of Table 120, and LPC2 corresponding to a calculation thereof at the end of Table 120. Table 122 is presumed to have been encoded by ACELP. The same applies to table 124.

Figure 6 is structured in a box numbered lines to the right of Figure 6. Each line represents a step of the processing that is performed in the encoder. It is to be understood that each line is temporarily aligned with the top line.

Line 1 in Figure 6 represents the original audio signal, segmented in Tables 122, 120 and 124 as indicated above. Thus, to the left of the “LPC1” marker, the original signal is encoded by ACELP. Between the "LPC1" and "LPC2" markers, the original signal is encoded using TC. As described above, in CT noise modeling is applied directly in the transform domain rather than the time domain. To the right of the LPC2 marker, the original signal is encoded by ACELP once again, i.e. a time domain encoding mode. This sequence of coding modes is chosen (ACELP then TC then ACELP) is to illustrate the processing in the FAC since the FAC relates to both transitions (from ACELP to TC and from TC to ACELP).

Note, however, that the transitions at LPC1 and LPC2 in Fig. 6 may occur within the interior of a current time slot or may coincide with the lead end thereof. In the first case, the determination of the existence of the associated FAC data can be performed by the parser 20, merely based on the first portion of syntax 24, whereas, in the case of loss of frames, the parser Syntax 20 may require syntax portion 26 to do so in the latter case.

Line 2 of Figure 6 corresponds to the decoded (synthesis) signals in each of Tables 122, 120 and 124. Accordingly, reference number 110 in Figure 5 is used within Table 122 corresponding to the possibility that the last sub-portion of Table 122 is a sub-portion encoded by ACELP as 92b in Figure 5, while a combination is used of reference numbers 108/78 to indicate the contribution of the signal to table 120, analogously to Figures 5 and 4. Once again, to the left of the LPC1 marker, the synthesis of that table 122 is presumed to have been encoded by ACELP. Therefore, synthesis signal 110 to the left of the LPC1 marker is identified as ACELP synthesis signal. There is, in principle, a great similarity between ACELP synthesis and the original signal in that table 122, since ACELP tends to encode the waveform as precisely as possible. Then, the segment between the LPC1 and LPC2 markers on line 2 of Figure 6 represents the output of the inverse MDCT of that segment 120 seen on the decoder. Again, segment 120 may be time segment 16b of an FD frame or a sub-portion of a TCX-encoded subframe, such as 90b in Figure 5, for example. In the figure, this segment 108/78 is called the "TC frame output". In Figures 4 and 5, this segment was called the retransformed signal segment. In case frame / segment 120 is a subpart of TCX segment, the TC frame output represents a reinvented TLP synthesis signal, where TLP stands for "Linear Prediction Transform Coding" to indicate that, in the case From TCX, noise modeling of the respective segment in the transform domain is obtained by filtering the MDCT coefficients using the spectral information from the LPC filters LPC1 and LPC2, respectively, which has also been described with respect to figure 5 with respect to the spectral weight 96. Note also that the synthesis signal, that is to say the signal reconstructed in a preliminary form that includes aliasing, between the markers "LPC1" and "LPC2" of line 2 of figure 6, ie signal 108/78, contains poisoning and aliasing effects in the time domain at the beginning and at the end. In the case of MDCT as a TDAC transform, time domain aliasing may be symbolized in the form of cleavages 126a and 126b, respectively. In other words, the upper curve of line 2 of figure 6 that extends from the beginning to the end of that segment 120 and is indicated by reference numbers 108/78, illustrates the effect of poisoning due to that the transformation poison is flat in the middle to leave the transformed signal unchanged, but not at the beginning and end. The bending effect is indicated by the Lower curves 126a and 126b at the beginning and end of segment 120 with the least sign at the beginning of the segment and the plus sign at the end of the segment. This effect of poisoning and aliasing (or doubling) in the time domain is inherent in the MDCT, which serves as an explicit example of TDAC transforms. Aliasing can be canceled when two consecutive frames are encoded using the MDCT as described above. However, in case the MDCT-120 encoded frame is not preceded or followed by other MDCT frames, its poisoning and aliasing in the time domain is not canceled and remains in the signal in the time domain after the MDCT reverse. Cancellation of the forward aliasing effect (FAC) can then be used to correct these effects in the manner outlined above. Finally, segment 124 after the LPC2 marker of Figure 6 is also presumed to be encoded using ACELP. Note that to obtain the synthesis signal in this table, the filter states of the LPC 102 filter (see Figure 5), that is, the memory of the long-term and short-term predictors, at the beginning of table 124 must be correctly which implies that the effects of temporary and poisoned aliasing at the end of the previous frame 120 between the LPC1 and LPC2 markers must be canceled by applying FAC in a specific way that is explained later. To summarize, line 2 of figure 6 contains the preliminary reconstructed signal synthesis of consecutive tables 122, 120 and 124, which includes the effect of poisoned aliasing in the time domain at the exit of the inverse MDCT corresponding to the box between the LPC1 and LPC2 markers.

To obtain line 3 of figure 6, the difference between line 1 of figure 6, that is to say in the original audio signal 18, and line 2 of figure 6, that is to say the synthesis signals 110 and 108/78, respectively, as described above. This gives a first difference signal 128.

The following explains the post-processing performed on the encoder side in relation to frame 120 with respect to line 3 in figure 6. At the beginning of frame 120, first, two contributions from the ACELP 110 synthesis are added together. to the left of the LPC1 marker on line 2 of Figure 6 as follows:

The first contribution 130 is a poisoned and time-reversed (folded) version of the last ACELP synthesis samples, ie the last samples of signal segment 110 illustrated in Figure 5. The length and window shape corresponding to this time reversed signal equals overlapped part of transformation window to left of frame 120. This contribution 130 can be considered a good approximation to the aliasing in the time domain present in the MDCT table 120 of line 2 in figure

6.

The second contribution 132 is a poisoned zero input response (ZIR) of the LPC1 synthesis filter where the Initial state is taken as the final state of this filter at the end of the ACELP 110 synthesis, that is, at the end of Table 122. The length and The window shape of this second contribution may be the same as that of the first contribution 130.

With the new line 3 of figure 6, that is, after adding the two contributions 130 and 132 mentioned above, the encoder takes a new difference to obtain line 4 of figure 6. Note that the difference signal 134 stops at the LPC2 marker. A rough view of the estimated envelope of the error signal in the time domain is indicated on line 4 of Figure 6. The error in the ACELP 122 table is estimated to be approximately flat in time domain. Subsequently, it is estimated that the error in the TC 120 table must exhibit the same general shape, that is, envelope in the time domain, as indicated in this segment 120 of line 4 in Figure 6. In this document, Polish this estimated form of the amplitude of error for explanatory purposes.

Note that if the decoder were to use only the synthesis signals on line 3 of Figure 6 to produce or reconstruct the decoded audio signal, then the quantization noise would generally be as the estimated envelope of the error signal 136 from line 4 of figure 6. Accordingly, it is to be understood that a correction should be sent to the decoder to compensate for this error at the beginning and end of the TC 120 frame. This error comes from the poisoning and time domain aliasing effects inherent in the MDCT / pair.

Reverse MDCT. The aliasing effect of the poisoned and the time domain has been reduced at the beginning of table TC 120 by adding the two contributions 132 and 130 from the previous ACELP table 122 as indicated above, although it cannot be entirely canceled as in the actual TDAC operation of consecutive MDCT frames. To the right of the TC 120 box on line 4 of Figure 6 immediately before the LPC2 marker, the entire aliasing effect of the poisoned and the time domain of the inverse MDCT / MDCT pair is preserved, and therefore must be canceled entirely by canceling the forward aliasing effect.

Before proceeding to describe the encoding process to obtain the forward aliasing effect cancellation data, reference is made to Figure 7 to briefly explain the MDCT as an example of TDAC transform processing. Both transformation directions are illustrated and described with respect to Figure 7. The transition from the time domain to the transform domain is illustrated in the upper half of Figure 7, while the retransformed is illustrated in the lower part of Figure 7.

In making the transition from the time domain to the transform domain, the TDAC transform involves a poison 150 applied to an interval 152 of the signal to be transformed, which extends beyond the time segment 154 with respect to which these last obtained transformation coefficients are actually to be transmitted within the data stream. The window applied in the poisoned 150 appears in figure 7 comprising an aliasing or overlapping part Lk that crosses the lead end of time segment 154 and an aliasing or overlapping part R_k at the rear end of time segment 154 with an unaliased part M_k that extends between them. An MDCT 156 is applied to the poison signal. In other words, a bend 158 is performed in order to fold a first quarter of interval 152 that extends between the lead end of interval 152 and the lead end of time segment 154 by returning along the left (lead) limit. ) of time segment 154. The same is done with respect to an aliasing portion R_k. Next, a DCT IV 160 of the poisoned and bent signal thus produced is run with as many samples as the temporal signal 154 to obtain transformation coefficients of the same number. At quantization a conversation is carried out at 162. Naturally, quantization 162 can be considered not understood by the TDAC transform.

A retransformed does the reverse. In other words, after dequantization 164, an IMDCT 166 is executed, which involves, first of all, a DCT¹ IV 168 in order to obtain time samples, the number of which is equal to the number of samples in time segment 154; to be rebuilt. Next, a unfolding process 168 of the inversely transformed signal portion received from module 168 is executed to thereby expand the time interval or the number of temporary samples of the IMDCT result by doubling the length of the overlapping portions. Then a poisoning at 170 is executed, using a retransformation window 172 that may be the same as that used by the poisoning 150, although it may also be different. The rest of the blocks in Figure 7 illustrate the TDAC or overlap and sum processing performed on the overlapping portions of the consecutive segments 154, i.e. the sum of the overlapped unfolded portions thereof, performed by the transition manipulator in the Fig. 3. As illustrated in Figure 7, the TDAC executed by blocks 172 and 174 results in the cancellation of the aliasing effect.

We now proceed to the description of figure 6. To efficiently compensate for the aliasing effects of poisoning and time domain at the beginning and at the end of the table of TC 120 on line 4 of figure 6, and assuming that the TC 120 frame uses frequency domain noise modeling (FDNS), forward aliasing effect correction (FAC) is applied following the processing described in Figure 8. First, it should be noted that Figure 8 describes this processing with respect to both the left side of the TC 120 frame around the LPC1 marker, and with respect to the right side of the TC 120 frame around the LPC2 marker. Recall that the TC 120 box in Figure 6 is presumed to be preceded by an ACELP 122 box at the boundary of the LPCT marker and followed by an ACELP box

124 at the limit of the LPC2 marker.

To compensate for the aliasing effects of poisoning and the time domain around the LPC1 marker, the processing is described in Figure 8.

First, a W (z) weight filter of the LPC1 filter is computed. The W (z) weighting filter could be either a modified analysis or a LPC1 bleach filter A (z). For example W (z) = Α (ζ / λ) where λ is a predetermined weighting factor. The error signal at the beginning of the CT frame is indicated with reference number 138 as in the case of line 4 of figure 6. This error is called the FAC target in figure 8. The error signal 138 is filtered by the filter W (z) at 140, where an Initial state of this filter, that is to say where an initial state of this filter memory, is the error of ACELP 141 in the table of ACELP 122 of line 4 in figure 6. The output of filter W (z) then forms the input of a transform 142 in figure 6. The transform is exposed by way of example as MDCT. The transformation coefficients emitted by the MDCT in the processing module 143 are then quantized and encoded. These encoded coefficients can make up at least part of the aforementioned FAC data 34. These encoded coefficients can be transmitted to the encoding side . The output of the Q process, that is, the quantized MDCT coefficients, is subsequently entered as input of an inverse transform such as an IMDCT 144 to constitute a signal in the time domain that is then filtered by the Inverse filter 1 / W (z) at 145 it has zero memory (Initial zero state). Filtering through 1 / W (z) extends beyond the length of the FAC blank using zero input, for samples that extend after the FAC blank. The filter 1 / W (z) output is a FAC 146 synthesis signal, which is a correction signal that can now be applied at the beginning of the TC 120 frame to compensate for the aliasing effect of the poisoning and the time domain effect that are produced there.

The correction processing of the aliasing effect of the poisoning and the time domain at the end of the table of TC 120 (before the LPC2 marker) is now described. For this purpose reference is made to figure 9.

The error signal at the end of the TC 120 box on line 4 of figure 6 5 bears the reference number 147 and represents the FAC target of figure 9. The FAC 147 target undergoes the same processing sequence than the FAC 138 blank of Figure 8, where the processing differs only in the Initial state of the W (z) 140 weighting filter. The initial state of filter 140 to filter the target from FAC 147 is the error from table TC 120 of line 4 of figure 6, indicated by reference number 148 in figure 6. Next, the other processing steps 142 to 145 are the same as in figure 8 which referred to the FAC blank processing at the beginning of the CT picture

120.

The processing of Figures 8 and 9 is fully executed from left to right if applied to the encoder to obtain local FAC synthesis and to compute the reconstruction thus obtained in order to determine whether the change in encoding mode involved in the Choosing the TC 120 encoding mode of the Box 120 is the optimal choice. In the decoder, the processing performed in Figures 8 and 9 is applied only from the midpoint to the right. In other words, the encoded and quantized transformation coefficients transmitted by the Q 143 processor are decoded to constitute the input of the IMDCT. See, for example, Figures 10 and 11. Figure 10 is the same as the right hand of Figure 8, while Figure 11 is; equal to the right hand of figure 9. The transition manipulator 60 of Figure 3 can be implemented, in accordance with the specific embodiment outlined below, in accordance with Figures 10 and 11 ·. That is, the transition manipulator 60 can subject the transformation coefficient information within the FAC 34 data present within the current frame 14b to a retransform to produce a first FAC synthesis signal 146 in the event of the transition of a sub -part of an ACELP time segment to a subpart of an FD or TCX time segment, or a second FAC 149 synthesis signal when transitioning from a sub-part of a time segment of

FD or TCX to a subpart of an ACELP time slice.

Please note once again that the FAC 34 data may refer to

I said transition takes place within the current time slot, in which case the parser 20 can derive the existence of the FAC data 34 only from the syntax portion 24, while the parser 20 has to take advantage of the portion of syntax 26 if the previous frame has been lost, in order to determine if there are FAC 34 data corresponding to the leading edge of the current time segment 16b.

Figure 12 illustrates how the complete synthesis or the reconstructed signal corresponding to the current frame 120 can be obtained using the FAC synthesis signals of Figures 8 to 11 and applying the inverse steps to those of Figure 6. Note also that even the steps now illustrated in Figure 12 are also performed by the encoder in order to determine whether the encoding mode corresponding to the current frame results in the best optimization, for example, of the transmission rate / sense of distortion or the like. In figure 12, it is summarized that the ACELP table 122 to the left of the LPC1 marker has already been synthesized or reconstructed, for example by module 58 of figure 3, up to the LPC1 marker, thus leading to the synthesis signal ACELP on the line

2 Figure 12 with reference number 110. Since an FAC correction is also used at the end of the TC frame, it is also assumed that the frame 124 after the LPC2 marker must be an ACELP frame. Then, to produce a synthesis or reconstructed signal in the TC 120 frame between the LPC1 and LPC2 markers of Figure 12, the following steps are performed.

These steps are also illustrated in Figures 13 and 14, where Figure 13 illustrates the steps performed by the transition manipulator 60 in order to cope with the transitions of a segment or part of a segment encoded by TC to a sub -part of a segment encoded by ACELP, while figure 14 describes the operation of the transition manipulator in the case of reverse transitions.

one. One step is to decode the MDCT-encoded TC frame and position the signal in the time domain thus obtained between the LPC1 and LPC2 markers, as illustrated in. line 2 of figure 12. Decoding is performed by module 54 or module 56 and Includes reverse MDCT as an example of TDAC retransforming, so that the decoded TC frame contains aliasing effects of poisoning and time domain. In other words, the time segment segment or sub-part to be currently decoded, and indicated by the index k in Figures 13 and 14, may be a time segment sub-part encoded by ACELP 92b as illustrated in FIG. 13 or a time slice 16b that is FD encoded or a TCX 92a encoded subpart as illustrated in FIG. 14. In the case of FIG. 13, the previously processed frame is therefore a TC-encoded time segment or sub-part, and in the case of FIG. 14, the previously processed segment is an encoded sub-part by ACELP. The reconstruction or synthesis signal produced as output from modules 54 to 58 is *

partially affected by aliasing effects. This is also the case for signal segments 78/108.

2. Another step of the processing of the transition manipulator 60 consists in the generation of the FAC synthesis signal according to Figure 10 in the case of Figure 14, and according to Figure 11 in the case of Figure 13. It is To say that the transition manipulator 60 can execute a retransform 191 on transformation coefficients within the FAC 34 data, in order to obtain the FAC synthesis signals 146 and 149, respectively. Synthesis signals FAC 146 and 149 are located at the beginning and end of the CT-encoded segment, which in turn is affected by aliasing effects and is level with time segment 78/108. In the case of Fig. 13, for example, transition manipulator 60 positions the FAC synthesis signal

149 at the end of the TC-k-1 encoded frame as also illustrated on the line in Figure 12. In the case of Figure 14, the transition manipulator 60 positions the FAC 146 Synthesis signal at the beginning of the TC-encoded frame k, as also illustrated on line 1 of figure 12. Note again that frame k is the frame to be currently decoded and frame k-1 is the frame previously decoded.

3. Regarding the situation in figure 14, where a coding mode change occurs at the beginning of the current CT frame k, the poisoned and doubled ACELP synthesis signal (inverted) 130 of the ACELP frame picture k-1 preceding the TC k frame, and the poisoned zero input response, or ZIR, of the LPC1 synthesis filter, i.e. signal 132, they are positioned so that they are at the same level as the retransformed signal segment 78/108 are affected by allaslng. This contribution is shown on line 3 of figure 12. As illustrated in Figure 14 and as already described above, transition manipulator 60 obtains the allaslng effect cancellation signal 132 by continuing LPC synthesis filtering of the preceding CELP subframe beyond the advance limit of the current time segment k and poisoning the continuation of signal 110 within current signal k, where both steps are indicated with reference numbers 190 and 192 in figure 14. To obtain the allaslng effect cancellation signal 130, the transition manipulator 60 also poisons, in step 194, the reconstructed signal segment 110 of the preceding CELP frame and uses this time-reversed and poisoned signal as signal 130.

Four. The contributions of lines 1, 2 and 3 of figure 12 and the contributions 78/108, 132, 130 and 146 of figure 14 and the contributions 78/108, 149 and 196 of Fig. 13, are added by the Transition manipulator 60 at the leveled positions explained above, to form the synthesis or reconstructed audio signal corresponding to the current frame k in the original domain, as indicated on line 4 of Fig. 12. Note that the processing of Fig . 13 and 14 produces a synthesis or reconstructed signal 198 in a CT frame in which the aliasing effects in the time domain are canceled and poisoned at the beginning and end of the frame, and where the potential discontinuity of the frame boundary around the marker LPC1 has been smoothed out and masked perceptually by filter 1 / W (z) of figure 12.

Accordingly, Figure 13 refers to the current processing of the CELP k-encoded frame and leads to cancellation of the forward aliasing effect at the end of the preceding TC-encoded segment. As stated in 196, the ultimately reconstructed audio signal is reconstructed without aliasing across the boundary between segments k-1 and k. The processing of Figure 14 leads to cancellation of the forward aliasing effect at the beginning of the current TC k-encoded segment indicated by reference number 198 illustrating the reconstructed signal across the boundary between segments k and k-1. The rest of the aliasing at the rear end of the current segment k is canceled by ta TDAC in case the next segment is encoded by TC, or by FAC according to figure 13 in case the subsequent segment is a segment encoded by ACELP . Figure 13 mentions this last possibility by assigning the reference number 198 to the signal segment of the time segment k-1.

Specific possibilities for how the second portion of syntax 26 can be implemented are mentioned in the following paragraphs.

For example, to deal with the appearance of missing frames, the syntax portion 26 may be configured in the form of the prev_mode 2-bit field, which explicitly signals, within the current frame 14b, the encoding mode to be applied in the previous table 14a according to the following table:

<td>prev_mode</td><td></td><td></td>

<td>ACELP</td><td> 0</td><td> 0</td>

<td>TCX</td><td> 0</td><td> , 1</td>

<td>FD_long</td><td> 1</td><td> 0</td>

<td>FD_short</td><td> 1</td><td> 1</td>

In other words, this 2-bit field can be called prev_mode, and therefore can indicate an encoding mode from the previous table 14a.

In the case of the example just mentioned, four different states are distinguished, namely;

1) The previous table 14a is an LPD table, whose last sub-table is an ACELP sub-table;

2) the previous frame 14a is an LPD frame, the last sub-frame of which is a TCX-encoded subframe;

3) The box above is an FD box that uses a long transform window and

4) The box above is an FD box that uses short transform windows.

The possibility of potentially using different window lengths of the FD encoding mode has already been mentioned above in connection with the description of Fig. 3. Naturally, the syntax portion 26 can have three different states and the FD encoding mode can be executed simply with a constant window length, thus summarizing the last two options 3 and 4 of those listed above.

In any case, on the basis of the 2-bit field outlined above, the parser 20 can decide whether FAC data is present corresponding to the transition between the current time segment and the previous time segment 16a within the current frame 14a or not. As described in more detail below, parser 20 and reconstructor 22 can even determine, based on prev_mode, if the previous frame 14a has been an FD frame using a long window (FD_lopg) or if the previous frame has been an FD frame using short windows (FD_short) and if the current frame 14b (if the current frame is a LPD box) follows an FD box or an LPD box whose differentiation is required according to the following embodiment in order to correctly parse the data flow syntax and reconstruct the information signal, respectively.

Accordingly, in accordance with the possibility just mentioned of using a 2-bit identifier as a syntax portion 26, each table 16a to 16c would be provided with an additional 2-bit identifier in addition to the syntax portion 24 that defines that the encoding mode of the current frame must be an FD or LPD encoding mode and the sub-frame structure in the case of the LPD encoding mode.

As for all the embodiments described above, it should be mentioned 5 that other dependencies between frames should also be avoided. For example, the decoder in Figure 1 might have SBR capability. In that case, a crossover frequency could be parsed by parser 20 of each table 16a to 16c within the respective SBR extension data instead of parsing said crossover frequency with a header of

SBR that could be transmitted within data stream 12 less frequently. In the same sense, other dependencies between tables could be eliminated.

It should be noted in regard to all of the embodiments described above, that the parser 20 could be configured to buffer at least the currently decoded frame 14b within a buffer with the passage of all of the frames 14a to 14c through this buffer in FIFO fashion (the first to enter is the first to exit). By using the buffer, parser 20 could perform frame removal from this buffer in frame units 14a through 14c. In other words, the loading and evacuation of the buffer of the parser 20 could be carried out in frame units 14a to 14c to comply with the restrictions imposed by the maximum available space in the buffer that can hold, for example, only one or more than one frame of maximum size at a time.

An alternative signaling possibility corresponding to syntax portion 26 with reduced bit consumption is described below. In accordance with this alternative, a different construction structure from syntax portion 26 is used. In the embodiment described above, syntax portion 26 was a 2-bit field that is transmitted in each frame

14a to 14c of the USAC encoded data stream: Since the FD part is only important for the decoder to know if it has to read FAC data from the bit stream in case the previous frame 14a has been lost, these 2 bits can be divided into two 1-bit flags, where one of them is marked in each frame 14a to 14c as fac_data_present. This bit can be entered into the single_channel_element and channel_pair_element structure, as appropriate, as illustrated in the tables in Figures 15 and 16. Figs. 1,5 and 16 can be considered as a definition of structure of. High-level syntax of Tables 14 according to the present embodiment, where the functions "function_name (...)" evoke subroutines and the names of syntax elements written in bold indicate the reading of the respective syntax element of the data flow. In other words, the marked portions or the shaded portions of Figures 15 and 16 indicate that each frame 14a to 14c is provided, according to this embodiment, with a fac_data_present flag. Reference number 199 indicates these portions.

The other 1-bit flag prev_frame_was_lpd is only transmitted, then, in the current frame if it has been encoded using the LPD portion of USAC, and also indicates whether the previous frame was encoded using the LPD path of USAC. This is illustrated in the table in figure 17.

The table in figure 17 illustrates a part of the Information 28 of figure 1 in the case where the current table 14b is an LPD table. As indicated in

200, each LPD frame is endowed with a prev_box_was_lpd flag. This information is used to parse the syntax of the current LPD table. From Figure 18 it can be deduced that the content and position of the FAC 34 data of the LPD frames depend on the transition at the lead end of the current LPD frame which is a transition between the TCX encoding mode and CELP encoding mode or a transition from FD encoding mode to CELP encoding mode. In particular, if the box currently decoding 14b is an LPD box immediately preceded by an FD box 14a, and fac_data_present indicates that FAC data is present in the current LPD box (since the progress sub-box is a sub-table of ACELP), then the FAC data is read at the end of the LPD table syntax at 202, where the FAC 34 data includes, in that case, a gain factor fac_gan! Indicated at 204 in figure 18. With this profit factor, contribution 149 in Figure 13 is adjusted to profit.

If, on the contrary, the current frame is an LPD frame, where the previous frame had also been an LPD frame, that is, if a transition occurs between the TCX and CELP subframes between the current frame and the previous frame, the FAC data is read at 206 without the gain adjustment option, that is, without the FAC 34 data including the FAC gain syntax element fac_gain. Furthermore, the position of the FAC data read at 206 differs from the position at which the FAC data is read at 202 in the case where the current table is an LPD table and the previous table is an FD table. . Although the reading position 202 takes place at the end of the current LPD table, the reading of the FAC data at 206 takes place before the reading of the specific data of the sub-table, that is, the ACELP or TCX data depending of sub-frame modes or sub-frame structure, at 208 and 210, respectively.

In the example of Figures 15 to 18, the LPC 104 information (Figure 5) is read after the specific data in the sub-frames such as 90a and 90b (compare with Figure 5) at 212.

For completeness only, the syntax structure of the LPD table according to Figure 17 is further explained with reference to the FAC data further contained, potentially within the LPD table, in order to provide FAC information regarding the. transitions between subframes of TCX and ACELP within the current LPD-encoded time segment. In particular, according to the embodiment of Figures 15 to 18, the LPD subframe structure is limited to subdlvidlr the current LPD-encoded time segment merely in units of rooms, these rooms being assigned to TCX or ACELP . The exact structure of LPD is defined by the lpd_mode syntax element read at 214. The first and second, and the third and fourth quarters may together form a subframe of TCX, while ACELP frames are limited to the length of one frame only. A TCX frame can also span the entire LPD-encoded time slot, in which case the number of sub-frames is simply one. The loop in Figure 17 traverses through the quarters of the current LPD-encoded time slot and transmits, provided the current quarter k is at the beginning of a new sub-frame inside the current LPD-encoded timeslot, the indicated FAC data at 216 as long as the immediately preceding subframe of the LPC frame currently at its start / decode is otherwise, ie TCX mode if the current subframe is from ACELP mode and vice versa.

To more fully detail only, Figure 19 illustrates a possible syntax structure of an FD table according to the embodiment of Figures 15 to 18. It can be seen that the FAC data is read at the end of the FD table with the determination of the presence or not of FAC 34 data, which merely implies the fac_data_present flag. In comparison with this, the parsing of fac_data 34 in the case of the LPD tables illustrated in Figure 17 requires, for a correct parsing, the knowledge of the flag prev_cuadro_was_lpd.

Consequently, the 1-bit flag prev_frame_was_lpd is only transmitted if the current frame is encoded using the USAC LPD part and signals if the previous frame was encoded using the LPD path of the USAC codec (see lpd_channel_stream () Syntax in Fig. 17)

Regarding the embodiment of Figures 15 to 19, it should also be noted that another syntax element could be transmitted in 220, that is, in the case that the current frame is an LPD frame and the previous frame be an FD box (where a first box in the current LPD box is an ACELP box) so the FAC data in 202 should be read to address the transition from the FD box to the ACELP sub-box at the end of the current LPD table. This additional syntax element read at 220 could indicate if the previous FD box 14a is FDJong or FD_short. Depending on this syntax element, the FAC 202 data might be affected. For example, the length of synthesis signal 149 could be affected depending on the length of the window used to transform the preceding LPD frame.

Summarizing the embodiment of the. Figures 15 and 19 and transferring the features mentioned therein to the embodiment described with respect to Figures 1 to 14, the following could be applied to these latter embodiments, either individually or in combination:

1) The FAC 34 data mentioned in the figures above was intended primarily to indicate the FAC data present in the current frame 14b in order to result in the cancellation of the forward aliasing effect taking place in the transition between the frame 14a and the current frame 14b, that is, between the corresponding time segments 16a and 16b. However, there may be other FAC data present. However, these additional FAC data refer to the transitions between TCX-encoded subframes and CELP-encoded subframes located within the current frame 14b if it is in the LPD mode. The presence or absence of these additional FAC data is independent of the syntax portion 26. In Figure 17, these additional FAC data are read at 216. The presence or absence thereof depends simply on lpd_mode read at 214. This last syntax element is, in turn, part of syntax portion 24 that reveals the encoding mode of the current frame. Ipd_mode along with core_mode read at 230 and 232 shown in Figures 15 and 16 correspond to syntax portion 24.

2) In addition, syntax portion 26 may be composed of more than one syntax element, as described above. Flag

FAC_data_present indicates whether or not there is fac_data data corresponding to the boundary between the previous frame and the current frame. This flag is present in an LPD frame, as well as in FD frames. Another flag, which in the foregoing embodiment is called prev_frame_was_lpd, is transmitted in LPD frames only to indicate whether the previous frame 14a was LPD mode or not. In other words, this second flag included in syntax portion 26 indicates whether the previous frame 14a was an FD frame. The parser 20 estimates and reads this flag only in case the current frame is an LPD frame. In Figure 17, this flag is read at 200. Depending on this flag, the parser 20 can expect the FAC data to comprise, and therefore read from the current table, a gain value fac_gain. The gain value is used by the reconstructor to establish the gain of the FAC synthesis signal corresponding to the FAC at the transition between the current and previous time slots. In the embodiment of Figures 15 to 19, this syntax element is read at 204, where the dependency on the second flag clearly arises from the comparison of the

<td></td><td>conditions leading to reading 206 and 202, respectively. On the other hand, or in addition, prev_cuadro_was_lpd can control a position in which the parser 20 estimates and reads FAC data. In the embodiment</td>

<td> 5</td><td>From Figures 15 to 19, these positions were 206 or 202. Furthermore, the second syntax portion 26 can also comprise another flag in case the current frame is an LPD frame. where the advance subframe of which is</td>

<td> 10</td><td>an ACELP box and a previous box is an FD box, to indicate if the previous FD box is encoded using a transform window long or a short transformation window. This last flag could be read at 220 in the case of the previous embodiment of Figures 15 to 19. Knowledge of this transform length FD can be used to determine the length of the FAC synthesis signals and the size of the FAC 38 data,</td>

<td> 15</td><td>respectively. By this measure, the FAC data can be adapted in size to the overlap length of the previous FD box window, to to get a better compromise between the quality of the coding and the encoding speed. ·</td>

<td> ‘ 20</td><td>3) Dividing the second portion of syntax 26 into the three flags just mentioned, it is possible to transmit only one flag or bit to signal the second portion of syntax 26 in case the current box is a FD, merely two flags or bits in case the current frame is a LPD box and the previous box is an LPD box, too. Only in the case of a transition from an FD frame to an LPD frame current, a third flag must be transmitted in the current frame. On the other hand,</td>

As indicated above, the second syntax portion 26 may be a 2-bit indicator transmitted by each frame and indicating the mode of the frame preceding this frame as necessary for the parser to decide whether the data of FAC 38 have to be read from the current table or not, and if so, where and how long is the FAC synthesis signal. In other words, the specific embodiment of Figures 15 to 19 could easily be extended to the embodiment using the aforementioned 2-bit driver to implement the second syntax portion 26. Instead of the FAC_data_present indicated in Figures 15 and 16, the 2-bit identifier would be transmitted. It would not be necessary to transmit the flags indicated at 200 and 220. Conversely, the content of fac_data_present in the “if clause leading to 206 and 218, could be derived by parser 20 from the 2-bit identifier. The following table could be accessed in the decoder to take advantage of the 2-bit flag.

'

<td>prev_mode</td><td>core_mode of the current box (superframe)</td><td>first_lpd_flag</td>

<td>ACELP</td><td> 1</td><td> 0</td>

<td>TCX</td><td> 1</td><td> 0</td>

<td>FD_long</td><td> 1</td><td> 1</td>

<td>FD_short</td><td> 1</td><td> 1</td>

A syntax portion 26 could also have merely three different possible values ​​in case the FD tables use only one possible length.

A slightly different structure, although similar to that described above with respect to FIGS. 15 to 19 is that set forth in FIGS. 20-22 using the same reference numbers used with respect to Figs. 15 to 19, so reference is made to this embodiment for the explanation of the embodiment of Fig. 20 to 22.

With respect to the embodiments described in relation to Fig. 3 10 and subsequent, it should be noted that any transform encoding scheme with the aliasing property can be used in connection with the TCX Tables, in addition to MDCT. Furthermore, a transform coding scheme such as FFT could also be employed, with no aliasing effect then in the LPD mode, ie without FAC corresponding to subframe transitions within the LPD frames and, therefore, without need to transmit FAC data corresponding to the boundaries between the edges of LPD.

The FAC data would then be included only for each transition of

FD to LPD and vice versa.

With respect to the embodiments described with respect to Fig. one 20 and subsequent, it should be noted that they referred to the case in which the additional syntax portion 26 was included in the line, that is, depending exclusively on a comparison between the coding mode of the current frame and the coding mode of the previous frame which is defined in the first syntax portion of that previous table, so in all the previously described embodiments, the decoder or parser can uniquely anticipate the content of the second syntax portion of the current frame by using or comparing the first syntax portion of these frames, that is, the previous frame and the current frame. In other words, in the case of no frame loss, it was possible that the decoder or parser derived, from the transitions between the frames, the existence of FAC data present or not in the current frame. If a frame is lost, the second syntax portion, such as the fac_data_present flag bit, explicitly provides that information. However, according to another embodiment, the encoder could take advantage of this explicit signaling possibility offered by the second syntax portion 26 to apply reverse encoding according to which the syntax portion 26 is adaptive, i.e. with the immediate execution decision frame by frame, for example

- established in such a way that, although the transition between the current frame and the previous frame is of the type that is usually accompanied by FAC data (such as FD / TCX, that is, any TC encoding mode, to ACELP, that is, any encoding mode in the time domain, or vice versa) the syntax portion of the current frame indicates the absence of FAC. The decoder could then be implemented to act strictly in accordance with syntax portion 26, thereby disabling, or effectively suppressing, the transmission of FAC data in the encoder that signals this suppression merely by setting, for example, fac_data_present = 0. The situation where this may be a favorable option is when encoding is performed at very low bit rates, where the additional FAC data may cost too many bits, while the aliasing distortion produced as a result may be tolerable in compared to the overall sound quality.

While some aspects have been described in the context of an apparatus, it is clear that these aspects also represent a corresponding method description, where a block or device corresponding to a method step or a characteristic of a method step. Similarly, the aspects described in the context of passing a method also represent a description of a corresponding block or item or characteristic of a corresponding apparatus. Some or all of the steps in the method may be performed (or using) a hardware device, such as a microprocessor, a programmable computer, or an electronic circuit. In some embodiments, one or more of the most important steps in the method may be performed by that type of apparatus.

The encoded audio signal in accordance with the present invention can be stored on a digital storage medium or can be transmitted on a transmission medium such as a wireless transmission medium or a cable-connected transmission medium such as the Internet.

Depending on certain implementation requirements, the embodiments of the invention can be implemented in hardware or software. The implementation can be executed using a means of_; digital storage, for example a floppy disk, a DVD, a Blue-Ray, a CD, a ROM, a PROM, an EPROM, an EEPROM or a FLASH memory, which have stored in the same electronically readable control signals ( or have the capacity to cooperate) with a programmable computing system in such a way that the respective method is executed. Therefore, the digital storage medium can be computer readable.

Some embodiments in accordance with the invention comprise a data carrier comprising electronically readable control signals, capable of cooperating with a programmable computing system such that one of the methods described herein is implemented.

In general, the embodiments of the present invention can be implemented as a computer program product with a program code, where the program code fulfills the function of executing one of the methods when executing the computer program on a computer. The program code can be stored, for example, on a machine-readable carrier.

Other embodiments comprise the computer program for executing one of the methods described herein, stored in a machine-readable carrier.

In other words, an embodiment of the method of the Invention therefore consists of a computer program consisting of program code to perform one of the methods described herein when the computer program is run on a computer.

Another embodiment of the methods of the invention therefore consists of a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for executing one of the methods described here. The data carrier, digital storage medium, or computer readable medium are generally tangible and non-transient

Another embodiment of the method of the invention is, therefore, a data bit stream 10 or a sequence of signals that the computer program represents to execute one of the methods described herein. The data stream or signal sequence may be configured, for example, to be transferred over a data communication connection, for example over the Internet.

Another embodiment comprises a processing means, for example a computer, a programmable logic device, configured or adapted to execute one of the methods described herein.

Another embodiment comprises a computer in which the computer program has been installed to execute one of the methods described here.

Another embodiment according to the invention comprises an apparatus or a system configured to transfer (for example, electronically or optically) a computer program to execute one of the methods described herein to a receiver. The receiver may be, for example, a computer, a mobile device, a memory device, or the like. The apparatus or system may comprise, for example, a file server for transferring the computer program to the receiver.

In some embodiments, a programmable logic device (for example an array of field programmable gates) may be used to execute some or all of the functionality of the methods described herein. In some embodiments, an array of field programmable gates can cooperate with a microprocessor to execute one of the methods described herein. In general, the methods are preferably executed by any hardware device.

The embodiments described above are merely illustrative of the principles of the present invention. It is understood that modifications and variations of the provisions and details described herein should be apparent to those skilled in the art. Therefore, it is only intended to be limited to the scope of the following patent claims and not to the specific details presented by way of description and explanation of the embodiments presented herein.