Method and arrangement for receiving digital video signals

Abstract: 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. A decoder for decoding such an ancillary signal is considerably simpler and less expensive than a full-spec MPEG decoder.

Description: FIELD OF THE INVENTION The invention relates to a method of and a television receiver for receiving a digital video signal encoded as an MPEG elementary video stream including autonomously encoded and predictively encoded pictures. 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, inter alia, to realize known features in MPEG receivers in a more cost-effective manner or to provide MPEG receivers with novel features. To this end, the invention provides a method of receiving and television receivers as defined in the independent claims. Advantageous embodiments are defined in the dependent claims. In an embodiment of the invention, only the autonomously encoded pictures are selected from the MPEG elementary stream. A signal including only the autonomously encoded pictures is called an ancillary signal. The ancillary signal can be decoded 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. A picture-in-picture television receiver according to the invention displays the ancillary signal as the picture-in-picture. In a multi-picture-in-picture television receiver, a plurality of ancillary signals is simultaneously displayed as a mosaic picture. An embodiment of the invention 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. The ancillary signal is locally created in the receiver. 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 Fig.1 shows a diagram of an arrangement for carrying out the method according to the invention. Fig.2 shows a flow chart illustrating the operation of the arrangement shown in Fig.1. Fig.3 shows a diagram of another embodiment of the arrangement for carrying out the method according to the invention. Fig.4 shows a flow chart illustrating the operation of the arrangement shown in Fig.3. Fig.5 shows a diagram of a digital picture-in-picture television receiver according to the invention. Fig.6 shows a diagram of a digital multi-picture-in-picture television receiver according to the invention. DESCRIPTION OF EMBODIMENTS Fig.1 shows a diagram of an arrangement according to the invention. The arrangement comprises a variable-length decoder 10 (hereinafter VLD), an inverse quantizer 11 and a picture memory 12 . 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 20 , 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 21 , the picture coding type accommodated in the picture header is decoded. In a step 22 , 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 20 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 23 ) and the reception of a macroblock (step 24 ). In a step 25 , the VLD decodes and outputs the DC coefficient of a block within the current macroblock. In a step 26 , the subsequent coefficients up to the detection of an end-of-block code are discarded. In a step 27 , 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 25. In a step 28, 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 24. Finally, it is ascertained in a step 29 whether all slices of the picture have been processed. As long as that is not the case, the VLD returns to step 23 . If all slices have been processed, the VLD returns to step 20 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 11 and then stored in memory 12. 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 25 and 26 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 30 and variable-length decoder 31 . 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 20-22 are the same as the corresponding steps shown in Fig.2. Thus, in the step 20 , the VLD reads the input bitstream and discards all data until a picture start code is encountered. In the step 21 , the picture coding type accommodated in the picture header is decoded. In the step 22, 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 20 to await the next picture start code. If the picture is an I-picture, a step 40 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 41 , the slice header is received and stored in the memory. A macroblock is now being received. In a step 42, all macroblock data up to the first block is written in the memory. In a step 43 , 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 44 , 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 45 , the end-of-block code is stored in the memory. Steps 27-29 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 30 , 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 30 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 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 stream TS comprising 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. The demultiplexer 60 extracts from the transport stream TS 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 61 and video decoder 62, respectively. The decoded audio signal is applied to a loudspeaker 65 . 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 63 and converted into a signal V2' with a reduced size and temporal resolution. Both video signals V1 and V2' are mixed in video mixer 64 and displayed on a display screen 66 . In an 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 63 takes the form of the circuit shown in Fig.1, already discussed. Fig.6 shows a diagram of a digital multi-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, a MPIP-decoder 74 , a display screen 75 and a control circuit 76. The demultiplexer 70 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 71 and video decoder 73 , respectively. The decoded audio signal is applied to loudspeaker 72 . The decoded video signal is displayable, via a switch 77 , on display screen 75 . 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. MPIP-decoder 74 is adapted to decode the ancillary 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 Fig.1. However, memory 743 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 76 receives from demultiplexer 70 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 video signal Vi associated therewith. For a plurality of different programs, the relevant PIDs are successively applied to the demultiplexer 70 so as to apply the associated video signal Vi to MPIP-decoder 74 . Each decoded small-size picture is stored in a section of memory 743 under the control of the write address WA generated by control circuit 76. The plurality of small-size pictures together constitutes a mosaic video picture which can be displayed under user control on display screen 75 via the switch 77. Upon user-selection of one of the displayed miniature pictures (e.g. by a cursor device not shown), the control circuit 76 converts the selected display screen position into the program number associated therewith, and controls demultiplexer 70 so as to select the audio stream Aj and video stream Vj associated with the selected program. The control circuit further controls switch 77 so as to display the selected program in full size and resolution on display screen 75 and to reproduce its sound via loudspeaker 72 . 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. A decoder for decoding such an ancillary signal is considerably simpler and less expensive than a full-spec MPEG decoder. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that 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 reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of other elements or steps than those listed in a claim. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim enumerating several means, several of these means can be embodied by one and the same item of hardware.

Claims: A method of receiving a first video signal (V1) representing a main picture and a second video signal (V2) representing a picture-in-picture, the second video (V2) signal being a compressed digital video signal, such as an MPEG elementary stream, including both autonomously encoded pictures and predictively encoded pictures, characterized in that the method comprises the step of: selecting and decoding only said autonomously encoded pictures for display as the picture-in-picture. A television receiver, comprising: means (60) for receiving a first video signal (V1) representing a main picture and a second video signal (V2) representing a picture-in-picture, the second video (V2) signal being a compressed digital video signal, such as an MPEG elementary stream, including both autonomously encoded pictures and predictively encoded pictures, characterized in that the receiver comprises means (63) for selecting and decoding only said autonomously encoded pictures for display as the picture-in-picture. A receiver as claimed in Claim 2. wherein the autonomously encoded pictures comprise transform-coded blocks having respective DC coefficients, the means (63) for selecting and decoding the autonomously encoded pictures being adapted to select and decode only said DC coefficients. A method of receiving a plurality of compressed digital video signals, such as MPEG elementary streams (Vj), each including both autonomously encoded pictures and predictively encoded pictures, characterized in that the method comprises the steps of: selecting and decoding only said autonomously encoded pictures and simultaneously displaying the respective plurality of pictures as a mosaic picture. A television receiver, comprising: means (70) for receiving a plurality of compressed digital video signals, such as MPEG elementary video streams (Vi, Vj), each including both autonomously encoded pictures and predictively encoded pictures, characterized in that the receiver comprises: means (74) for selecting and decoding only said autonomously encoded pictures and means (74,75,76,77) for simultaneously displaying the respective plurality of pictures as a mosaic picture. A receiver as claimed in Claim 5, wherein the autonomously encoded pictures comprise transform-coded blocks having respective DC coefficients, the means (74) for selecting and decoding the autonomously encoded pictures being adapted to select and decode only said DC coefficients. A receiver as claimed in Claim 5, further comprising user control means (76) for selecting one of the displayed plurality of pictures and adapted to select, decode and display (76,73,75,77) the video signal (Vj) from which the selected picture has been derived.

Inventors: ROSENGREN JURGEN FRITZ, SAEIJS RONALD WILHELM JOHAN JO, ROSENGREN, JURGEN FRITZ, SAEIJS, RONALD WILHELM JOHAN JOZEF
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1995
Main classification: H04N5
IPC codes: H04N7, H04N5, G06T9