For real-time video services, excessively delayed ATM cells cannot be used later, and they are considered as lost cells. For compressed video services the effect of lost cells is very annoying, as each cell covers a large area of the picture. One method to overcome the problem of lost cells is the two-layer coding technique. Although two-layer coding is very immune to cell losses, its main drawback is the high bit rate. Experiments have shown that the average bit rate of two-layer codecs can go up to two times that of one-layer for the same coded picture quality (depending on the assigned base layer bit rate). Some techniques for reducing two-layer codec bit rates are introduced, and their performance is compared with the normal two-layer and the conventional one layer counterparts.
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