MXPA01011409A - Resynchronization method for decoding video - Google Patents

Abstract

The present invention relates to a method of resynchronization that attempts to enable a resynchronization of a compressed video data signal received by a decoder, after a transmission error has been detected in said compressed video data signal. The method of resynchronization is based on the use of a resynchronization word. In order to ensure a proper decoding of the compressed video data signal, the resynchronization word is distinguishable from the known VLC words as well as the VOP start code.

Description

RESYNCHRONIZATION METHOD FOR DECODIFIES VIDEO FIELD OF INVENTION - The present invention relates to a method of resynchronizing a compressed video data signal received by a decoder. Such a resynchronization method can be used in, for example, a channel, MPEG-4 decoder for a resynchronization of the decoding after an error i in the transmission has been detected in the received compressed video data signal.

ANTECEDENTS OF THE. INVENTION A resynchronization method is known by the standard Visual MPEG-4, referred to as the MPEG-4 Visual version 1, ISO / IEC 14496-2. The standard MPEG-4 uses video objects, which are entities in a scene that a user can access and manipulate. To allow the video object to be accessed, it is necessary to have a coded representation of its form. Instances of video objects at a given time are called planes of video objects. { from now on referred to as VOPs). A layer of a video object is a set of VOPs, whose type of shape is identified by an integer called a video object in the form of layers.

Internally coded VOPs (hereinafter referred to as I-VOPs) are encoded without reference to other figures. They provide access points to the decoded sequence where decoding can begin, but are encoded only with moderate compression. The predicted coded VOP 's (hereafter referred to as P-VOPs) are coded more efficiently, using a motor prediction compensated by past internal or predicted coded VOPs, and are generally used as a reference I for other predictions. The predicted bidirectional coded VOPs (hereafter, referred to as B-VOPs) provide the highest degree of compression, but require both past and future reference VOPs for motor compensation. The motor vectors are defined by each 16 samples by a region of 16 lines of a VOP, hereinafter referred to as a macro block, or 8 samples by 8 lines of a region of a VOP, hereinafter referred to as in block, as required . A VOP_code_of_avance and a VOP_code_of_again 'are integers used in the decoding of a motor vector. The resynchronization method described in the standard MPEG-4 is based on a resynchronization word, referred to as a resyn_marker, which is inserted into the signal of. compressed video data. A one-bit flag called wmarcado_resinc_inhabilitads "is set to" 1"to indicate that there is no arcadsr_resinc e ^ .- the VOPs code, and" 0"to indicate that the marker exists there. MPEG-4 standard is a binary instrument of at least 16 zeros followed by a "0 0000 0000 0000 0001". For an I-VOP or a VOP in which the video_object_in_format_of_layers has the value "binary_only", the_resin_marker is 16 zeros followed by a 1. The length of the rescale_marker depends on the value of the next vop_code for a P-VOP, and the greatest value of either, vop_cigigof_advance or vop_code_of_return, for a B-VOP. the correct code is given by 16 + code The score_resin is (15 + code) zeros followed by a one, it only appears when the flag of the_restr_marker_inhibited flag is set to w0". The_resink_marker can only be located in front of a macro block and aligned to a byte.

The object of this invention is to achieve a more reliable resynchronization of the compressed video data signal received by a decoder. The invention takes the following aspects into consideration. A word of resi; The timing should be distinguishable from all the possible bit sequences which can be contained in the compressed video data signal, because we do not have any a priori knowledge of where the resynchronization word can be. For example, in the standard MPEG-4, a resynchronization word must be distinguishable from any variable code word (hereinafter referred to as VLC) as well as the VOP Start Code, which marks the start of a VOP. . However, the standard MPEG-4 in its current stage i allows some combinations of VLC words that can lead to the existence of an unexpected resynchronization word in the compressed video data signal and, as a consequence, to a false resynchronization. A combination of bits, which is obtained from the decoder of a macro block belonging to a B-VOP and which can lead to an unwanted resynchronization as the following: Macro block mode for B-blocks (MODB): 00 Type of macro block (MB TYPE): 0001 Block pattern encoded for B-blocks (CBPB): 1000 00 Quantized information (DBQUANT): 0 Motor vector VLC word: 0000 0000 0011 1 This example shows a particular combination of VLC words comprising a pair of 16 consecutive zeros followed by a one: 5 zeros corresponding to the word CBPB VLC, 1 corresponding to the word DBQUANT VLC, and 10 corresponding to the -15.5 word VLC of vector motor. In this case, there is an ambiguity between this particular combination of VLC words and the resynchronization word, which is also constituted by 16 consecutive zeros (15 + code, code being equal to 1 in this case) followed by a one. The particular combination of VLC words is capable of emulating a resynchronization that is not expected. In this case, an MPEG-4 video decoder can not • properly decode the compressed video data signal. The resynchronization method according to the invention is characterized in that it comprises a term for detecting a resynchronization word comprising at least 17 successive zeros followed by a one for a predicted bidirectional coded video object plane. As a consequence, said resynchronization method will provide a more reliable resynchronization of the compressed video data signal, because it is based on a resynchronization word containing at least 17 successive zeros, even if the code equals one, which is distinguishable from the particular combination of VLC words previously described. Said resynchronization method will hereby ensure proper decoding of the compressed video data signal containing said particular combination of VLC words. • The present invention also applies to a video decoder implementing said resynchronization method and a compressed video data signal that loads a resynchronization word as described in the resynchronization method. The present invention finally relates to a method of inserting a resynchronization word into a compressed video data signal and to a video encoder implementing said method. These and other aspects of the invention will be apparent and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein, FIGURE 1 shows a video package containing a resynchronization word according to the invention, and FIGURE 1 2 is a flow chart illustrating a resynchronization method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention is applied to a resynchronization method that attempts to enable a resynchronization of a data signal (compressed video received by a decoder, after a transmission error has been detected in said data signal). of compressed video.

Generally, the data between the synchronization point before the error and the first point where the resynchronization is restored are discarded. If the resynchronization method locates the amount of data discarded by the decoder, it will greatly increase the ability of other types of tools that recover data and / or that disguise the effects of errors. This resynchronization method is described in the context of an MPEG-4 video data signal but is also applicable to other types of compressed video data signals, which manipulate the video objects. The resynchronization method is based on a division of a VOP into video packets. Said method allows periodic resynchronization words to be provided through the compressed video data signal. Thus, the length of the video packets is not based on the number of macro blocks, but on the number of bits contained in the packet, which makes the resynchronization method more accurate. FIGURE 1 shows a typical video package. This video package comprises: - The resynchronization word (RW-) used to mark the start of a new video packet, Collector information such as the addresses of the macro blocks (NUM) of the first macro block contained in the packet, the quantization parameter (Q) and the Collector Extension Code (HEC); the collector information is necessary to restart the coding process, - macro data ^ block (MBD). The word of res I synchronization must be distinguishable 1 from any possible XL words as well as from the VOP start code. As described in the summary of the invention, a particular sequence of VLC words create an ambiguity in the case of the art resynchronization word. - "- 5 Said sequence comprises: A macro block mode for VLC blocks B-words (MODB).

This word MODB VLC is present only in macro blocks encoded of B-VOPs and is equal to: • 1 if no CBPB data or MB type data are present for a macro block, • 01 if only Type MB data are present for a macro block, • 00 if CBPB data and MB type are present for a macro block. • 25 One word VLC type (Type MB) macro block. This word Type MB VLC is present only in macro encoded blocks of B-VOPs for which a motor vector is included. The codes for Type MB are 1, 01, 001 or 0001 depending on the type of motor vector used. A macro block pattern for VLC block-B words (CBPB). This word CBPB is a 3 to 6 bit code, each bit in the code representing an encoded / uncoded status of a block. For each of the transparent non-I blocks with coefficients, the corresponding bit in the code is set to "1". A VLC word of quantizing information (DBQUANT).

This DBQUANT word specifies the change in quantification for B-VOPs. The codes for DBQUANT are 10, 0 or 11. A motor VLC word. This VLC word is a 1 to 13 bit code and contains up to 10 consecutive zeros as described in the following examples: i Codes! Differences of Vectors 0000 0000 0010 1 -16 0000 0000 0011 1 -15.5 0000 0000 0011 0 15.5 0000 0000 0010 0 16 The definition of the above-described VLC words guides a combination of bits comprising at most a set of 17 consecutive zeros followed by a one: 6 zeros corresponding to the word CBPB VLC, 1 corresponding to the word DBQUANT VLC, and 10 corresponding to the word VLC of motor vector. Such a combination of bits can be ambiguous in the case of a resynchronization word having (15 + code) zeros if code is equal to 1 or 2. It is a possible case because the value of the code, corresponding to vop__códigof_de_avanee or vop_code_of_return, is an integer i 10 of 3 bits that acquires values from 1 to 7, the value 0 being prohibited, and said values depending on the range of the driving vector.

As a consequence, the longitxid of the word of , _ resynchronization must be increased and must contain at least 17 successive zeros. The resynchronization word score_resin according to the invention is a row binary of at least 16. zeros followed by a one '0 0000 0000 0000 0001 '. For an I-VOP or a VOP where the video_object_in_format_of_layers has the value "solo_binary", the resins_marker are 16 zeros followed by a one. The length of this rescale_marker is dependent on the value of the vop_code_of_avance, for a P-VOP, and the longest value of any vop_code_of_a_nee and 5 cop_code_of_return for a B-VOP. In a preferred embodiment, the ratio between the length of the rescale_ marker and the appropriate code is given by 16 + code for a P-VOP and 16 + n + code for a B_VOP. The resin_marker is (15 + code) zeros followed by a one for a P_VOP and (15 + n + code) zeros followed by a one for a B_VOP, where n is an integer strictly greater than 0. It is only present when the flag marked_resin_inhibited it is set to "0". A Resine marker should only be located immediately before a macro block and be aligned with a byte. i FIGURE 2 is a flow chart illustrating a resynchronization method according to the invention. During the decoding step (DEC) a compressed video data signal (CS) is received and an output signal (OS) is supplied. The output signal is analyzed during an error analysis step (ERR). When an error is detected in the compressed video data signal, as it is, for example, an illegal VLC word or a semantic error, a detection step (DET) searches for a resynchronization word in the compressed video data signal. Said resynchronization word depends on the type of VOP and comprises: 16 zeros followed by a one for an I-VOP, (15 + code) zeros followed by a one for a P-VOP, (15 + n + code) zeros followed for a one for a B-VOP, where n is a strictly integer. greater than 0. Finally, when a resynchronization word is detected, a synchronization step (SYNC) allows the decoding process to be restarted from the collector information following the resynchronization word. The macro block number (NUM) provides spatial resynchronization while the quantization parameter (Q) allows the differential decoding process to be resynchronized. . i In a first embodiment of the invention, a word of CBPB VLC containing 6 consecutive zeros is not a possible combination, because this word VLC, which must correspond to 6 uncoded blocks, could then comprise a word MODB VLC equal to 1 or 01 (data not CBPB). In this case, the most unfavorable word CBPB VLC is "100000" and a value of n equal to 1 corresponding to a resynchronization word of (16 + code) zeros followed by a one, is sufficient to prevent any ambiguity between said word of resynchronization and the known VLC words. In a second embodiment of the invention, a VB MODB word equal to 00 associated with a CBPB VLC word containing 6 consecutive zeros is represented, because this combination of VLC words is allowed by the standard MPEG-4. Consequently, the value of n is. 2 corresponding to a _ resynchronization word of (17 + code) zeros followed by a one, which preclude any ambiguity in the case of a combination of VLC words comprising a set of 17 consecutive zeros followed by a one: 6 zeros corresponding to the word CBPB VLC, 1 zero corresponding to the word DBQUANT VLC, and 10 zeros corresponding to the word VLC of driving vector.

A resynchronization method as described in FIGURE 2 is imp1ementad < 'in an integrated circuit to be integrated into, for example, a video decoder. The video decoder is adapted to receive a compressed video data input signal and comprises a detector for detecting the resynchronization word in the compressed video data input signal and a synchronizer for resynchronizing the decoding of the video input signal. compressed video data from the detection of the resynchronization word.

The present invention also applies to an insertion method, within a compressed video data signal, a resynchronization word containing at least 17 successive zeros, for example (15 + n + code) zeros in the preferred embodiment, followed by a one for video data corresponding to a B-VOP. Said insertion method is implemented in an integrated circuit to be integrated in, for example, a video encoder. The corresponding video encoder is adapted to supply a compressed video data output signal and comprises means for inserting the resynchronization word into the compressed video data output signal.

. On the other hand, the signal supplied by said video encoder I or received by said video decoder is recognizable since it contains a resynchronization word comprising at least 17 successive zeros, for example (15 + n + code) zeros in the preferred embodiment, followed by a one for video data corresponding to a B-VOP.

It will be obvious that the use of the verb "to understand" and its conjugation does not exclude the presence of any other steps or elements that those defiin any clause. The word "a" preceding an element or step does not exclude the presence of a plurality of said elements or steps.

Claims (7)

1. CHAPTER CLAIMING Having described the invention, it is considered as a novelty and, therefore, what is contained in the following clauses is claimed: 1. A method of resynchronization of a compressed video data signal received by a decoder, which contains a step for detecting a resynchronization word in the compressed video data signal and a step of synchronizing a decoder of the compressed video data signal from the detection of the resynchronization word, by being characterized in that way , the discovery step is adapted to detect a resynchronization word comprising at least 17 successive zeros, followed by one for video information corresponding to a plane of the video object code, bidirectionally predicted.
2. 2. A video decoder for receiving a compressed video data signal containing a detector for detecting a re-recording word in the compressed video data signal and a synchronizer for resynchronizing a decoding of the compressed video data signal from detection of the resynchronization word, characterized in such a way, that said detector is adapted to detect a • resynchronization word, which comprises at least 17 successive zeros, followed by one for video information corresponding to a plane of the video object code, bidirectionally predicted.
3. 3. A method for inserting a resynchronization word into a compressed video data signal supplied by an encoder, characterized in such a way, that said method is adapted to insert a resynchronization word comprising at least 17 successive zeros, followed one for information from video corresponding to a flat map of the video object code, bidirectionally predicted.
4. 4. A video encoder for supplying a compressed video data signal and containing means for inserting a resynchronization word into the compressed video data signal, characterized in such a manner, that said means for inserting is adapted to insert a resynchronization word comprising at least 17 successive zeros, followed by one for video information corresponding to a plane of the video object code, bidirectionally predicted.
5. 5. A compressed video data signal containing a resynchronization word, so characterized, that the resynchronization word comprises at least 17 successive zeros, followed by one for video information corresponding to 'a plane of the video object code, bidirectionally predicted .
6. 6. A computer program product for a video decoder comprising a set of instructions which, when loaded in the decoder, causes the decoder to perform the method as set out in clause 1. I 1.
7. 7. A computer program product for a video encoder comprising a set of instructions that, when loaded in the encoder, cause the encoder to perform the method as set forth in clause 3.