TABLE OF CONTENTS H.263 pays special attention to variable length coding (VLC) for two different reasons. First, since H.263 is a low bit rate codec, it uses any means as well as arithmetic coding as an efficient VLC to enhance the compression efficiency. On the other hand, since H.263 is intended for mobile applications, where the channel error can be very severe and VLC coded data is very prone to the effects of errors, it uses a less compression efficient VLC to localise the side effect of channel errors. These two contradictory requirements are of course for two different applications and both are optional. In the normal mode, H.263 like the other standard video codecs uses the conventional VLC.
In the optional arithmetic coding mode of H.263 [25-E], all the corresponding variable length coding/decoding operations may be replaced by binary arithmetic coding/decoding. This mode is used to improve the compression efficiency. It is shown that use of arithmetic coding will improve the compression efficiency over the conventional Huffman coded VLC by approximately 5 10 per cent, depending on the type of data to be coded [10]. However, arithmetic coded data is as prone to channel errors as normal VLC data. Hence, care should be taken to protect data against channel errors.
The type of arithmetic coding is called syntax-based, since a symbol is VLC encoded using a specific table based on the syntax of the coder. This table typically stores lengths and values of the...
