- This vulnerability is represented by the rapid propagation of errors in both time and space and the quick degradation of the reconstructed video quality. - In this chapter, we cover a variety of the error concealment and resilience techniques used in video communications today, and the transport-based error control schemes will be examined in the next chapter.. - This kind of error is the least destructive of the three to the quality of service.. - This effect can be mitigated if the actual MVs are encoded instead of the prediction residual. - In other words, the decoder fails to detect any valid VLC word for a particular video parameter within a segment of the bit stream that corresponds to the maximum length of the corrupted code word. - Error concealment techniques take advantage of the human eyes tolerance to distortion in the high-frequency components more than the low-frequency compo- nents of a video frame. - Techniques have been proposed for the error concealment of the damaged shape data of MPEG-4 video coded sequences (Shirani, Erol and Kossentini, 2000). - Another method is to replace the lost MV by the MV of the spatially corresponding MB in the previous frame. - The MV that best moves its corresponding MB in the direction of the damaged MB (MB with lost MV) is used as the value of the lost MV. - Figure 4.4 One-hundredth frame of Foreman coded with H.263 and subject to random errors with BER : 0.01 per cent: (a) no concealment, (b) zero-MV technique, (c) MV of spatially corresponding MB in previous frame, (d) MV of MB in previous frame that best moves in the direction of the lost MV. - When only some coefficients in a block are damaged, coefficients in the same block could be used for the interpolation of the lost coefficient value. - The pixels in all of the four one-pixel wide boundaries could be used, or alternatively only those pixels in the two nearest boundaries, as shown in Figure 4.5. - At the decoder, a simple error concealment technique sets both MVs and texture blocks of the concealed INTER. - UEP was initially proposed as one of the error resilience techniques applied on MPEG-4 video data during the. - development process of the standard. - This results in some of the motion information receiving less protection than required. - As established in the previous section, the content of the first partition of an MPEG-4 video packet is much more sensitive to channel errors than that of the second partition. - This results in a huge waste of bandwidth, resulting from discarding all the error-protected parameters in the skipped video segment, there- by reducing the efficiency of the employed FEC scheme.. - Therefore, the rate of the convolutional coder varies depending on the decoder ability to correct the corrupted bits. - The inserted data can therefore be located by reading backwards from the synch word at the beginning of the following video packet. - Errors detected in either one of the two CRC check codes or in the first partition of a video packet lead to a whole packet loss.. - To enable the video decoder to locate the duplicate motion data in the bit stream, a specific bit pattern is sent just prior to the start of the duplicated MVs. - Figure 4.14 Seventy-fifth frame of the Foreman sequence encoded with MPEG-4 and sent over a mobile channel of BER : 3 . - One or more bit errors hit a motion vector component in such a way that the decoder is unable to find a legitimate codeword at this position of the bit stream. - In this case, the decoder assumes an error is detected, moves forward in the bit stream to locate the start of the duplicate data segment, reads the second version of the MV component and resumes decoding after returning to the position of errors and flushing the number of bits that correspond to the length of the decoded MV components. - Obviously, the possibility that both copies of the same MV component are corrupted is not annihilated but the likelihood of a bit error in the same component is reduced. - If the decoder finds no discrepancy between the calculated parity and the value of the checksum word, it. - In order to reduce the bit overhead of the above mechanism, motion data duplication can be applied on a GOB level, as shown in Figure 4.17.. - The number of INTRA frames should be a compromise between the error resilience of the video coder and. - Figure 4.19 Two-hundredth frame of the Foreman sequence coded with H.263 and transmit- ted over a channel with random errors at BER : 10. - However, if a VLC word in the first INTRA frame is hit by errors, the decoder fails to complete the reconstruction of the following part of the frame. - Figure 4.20 depicts the luminance PSNR values of the Fore- man sequence with increased I-frame frequency when the first frame is subject to errors. - The damaged I-frame will also entail the corruption of the next P-frames which are all temporally predicted. - This is demonstrated in the low PSNR values of the first 20 frames of Figure 4.20.. - The INTRA frame refresh technique described earlier entails a large increase of the output bit rate of a video encoder. - AIR is a technique that is defined in AnnexE of the MPEG-4 standard. - The number of coded MBs is determined based on the bit rate and frame rate requirements of the video application. - A bit error that corrupts one of the variable-length codes (such as CBPY, MCBPC, TCOEFF runs and levels) of an I-frame leads to the loss of synchronisation even when INTRADC coefficients are protected with FEC techniques. - In this situation, the decoder terminates the processing of the damaged video frame and discards the next segment of the bit stream until it detects the first error-free synch word. - For a QCIF-size sequence, the fixed length section of the I-frame contains 594 INTRADC coeffi-. - If an error is still detected in the fixed-length segment of the I-frame, the decoder preserves its synchronisation and goes to the next 16-bit protected INTRADC coefficient in the I-frame. - However, if an error is detected in one of the VLC words in the variable-length section, the decoder sets to zero all the AC coefficients following the position of error. - Figure 4.26 Luminance PSNR values for 150 frames of the Miss America sequence encoded with H.263 at 47 kbit/s, 1.2 I-f/s, BER : 10. - To enhance the error resilience of a video coder, the factors that lead to the variability of the bit rate must be counteracted (Sadka, Eryurtlu and Kondoz, 1997). - One method is to place some relatively long synch words that can notify the decoder of the start of a specific block of data in the bit stream. - H.263/M is a modified version of the standard H.263 coder for mobile applications. - Undoubtedly, FLC incurs some overhead on the coded stream and reduces the coding efficiency of the compression algorithm. - The major role of fixed-length coding is to reduce the possibility of the decoder losing its synchronisation on the occurrence of an error. - This implies the failure of the fixed-length coding scheme when an error hits one of the VLC words of TCOEFF. - If an error is encountered in the variable-length section of the frame (AC coefficients), the decoder can skip the remaining bits, setting all the following AC parameters to zero. - The mode of each MB in a P-frame is an important parameter that affects the resilience of the video stream to errors. - In this case, the part of the stream that follows the position of error is discarded until an error-free synch word is detected.. - The number of discarded bits is a function of the distance between the position of error and the location of the next error-free synch word. - Figure 4.29 Frame 200 of the Foreman sequence encoded at 64 kbit/s and transmitted over a mobile channel with random error distribution and BER : 10. - Figure 4.30 PSNR values for 200 frames of the Foreman sequence encoded at 64 kbit/s and transmitted over a mobile channel with random error distribution and BER : 10. - its synchronisation at the synch word, it resumes its operation in the reverse direction, decoding the segment of the bit stream that was initially skipped in the forward direction. - Figure 4.31 Methods used in two-way decoding to determine what segment of the bit stream to discard: (a) separated error points, (b) crossed error points, (c) error detected in only one direction, (d) errors isolated to a single MB. - Another method to generate a constant weight RVLC table is to use a fixed number of the first symbol of a code. - Figure 4.32 One frame of the Carphone sequence coded with H.263 at 28 kbit/s and 12.5 f/s using: (a) ordinary decoding, (b) two-way decoding. - Since the number of the first detected symbol in each code can be identified, these codewords can be two-way decoded. - A synch word is transmitted once every GOB to allow the decoder to restore synchronisation, upon detection of an error, at the beginning of the next GOB. - Using two-way decoding, the decoder is able to recover the bits situated between the corrupted MB and the start code of the following GOB. - Therefore, the effective error rate is likely to be higher than the actual bit error ratio of the communication link (Eryurtlu et al., 1997). - where r is the overall bit rate of the video coder, f is the frame rate, s is the number of synch words per frame and l. - Q is the length of the synch word. - AF values and for different values of the block length L. - The size of the fixed-length slots is determined by the encoder and sent to the decoder at the beginning of each block of video prior to transmission of the slots. - If the decoder detects a bit error in one of the VLC words, the syn- chronisation can always be recovered at the beginning of the next slot, thereby limiting the damage to the bits contained in the corrupted slot.. - This can be determined by evaluating the sum of the lengths b. - EREC is an algorithm that consists of N number of stages whereby a VLC word is placed in each of the fixed-length slots. - The reorganisation process of the four MV components is depicted in Figure 4.38.. - In an attempt to recover synchronisation at the beginning of the next slot, the decoder skips all the embedded bits in the slot, including those bits that might belong to other VLC words in the video block. - Figure 4.39 Frame 155 of the Carphone sequence coded with H.263 at 80 kbit/s and 25 f/s and transmitted over a channel with BER : 0.1 per cent: (a) without error resilience, (b) with EREC applied on VLC words in the bit stream. - Figure 4.40 Frame 124 of the Suzie sequence coded at 110 kbit/s and 25 f/s transmitted over a channel with BER : 10. - For instance, the first partition in the packet consists of all the encoded shape and motion data of the contained MBs in addition to the corresponding administrative data such as the COD flags and MCBPC. - EREC is applied to the motion data of the first partition while the DCT data is coded with reversible VLC words to allow two-way decoding of DCT coefficients when the decoder flags an error in the bit stream of the second partition. - Moreover, a one-half rate Turbo coder is used to protect the header data of the first partition.. - Figure 4.41 Average PSNR values for the Suzie sequence encoded with MPEG-4 at 32 kbit/s and 10 f/s with and without the combined error-resilience tools of the MPEG-4 standard. - These error-resilience techniques are described in the annexes that are associated with the definition of the core H.263 standard. - A slice contains a number of MBs that are coded independently of the content of other slices. - In this case, the functionality of this mode becomes similar to that of the object-oriented VOP structure in the MPEG-4 standard. - adopts AnnexN on reference picture selection to enhance the error resilience of the compressed stream by enabling more than just one reference frame in the motion compensation process, as described in the next section. - specifies a set of three optional error resilience techniques in Annexes U, V and W of the H.263 video coding standard (Sullivan, 2000). - AnnexV is a combination of the data partitioning scheme described in Section 4.4 and the slice structure mode specified by annexK of H.263. - The fixed-point IDCT indicates that a particular IDCT approximation is used in the construction of the bit stream. - However, if the decoder is capable of the fixed-point IDCT and the encoder indicates the fixed-point IDCT function type in the bit stream, then the forced updating requirement is removed and the frequency of. - is an error-resilience technique that is similar to the NEWPRED approach suggested as a part of the MPEG-4 suite of error-resilience strategies. - However, to do that the decoder must be informed of the reference picture that was selected to predict a particular segment of frame.. - During error-free transmission, the operation of the encoder is not altered and the previously decoded image is used as a reference picture.. - NACKs report the temporal and spatial location of the video content that could not be decoded successfully. - of this anneximproves the coding efficiency of the video encoder and renders the coded stream more robust to random transmission errors. - Alternatively, the decoding of the corrupted thread is ceased and the next sync frame is predicted only from. - A critical factor of the evaluation is the delay that an error control scheme incurs on the two-way and multipoint video communications. - Two-way decoding enables the decoder to decode the bit stream in the reverse direction to salvage all or part of the bit stream which is skipped in the forward direction after an error is detected. - of the IEEE, Special Issue on Video for Mobile Multimedia, 1999.. - of the International Conferences on Acoustics, Speech and Signal Processing ICASSP, III . - of the Packet Video Workshop 2000, Sardinia, Italy, May 2000.. - of the IEEE International Conference on Image Processing . - of the IEEE, 86, No. - of the 2000 International Conference on Image Processing Jan
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