Arrangement for decoding digital video signals

FIELD OF THE INVENTION

The invention relates to a transmitter and method for transmitting a digital video signal encoded as an MPEG elementary video stream including autonomously encoded and predictively encoded pictures. The invention also relates to a television receiver and video recorders for receiving MPEG elementary video streams.

BACKGROUND OF THE INVENTION

MPEG is a packet-based time multiplex system for transmitting digital video programs. 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 elementary streams sharing the same time base make up a program. A typical program might consist of one video stream and one 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", 1993-12-01. Part 1 of this standard relates to the system aspects of digital transmission, Part 2 relates more particularly to video encoding.

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

EP 0 534 139 discloses a system for displaying a number of video windows. Each source is divided into "coding intervals" and each coding interval is JPEG compressed. The coding intervals not to be displayed because they are overlapped by other windows are filtered out.

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

Television receivers are known having features, for example, picture-in-picture, which require some video processing circuits to be duplicated. Implementing the same features in an MPEG television receiver would require the MPEG decoder to be duplicated. 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 alleviate the above mentioned problem. One object is, <i>inter alia,</i> to realize known features in MPEG receivers in a more cost-effective manner or to provide MPEG receivers with novel features. It is also an object of the invention to provide a transmitter and method for transmitting a digital video signal which renders it possible to implement said known or new receiver features in a cost effective manner.

To this end, the invention provides a transmitter, a method of transmitting, television receivers, video recorders, an MPEG transport stream, and a storage medium as defined in the independent claims. Advantageous embodiments are defined in the dependent claims. Herewith is achieved that the ancillary signal can be decoded at the receiver end by a decoder, which is considerably simpler and less expensive than a full-spec MPEG decoder. More in particular, motion compensation circuitry and a large amount of memory can be dispensed with. The ancillary signal requires only a low bit rate.

Various applications of the ancillary signal are conceivable. A picture-in-picture television receiver, for example, may display it as the picture-in-picture. In a multi-picture-in-picture television receiver, a plurality of ancillary signals may be simultaneously displayed as a mosaic picture. In a video recorder, the ancillary signal may be separately recorded so as to be reproduced at higher playback speeds.

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

MPEG also allows parts of the signal to be scrambled, whereas other parts remain unscrambled. In a further embodiment of the invention, the MPEG elementary video stream is scrambled and the ancillary elementary video stream is unscrambled. This allows a video program to be viewed free of charge but at a low quality. The ancillary signal thus acts as an "appetizer", attracting the viewer's attention to the presence and contents of the main signal. When recorded simultaneously with the main signal on a digital video recorder, the ancillary signal also assists the user in finding the beginning of a scrambled program on tape.

The ancillary signal can also locally be created in the receiver or video recorder. Although receiving a full-spec MPEG signal, the decoder may be simple because motion compensation circuitry and a large amount of memory can be dispensed with.

BRIEF DESCRIPTION OF THE DRAWINGS

<ul id="ul0001" list-style="none" compact="compact"><li>Fig. shows a diagram of an arrangement for carrying out the method according to the invention.</li><li>Fig.2 shows a flow chart illustrating the operation of the arrangement shown in Fig.1.</li><li>Fig.3 shows a diagram of another embodiment of the arrangement for carrying out the method according to the invention.</li><li>Fig.4 shows a flow chart illustrating the operation of the arrangement shown in Fig.3.</li><li>Fig.5 shows a diagram of an arrangement for transmitting the ancillary video signal as an elementary MPEG bitstream of the same program as the main video signal.</li><li>Fig.6 shows a diagram of a digital picture-in-picture television receiver according to the invention.</li><li>Fig.7 shows a diagram of a digital multi-picture-in-picture television receiver according to the invention.</li><li>Figs. 8A and 8B show embodiments of a digital video recorder according to the invention.</li></ul>

DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a diagram of an arrangement according to the invention. The arrangement comprises a variable-length decoder <b>10</b> (hereinafter VLD), an inverse quantizer <b>11</b> and a picture memory <b>12.</b> The arrangement receives an elementary video stream representing a main video signal Vm and derives therefrom an ancillary video signal Va. The main video signal Vm is assumed to have been encoded according to "ISO/IEC CD 13818-2: Information technology - Generic coding of moving pictures and associated audio information - Part 2: Video", 1993-12-01, also referred to as the MPEG2 video coding standard. For understanding the invention, it suffices to mention that the main signal Vm includes autonomously encoded pictures (I-pictures) and predictively encoded pictures (P- and B-pictures). Each picture has been divided into blocks of 8*8 pixels and each block has been transformed to spectral coefficients. The relevant coefficients are subjected to a combination of Hufman coding and runlength coding. Four luminance blocks and associated chrominance blocks constitute a macroblock and a plurality of macroblocks constitute a slice. The first (DC) coefficient of blocks of I-pictures represents the average luminance and chrominance of an 8*8 pixel block. The bitstream Vm further includes overhead information such as syncwords, picture type parameters, and the like.

The operation of the arrangement shown in Fig. will now be elucidated with reference to a flow chart shown in Fig.2. In a first step <b>20,</b> the VLD reads the input bitstream and discards all data until a picture start code is encountered. Data defining a picture is now being received. In a step <b>21</b>, the picture coding type accommodated in the picture header is decoded. In a step <b>22,</b> it is established whether said picture coding type indicates that an I-picture is being received. If that is not the case, the VLD returns to step <b>20</b> to await the next picture start code. If the picture is an I-picture, the VLD successively awaits the reception of a slice header (step <b>23)</b> and the reception of a macroblock (step <b>24).</b>

In a step <b>25,</b> the VLD decodes and outputs the DC coefficient of a block within the current macroblock. In a step <b>26,</b> the subsequent coefficients up to the detection of an end-of-block code are discarded. In a step <b>27,</b> it is ascertained whether all blocks of a macroblock have been processed. As long as that is not the case, the VLD returns to step <b>25.</b> In a step <b>28,</b> it is ascertained whether all macroblocks of a slice have been processed. As long as that is not the case, the VLD returns to step <b>24.</b> Finally, it is ascertained in a step <b>29</b> whether all slices of the picture have been processed. As long as that is not the case, the VLD returns to step <b>23.</b> If all slices have been processed, the VLD returns to step <b>20</b> in order to search the next I-picture in the bitstream.

The VLD thus extracts the DC coefficients of I-pictures from the input bitstream. As shown in Fig.1, said coefficients are supplied to the inverse quantizer <b>11</b> and then stored in memory <b>12.</b> Each DC coefficient represents the average luminance and chrominance value of an 8*8 pixel block of the main video I-pictures. The ancillary video signal is obtained by reading out said memory with an appropriate time basis.

In an alternative embodiment, steps <b>25</b> and <b>26</b> are modified so as to decode all coefficients of a block. In that case, the ancillary signal comprises I-pictures and is a temporally reduced version of the main signal.

Fig.3 shows a diagram of another embodiment of the arrangement for carrying out the method according to the invention. In this arrangement the bitstream representing the main video signal Vm is supplied to a memory <b>30</b> and variable-length decoder <b>31.</b> The variable-length decoder analyses the bitstream and generates a write enable signal WE so as to determine which part of the bitstream is stored in the memory. The memory is read out at a lower bitrate so as to constitute an elementary video stream representing the ancillary video signal Va.

The operation of the arrangement shown in Fig.3 will now be elucidated with reference to a flow chart shown in Fig.4. The steps <b>20-22</b> are the same as the corresponding steps shown in Fig.2. Thus, in the step <b>20</b>, the VLD reads the input bitstream and discards all data until a picture start code is encountered. In the step <b>21</b>, the picture coding type accommodated in the picture header is decoded. In the step <b>22</b>, it is ascertained whether said picture coding type indicates that an I-picture is being received. If that is not the case, the VLD returns to step <b>20</b> to await the next picture start code.

If the picture is an I-picture, a step <b>40</b> is performed in which the VLD allows the picture header to be kept in the memory by generating an appropriate write enable signal. In a step <b>41</b>, the slice header is received and stored in the memory. A macroblock is now being received. In a step <b>42</b>, all macroblock data up to the first block is written in the memory.

In a step <b>43</b>, the VLD detects the presence of a DC coefficient of a block within the current macroblock and allows this coefficient to be stored in the memory. In a step <b>44</b>, the subsequent coefficients of the block up to the detection of an end-of-block code are discarded. The VLD refrains from generating the write enable signal while said coefficients are being received. In a step <b>45</b>, the end-of-block code is stored in the memory. Steps <b>27-29</b> are the same as the corresponding steps shown in Fig.2. They facilitate the check of whether or not the current I-picture has been processed.

The arrangement shown in Fig.3 thus copies the main bitstream Vm in memory <b>30,</b> thereby ignoring the P- and B-pictures as well as the non-DC-coefficients of I-pictures. The ancillary video signal Va obtained by reading out the memory <b>30</b> is the same as that created by the arrangement shown in Fig.1 but is now suitable for transmission as a further elementary signal. It is a low bitrate replica of the main signal, with a reduced spatial and temporal resolution.

Fig.5 shows a diagram of a transmitter according to the invention. The transmitter comprises a demultiplexer <b>50,</b> a transcoder <b>51,</b> a circuit <b>52</b> for regenerating program specific information, and a remultiplexer <b>53.</b> The arrangement receives a packetized transport stream TS1. Said transport stream comprises a plurality of audiovisual programs, each program being formed by one or more elementary streams (e.g. video, audio, data). The transport stream also comprises packets accommodating program-specific information (PSI). PSI packets specify which programs are available, as well as 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 13818-1: Information technology - Generic coding of moving pictures and associated audio information - Part 1: Systems", 1993-12-01, also known as the MPEG2 systems standard.

Demultiplexer <b>50</b> selects an elementary video stream Vm from which an ancillary signal Va is to be derived. Other elementary streams E1,E2,E3 remain unprocessed in this embodiment. The main video signal Vm is applied to transcoder <b>51</b> which may take the form of the arrangement shown in Fig.3, already discussed. The transcoder outputs an ancillary video signal Va in the form of a further elementary stream. Demultiplexer <b>50</b> also extracts program-specific information packets PSI1 from the transport stream TS1 and applies them to circuit <b>52</b>. This circuit updates the program-specific information so as to specify that ancillary signal Va is present in output transport stream TS2. The circuit further specifies that Va is associated with the same audiovisual program as the main elementary stream Va from which it has been derived. The updated program-specific information PSI2 and the ancillary elementary stream Va are then added by remultiplexer <b>53</b> to the original elementary streams and retransmitted as a new transport stream TS2.

Fig.6 shows a diagram of a digital picture-in-picture (PIP) television receiver according to the invention. The receiver comprises a demultiplexer <b>60,</b> an MPEG2 audio decoder <b>61,</b> an MPEG2 video decoder <b>62,</b> a PIP-decoder <b>63,</b> and a video mixer <b>64.</b> The demultiplexer <b>60</b> receives an MPEG2 transport stream TS and extracts therefrom 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 audio decoder <b>61</b> and video decoder <b>62</b>, respectively. The decoded audio signal is applied to a loudspeaker <b>65</b>. The demultiplexer further extracts from the transport stream TS a further elementary video stream V2 associated with a different program which is to be displayed as picture-in-picture. The further elementary stream V2 is decoded by PIP-decoder <b>63</b> and converted into a signal V2' with a reduced size and temporal resolution. Both video signals V1 and V2' are mixed in video mixer <b>64</b> and displayed on a display screen <b>66</b>.

In a first embodiment of the PIP-receiver, elementary stream V2 defines a full-size, full-resolution MPEG-encoded video signal, including I, P and B-pictures. In this embodiment, PIP-decoder <b>63</b> takes the form of the circuit shown in Fig.1, already discussed. In a second embodiment of the PIP-receiver, the elementary stream V2 is assumed to be an ancillary video stream transmitted by an arrangement as shown in Fig.5. As explained with reference to Fig.5, the elementary stream V2 comprises DC-coefficients of I-pictures only. In this embodiment, PIP-decoder <b>63</b> also takes the form of the circuit shown in Fig.1, already discussed. However, the variable-length decoder (<b>10</b> in Fig.1) is simpler because various types of overhead data are absent in the bitstream and thus do not need to be processed.

Fig.7 shows a diagram of a digital multi-picture-in-picture (MPIP) television receiver according to the invention. The receiver comprises a transport stream demultiplexer <b>70,</b> an MPEG2 audio decoder <b>71,</b> a loudspeaker <b>72,</b> an MPEG2 video decoder <b>73</b>, a MPIP-decoder <b>74</b>, a display screen <b>75</b> and a control circuit <b>76</b>. The demultiplexer <b>70</b> receives an MPEG2 transport stream TS and extracts therefrom 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 audio decoder <b>71</b> and video decoder <b>73</b>, respectively. The decoded audio signal is applied to loudspeaker <b>72</b>. The decoded video signal is displayable, via a switch <b>77</b>, on display screen <b>75</b>. The demultiplexer further extracts from the transport stream TS a further elementary video stream Vi associated with a program i. The further elementary stream Vi may define a full-size, full-resolution MPEG-encoded video signal, including I, P and B-pictures. The further elementary video stream Vi may also be an ancillary video stream transmitted by an arrangement as shown in Fig.5. In the latter case, the ancillary video signal comprises, as explained above, DC-coefficients of I-pictures only.

MPIP-decoder <b>74</b> is adapted to decode the ancillary signal Vi. The decoder comprises a variable-length decoder <b>741</b>, an inverse quantizer <b>742</b> and a memory <b>743.</b> The decoder has the same structure as the arrangement shown in Fig.1. However, memory <b>743</b> now has a plurality of memory sections, addressed by a write address WA, each section having the capacity to store a respective small-size picture.

In operation, control circuit <b>76</b> receives from demultiplexer <b>70</b> the transport packets accommodating program-specific information PSI. As already mentioned before, said packets 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 PID defining the transport packets conveying the ancillary video signal Vi associated therewith. For a plurality of different programs, the relevant PIDs are successively applied to the demultiplexer <b>70</b> so as to apply the associated ancillary video signal Vi to MPIP-decoder <b>74</b>. Each decoded small-size picture is stored in a section of memory <b>743</b> under the control of the write address WA generated by control circuit <b>76</b>. The plurality of small-size pictures together constitutes a mosaic video picture which can be displayed under user control on display screen <b>75</b> via the switch <b>77</b>.

Upon user-selection of one of the displayed miniature pictures (e.g. by a cursor device not shown), the control circuit <b>76</b> converts the selected display screen position into the program number associated therewith, and controls demultiplexer <b>70</b> so as to select the audio stream Aj and video stream Vj associated with the selected program. The control circuit further controls switch <b>77</b> so as to display the selected program in full size and resolution on display screen <b>75</b> and to reproduce its sound via loudspeaker <b>72</b>.

Figs. 8A and 8B show two embodiments of a digital video recorder according to the invention. The recorder receives an MPEG2 transport stream TS and comprises a demultiplexer <b>80</b> to select therefrom an elementary audio stream A and an elementary video stream V. The elementary video stream is assumed to have been scrambled such that only the predictively encoded (P and B) pictures are scrambled. Both elementary streams are recorded on a digital storage medium <b>81</b>.

In the embodiment shown in Fig.8A, the video stream V is further applied to a transcoder <b>82</b> which may take the form as shown in Fig.1. In the embodiment shown in Fig.8B, the demultiplexer further selects an ancillary elementary stream Va, which is transmitted by an arrangement as shown in Fig.5. The ancillary stream is applied to a "simple" MPEG decoder <b>89</b> as explained hereinbefore with reference to Fig.6.

In both embodiments, the ancillary signal Va comprises the DC-coefficients of I-pictures of the same program as video signal V. This ancillary signal is separately recorded on storage medium <b>81</b>. Upon normal playback, the recorded audio and video elementary streams A and V are decoded by MPEG2 audio decoder <b>83</b> and MPEG2 video decoder <b>85</b>, respectively. The audio signal is applied to an audio output terminal <b>84</b>. If the video signal has been scrambled, it can only be displayed when processed by a descrambler <b>86</b>. The descrambler may take the form of a circuit which is activated only upon insertion of a smart card holding a sufficient amount of credit. These types of descramblers are known <i>per se</i> in the art. The decoded and descrambled video signal is then applied, via a switch <b>87,</b> to a video output <b>88.</b>

The video recorder can optionally output, via switch <b>87</b>, the separately recorded ancillary signal Va. Said signal comprises DC-coefficients of I-pictures only and is thus not scrambled. Displaying this signal in reduced size or, after suitable upsampling (not shown), in full size but with reduced resolution, allows the user to scan the storage medium <b>81</b> for locating the start of a particular scrambled program without having yet to pay therefor. It is to be noted that in the embodiment of Fig.8B the "simple" decoder <b>89</b> can also be located in the reproduction part (i.e. between storage medium <b>81</b> and switch <b>87</b>) of the video recorder.

In summary, the invention relates to a method and arrangement for deriving an ancillary signal from a compressed digital video signal (e.g. MPEG). The DC coefficients of autonomously encoded pictures (I-pictures) are selected from the compressed signal. The ancillary signal thus obtained can be used for display in a (multi-) picture-in-picture television receiver. If the main signal is scrambled, the ancillary signal can be used as an "appetizer" in order to encourage the user to pay a subscription fee. The ancillary signal can separately be recorded in digital video recorders so as to assist the user in finding the beginning of a scrambled program on tape. The ancillary signal can also be generated at the transmitter end and transmitted at a low bit rate. A decoder for decoding such an ancillary signal is considerably simpler and less expensive than a full-spec MPEG decoder.