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U. Glaeser

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Error Resilience for Mobile Video<br />

Mobile channels are characterised by a high level of hostility resulting from high bit error ratios (BER)<br />

and information loss. Because of the bit rate variability and the spatial and temporal predictions, coded<br />

video streams are highly sensitive to transmission errors. This error sensitivity can be the reason for an<br />

ungraceful degradation of video quality, and hence the total failure of the video communication service.<br />

A single bit error could lead to a disastrous damage to perceptual quality. The most damaging effect of<br />

errors is that which leads to a loss of synchronisation at the decoder. In this case, the decoder is unable<br />

to determine the size of the affected variable-length video parameter and, therefore, drops the stream<br />

bits following the position of error until it resynchronises at the next synch word. Consequently, it is<br />

vital to employ an error resilience mechanism for the success of the underlying video communication<br />

service.<br />

A popular technique used to mitigate the effects of errors is called error concealment [9]. It is a decoderbased<br />

zero-redundancy error control scheme whereby the decoder makes use of previously received errorfree<br />

video data for the reconstruction of the incorrectly decoded video segment. A commonly used approach<br />

conceals the effect of errors on a damaged MB by relying on the content of the spatially corresponding MB<br />

in the previous frame. In the case where motion data is corrupted, the damaged motion vector can be<br />

predicted from the motion vectors of spatially neighboring MBs in the same picture. On the other hand,<br />

transform coefficients could also be interpolated from pixels in neighboring blocks.<br />

However, error concealment schemes cannot provide satisfactory results for networks with high BERs<br />

and long error bursts. In this case, error concealment must be used in conjunction with error resilience<br />

schemes that make the coded streams more robust to transmission errors and video packet loss. In the<br />

literature, there are a large number of error resilience techniques specified in the standard ISO MPEG-<br />

4 [10] and the annexes to ITU-T H.263 defined in recommendations H.263+ [11] and H.263++ [12].<br />

One of the most effective ways of preventing the propagation of errors in encoded video sequences is<br />

the regular insertion of INTRA-coded frames, which do not make use of any information from previously<br />

transmitted frames; however, this method has the disadvantage of making the traffic characteristics of a<br />

video sequence extremely bursty since a much larger number of bits are required to obtain the same<br />

quality levels as for INTER (predictively coded) frames. A more efficient improvement to INTRA-frame<br />

refresh consists of regular coding of INTRA MBs per frame, referred to as Adaptive INTRA Refresh (AIR),<br />

where the INTRA coded MBs are identified as part of the most active region in the video scene. The<br />

insertion of a fixed number of INTRA coded MBs per frame can smooth out the bit rate fluctuations<br />

caused by coding the whole frame in INTRA mode. In the following subsections, we present two major<br />

standard-compliant error resilience algorithms specified in the MPEG-4 video coding standard, namely<br />

data partitioning and two-day decoding with reversible codewords.<br />

Video Data Partitioning<br />

The non error-resilient syntax of video coding standards suggests that video data is transmitted on a MB<br />

basis. In other words, the order of transmission is established such as all the parameters pertaining to a<br />

particular MB are sent before any parameter of the following MB is transmitted. This implies that a bit<br />

error detected in the texture data of an early MB in the video frame leads to the loss of all forthcoming<br />

MBs in the frame. Data partitioning changes the order of transmission of video data from a MB basis<br />

to a frame basis or a Visual Object Plane (VOP) basis in MPEG-4 terminology. Each video packet that<br />

corresponds to a VOP consists of two different partitions separated by specific bit patterns called markers<br />

(DC marker for INTRA coded VOPs and motion marker for INTER coded VOPs). The first partition<br />

contains the shape information, motion data, and some administrative parameters such as COD for<br />

INTRA frames and MCBPC of all the MBs in the VOP, while the second partition contains the texture<br />

data (i.e., the transform coefficients TCOEFF) of all the MBs inside the VOP and other control parameters<br />

such as CBPY. Using this partitioning structure as illustrated in Fig. 42.31, the errors that hit the data<br />

bits of the second partition do not lead to the loss of the whole frame since the error-sensitive motion<br />

data would have been correctly decoded upfront.<br />

© 2002 by CRC Press LLC

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