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Compressed Video Communications - Sadka A.

Sadka A. Compressed Video Communications - John Wiley & Sons, 2002. - 283 p.
ISBN: 0-470-84312-8
Download (direct link): compressedvideo2002.pdf
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This could be done by placing the fixed-length codes of INTRADC coefficients, with a Hamming distance of one, as close together as possible in the corresponding FLC table at both the encoder and decoder. The effect is that the most likely INTRADC codes are less sensitive to a single bit error than the less likely codes. Another possible way of protecting INTRADC coefficients is to make use of their fixed-length coding for FEC protection. In the H.263 standard, each INTRADC coefficient is eight-bit long, and therefore applying half-rate convolutional coding on each INTRADC coefficient leads to a total overhead of 5.94 kbit/s (4752 bits per QCIF I-frame) for a frame rate of 25 f/s and INTRA frame rate of 1.251-f/s. The remaining 63 AC coefficients of each block in an I-frame can be coded with a coarse quantiser to counter the bit rate overhead imposed by the I-frames and the FEC protection of INTRADC coefficients.
4.7.1 Adaptive INTRA refresh (AIR)
The INTRA frame refresh technique described earlier entails a large increase of the output bit rate of a video encoder. The reason for that is the low compression efficiency achieved by the INTRA coding mode and the large number of MBs to be INTRA coded. For instance, refreshing a QCIF video scene with an I-frame requires the transmission of 99 INTRA coded MBs. This process leads to the
144
ERROR RESILIENCE IN COMPRESSED VIDEO COMMUNICATIONS
formation of undesirable spikes in the bit rate each time an I-frame is transmitted. Therefore, encoding a frame in INTRA mode produces a burst of bits that causes inevitable delays and helps build up a state of congestion in the network. Moreover, if the moving area of the image is corrupted by errors, the degradation propagates temporally, giving rise to long periods of quality deterioration until the next INTRA refresh takes place. To reduce the bit rate of INTRA coded frames while still maintaining error robustness and limiting temporal propagation of errors, a scheme known as adaptive INTRA refresh is normally used. AIR is a technique that is defined in Annex E of the MPEG-4 standard. It involves sending a limited number of INTRA MBs in each VOP, as opposed to the conventional Cyclic INTRA Refresh (CIR) where all MBs of a VOP are uniformly INTRA coded. The number of MBs to be INTRA coded in AIR is much smaller than the total number of MBs per VOP or frame. AIR selectively INTRA codes a fixed and predetermined number of MBs per frame according to a refresh map. The generation of this refresh map is achieved by marking the position of MBs which are subjected to motion, as illustrated in Figure 4.21 where the number of MBs to be INTRA coded per VOP is 2. The motion evaluation is carried out by comparing the sum of absolute differences (SAD) of a MB with a threshold value SAD-th. SAD is calculated between the MB and its spatially corresponding MB in the previous VOP and SAD-th is the average SAD value of the entire MBs in the previous VOP. If the SAD of a particular MB exceeds SAD-th, the encoder decides the MB belongs to a high motion area that is sensitive to transmission errors and thus marks the MB for INTRA coding. If the number of MBs marked for INTRA coding exceeds the number of MBs set to be INTRA coded, then the video coder moves down the frame in vertical scan order encoding INTRA MBs until the preset number of MBs have been encoded. For the next frame, the encoder starts in the same position and begins coding INTRA MBs including those marked for INTRA coding in the previous frame. The number of coded MBs is determined based on the bit rate and frame rate requirements of the video application. However, for improved robustness, the number of MBs can be made adaptive in accordance with the motion characteristics of each video frame (Worrall et al., 2000). Since the moving area of the picture is frequently encoded in INTRA mode, it is possible to quickly refresh the corrupted moving area.
Obviously, increasing the number of MBs that are refreshed in each frame speeds up the recovery from errors, but results in a decrease in error-free quality at a given target bit rate. This is due to the coarser quantisation process used to achieve the target bit rate. However, AIR provides a better and more consistent objective error-free quality than the conventional INTRA refresh technique for the same target bit rate, as shown in Figure 4.22. On the other hand, AIR produces a more stable output rate, as shown in Figure 4.23, since the INTRA coded information is sent more regularly (a fixed number of MBs per frame as opposed to 99 MBs once every number of frames). Therefore, the number of MBs to be INTRA coded
4.7 INTRA REFRESH 145
Figure 4.21 Generation of a motion map for AIR coding
Frame Number
Figure 4.22 F-PSNR values for 50 frames of Suzie sequences coded with MPEG-4 at the same target bit rate for both AIR and conventional INTRA frame refresh schemes in error-free conditions
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