Practical considerations for variable length source coding

Variable length source coding schemes offer substantial improvements in data rate for a wide variety of data compression techniques. Unfortunately, practical considerations have limited their use. This paper describes several techniques which allow these powerful codes to be effectively applied. In particular, the problem of loss of code word synchronization due to channel errors is addressed. It is shown how self-synchronizing codes limit error propagation. Also, variable delay and the associated difficulties of buffer synchronization and buffer overflow are considered. The variable delay at the transmitter can be coupled with a variable delay at the receiver to give a fixed system delay. Buffer control then becomes the allocation of this delay between transmitter and receiver. Finally, a switching property of variable length codes to better match the bimodal operation of the source is presented.