Method and arrangement for receiving digital video signals

ES 2 215 360 T3

DESCRIPTION

Method and arrangement for receiving digital video signals.

Field of the invention

The invention relates to a method and television receiver for receiving an encoded digital video signal as an MPEG elementary video stream including autonomously encoded and predictively encoded images.

Background of the invention

MPEG is a packet-based time multiplexing system for transmitting digital video programs. The data is transmitted in transport packets. Each transport packet contains data from exactly one elementary stream with which it is associated by means of its packet identifier. Examples of elementary streams are video streams, audio streams, and data streams. One or more elemental streams sharing the same time base form a program. A typical program might consist of a video stream and an audio stream. One or more programs constitute a transport stream. The MPEG standard is described in "ISO / IEC CD 13818: Information technology - Generic coding of moving pictures and associated audio information", 01-12-1993. Part 1 of this standard deals with the system aspects of digital transmission, Part 2 deals more particularly with video coding.

EP-A-0 505 985 describes a video recorder, which encodes and subsequently records a video signal as an MPEG bit stream comprising autonomously encoded images (I) and autonomously encoded images (P, B). predictive. When playing back the recorded signal at high speeds, the head is directed from one I-image to the next.

Document EP 0 534 139 describes a system for displaying a number of video windows. Each source is divided into "encoding ranges" and each encoding range is compressed in JPEG. Coding ranges that are not to be displayed are filtered because they are overlapped by other windows.

Document EP 0 606 857, state of the art according to article 54 (3) CPE only for designated contracting states DE, FR and GB, describes a digital video tape recorder that selects useful data to generate images during the operation of Trick Play and records the data on trick play tape segments arranged on a tape to form fast scan tracks and multi-speed play tracks. It also describes the transmission of trick play data in addition to normal data.

Television receivers are known which have characteristics, eg picture-in-picture, which require duplication of certain video processing circuitry. Implementation of the same features in an MPEG television receiver would require duplication of the MPEG decoder. However, an MPEG decoder is a very complicated and expensive device.

Object and summary of the invention

It is an object of the invention to solve the aforementioned problem. An object is, inter alia, to develop known features in MPEG receivers in a more economical way or to provide MPEG receivers with novel features. To this end, the invention provides a method of reception and television receivers, as defined in the independent claims. Advantageous embodiments are defined in the dependent claims. In one embodiment of the invention, only the autonomously encoded images are selected from the MPEG elemental stream. A signal that includes only the autonomously encoded images is called an auxiliary signal. The auxiliary signal can be decoded by a decoder, which is considerably simpler and less expensive than a full spectrum MPEG decoder. More particularly, motion compensation circuitry and a large amount of memory can be dispensed with. The auxiliary signal only requires a low bit rate.

A picture-in-picture television receiver according to the invention displays the auxiliary signal as picture-in-picture. In a multi-picture picture-in-picture television receiver, a plurality of auxiliary signals are simultaneously displayed as a mosaic picture.

An embodiment of the invention is characterized in that only the DC coefficients of autonomously encoded images constitute the auxiliary signal. Such a signal requires a very low bit rate and a decoder for such a signal is extremely simple.

The auxiliary signal is created locally at the receiver. Although a full spectrum MPEG signal is received, the decoder can be simple in that motion compensation circuitry and a large amount of memory can be dispensed with.

Brief description of the drawings

Figure 1 shows a diagram of an arrangement for carrying out the method according to the invention.

Figure 2 shows a flow chart illustrating the operation of the arrangement shown in Figure 1.

Figure 3 shows a diagram of another embodiment of the arrangement for carrying out the method according to the invention.

Figure 4 shows a flow chart illustrating the operation of the arrangement shown in Figure 3.

Figure 5 shows a diagram of a digital picture-in-picture television receiver according to the invention.

Fig. 6 shows a diagram of a digital multiple picture television receiver according to the invention.

Description of achievements

Figure 1 shows a diagram of an arrangement according to the invention. The arrangement comprises a variable length decoder 10 (hereinafter DLV), an inverse quantizer 11 and an image memory 12. The arrangement receives an elementary video stream representing a main video signal Vp and derives therefrom an auxiliary video signal Va. The main video Vp signal is assumed to have been encoded according to “ISO / IEC CD 138182: Information technology - Generic coding of moving pictures and associated audio information - Part 2: Video”, 01-12-1993, also called the coding standard MPEG2 video. To understand the invention, it suffices to mention that the signal Vp prin2

Cipal ES 2 215 360 T3 includes autonomously coded images (I images) and predictively coded images (B and P images). Each image has been divided into 8 * 8 pixel blocks and each block has been transformed to spectral coefficients. The relevant coefficients are subjected to a combination of Hufman encoding and blank segment encoding. Four luminance blocks and associated chrominance blocks make up a macroblock and a plurality of macroblocks make up a slice. The first image block coefficient (DC) I represents the mean chrominance and luminance of an 8 * 8 pixel block. The bit stream Vp includes header information such as sync words, picture type parameters, and the like.

Next, the operation of the arrangement shown in Figure 1 will be clarified with reference to a flow chart shown in Figure 2. In a first step 20, the DLV reads the input bit stream and discards all data until an image start code is found. Now, some data is received that defines an image. In a step 21, the encoding image type in the image header is decoded. In a step 22, it is established whether said type of encoding image indicates that an I image is being received. If this is not the case, the DLV returns to step 20 to wait for the next image start code. If the image is an I image, the DLV waits successively for the reception of a slice header (step 23) and the reception of a macroblock (step 24).

In a step 25, the DLV decodes and outputs the DC coefficient of a block within the current macroblock. In a step 26, subsequent coefficients are discarded until an end-of-block code is detected. In a step 27, it is determined whether all the blocks of a macroblock have been processed. Provided that this is not the case, the DLV returns to step 25. In a step 28, it is determined whether all macroblocks of a slice have been processed. Provided that this is not the case, the DLV returns to step 24. Finally, it is determined in a step 29 whether all slices of the image have been processed. Whenever this is not the case, the DLV returns to step 23. If all slices have been processed, the DLV returns to step 20 to search for the next I-image in the bit stream.

In this way, the DLV extracts the image DC coefficients I from the input bit stream. As shown in figure 1, said coefficients are supplied to the inverse quantizer 11 and then stored in memory 12. Each DC coefficient represents a mean value of luminance and chrominance of a block of 8 * 8 pixels of the I images of main video. The auxiliary video signal is obtained by reading said memory with an appropriate time base.

In an alternative embodiment, steps 25 and 26 are modified to decode all coefficients in a block. In that case, the auxiliary signal comprises I-images and is a temporally reduced version of the main signal.

Figure 3 shows a diagram of another embodiment of the arrangement for carrying out the method according to the invention. In this arrangement, the bit stream representing the main video signal Vp is supplied to a memory 30 and a variable length decoder. The variable length decoder analyzes the bit stream and generates a write enable signal HE to determine how much of the bit stream is stored in memory. The memory is read at a lower bit rate to constitute an elementary video stream representing the auxiliary video signal Va.

Next, the operation of the arrangement shown in Figure 3 will be clarified with reference to a flow chart shown in Figure 4. Steps 20-22 are the same as the corresponding steps shown in Figure 2. Thus, In step 20, the DLV reads the input bit stream and discards all data until a picture start code is found. In step 21, the encoding image type provided in the image header is decoded. In step 22, it is determined whether said encoding image type indicates that an I image is being received. If this is not the case, the DLV returns to step 20 to wait for the next image start code.

If the image is an I image, a step 40 is executed in which the DLV allows the image header to remain in memory by generating an appropriate write enable signal. In a step 41, the slice header is received and stored in memory. Now, a macroblock is being received. In a step 42, all macroblock data up to the first block is written to memory.

In a step 43, the DLV detects the presence of a DC coefficient of a block within the current macroblock and allows this coefficient to be stored in memory. In a step 44, subsequent block coefficients are discarded until an end-of-block code is detected. DLV refrains from generating the write enable signal while those coefficients are being received. In a step 45, the end-of-block code is stored in memory. Steps 27-29 are the same as the corresponding steps shown in Figure 2. These facilitate checking whether or not a current I-image has been processed.

The arrangement shown in Figure 3 thus copies the main bit stream Vp into memory 30, thereby ignoring the P and B images, as well as the non-DC coefficients of the images.

I. The auxiliary video signal Va, obtained by reading the memory 30, is the same as that created by the arrangement shown in figure 1, but is now suitable for transmission as an additional elementary signal. It is a low bitstream replica of the main signal, with reduced spatial and temporal resolution.

Figure 5 shows a diagram of a digital picture-in-picture (PIP) television receiver according to the invention. The receiver comprises a demultiplexer 60, an MPEG2 audio decoder 61, an MPEG2 video decoder 62, a PIP decoder 63, and a video mixer 64. The demultiplexer 60 receives an MPEG2 transport CT stream comprising a plurality of audiovisual programs, each consisting of one or more elementary streams (eg, audio, video data). The transport stream also comprises packets that accommodate program specific information (PSI). PSI packages specify which programs are available, as well as

ES 2 215 360 T3 how many and which elementary streams each program comprises. A detailed description of transport streams and program-specific information can be found in "ISO / IEC CD 138181: Information technology - Generic coding of moving pictures and associated audio information - Part 1: Systems", 01-12-1993, also known as the MPEG2 system standard. The demultiplexer 60 extracts from the transport stream CT an elementary audio stream A1 and an elementary video stream V1 associated with a desired program. The elementary streams A1 and V1 are decoded by an audio decoder 61 and a video decoder 62, respectively. The decoded audio signal is applied to a speaker 65. The demultiplexer further extracts from the transport stream CT an additional elementary video signal V2 associated with a different program, to be displayed as picture in picture. The additional elementary stream V2 is decoded by a PIP decoder 63 and converted into a signal V2 'with reduced size and time resolution. Both video signals V1 and V2 'are mixed in a video mixer 64 and displayed on a display screen 66.

In one embodiment of the PIP receiver, an elementary stream V2 defines a full-resolution, full-size MPEG-encoded video signal including B, P, and I pictures. In this embodiment, the PIP decoder 63 takes the form of the circuit shown in Figure 1, already exposed.

Figure 6 shows a diagram of a multiple picture-in-picture (MPIP) television receiver according to the invention. The receiver comprises a transport stream demultiplexer 70, an MPEG2 audio decoder 71, a loudspeaker 72, an MPEG2 video decoder 73, an MPIP decoder 74, a display screen 75 and a control circuit 76. Demultiplexer 70 receives an MPEG2 transport stream CT and extracts from it an elementary audio stream Aj and an elementary video stream Vj, both associated with a program number j. The elementary streams Aj and Vj are decoded by an audio decoder 71 and a video decoder 73, respectively. The decoded audio signal is applied to speaker 72. The decoded video signal can be displayed, through a switch 77, on the display screen 75. The demultiplexer further extracts from the transport stream CT an additional elementary video stream Vj associated with a program i. The additional elemental stream Vi can define a full-size, full-resolution MPEG2 encoded video signal, including I, P, and B pictures.

The MPIP decoder 74 is adapted to decode the auxiliary signal Vi. The decoder comprises a variable-length decoder 741, an inverse quantizer 742, and a memory 743. The decoder has the same structure as the arrangement shown in Figure 1. However, memory 743 now has a plurality of memory sections, routed by a write address DE, with each section having the ability to store a respective small size image.

In operation, the control circuit 76 receives from a demultiplexer 70 the transport packets carrying program-specific PSI information. As already mentioned above, such packages specify which programs are available, as well as how many and which elementary streams each program comprises. The control circuit is adapted to read from the PSI data, for each available program i, the packet identifier PDI that defines the transport packets that carry the video signal Vi associated therewith. For a plurality of different programs, the relevant PIDs are successively applied to demultiplexer 70 to apply the associated video signal Vi to MPIP decoder 74. Each small decoded image is stored in a memory section 743 under the control of the write address DE generated by the control circuit 76. The plurality of small size images together constitute a mosaic video image that can be displayed under the control of the user on the display screen 75 through the switch 77.

As the user selects one of the displayed thumbnail images (for example, by a cursor device not shown), the control circuit 76 converts the selected position on the display screen to the program number associated therewith, and controls demultiplexer 70 to select the audio stream Aj and video stream Vj associated with the selected program. The control circuit further controls switch 77 to display the selected program in full resolution and size on the display screen 75 and to reproduce its sound through a speaker 72.

In summary, the invention relates to a method and arrangement for deriving an auxiliary signal from a compressed digital video signal (eg MPEG). The DC coefficients of the autonomously encoded images (I images) are selected from the compressed signal. The auxiliary signal thus obtained can be used for display on a picture-in-picture (multi) television receiver. A decoder for decoding such an auxiliary signal is considerably simpler and less expensive than a full spectrum MPEG decoder.

It should be noted that the aforementioned embodiments illustrate rather than limit the invention, and those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any of the reference signs in parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of other elements or steps than those enumerated in a claim. The invention can be carried out by means of hardware comprising several different elements, and by means of an appropriately programmed computer. In a device claim listing multiple media, some of these media can be realized with the same item of hardware.