A fast rate allocation algorithm of joint source and channel coding is proposed for progressive image transmission on band-limited and time varying channels. Firstly the channel is segmented into several operating regions according to the error-probability performance of a set of rate-compatible channel codes. In each operating region

the number of available channel codes in the rate set is reduced

and the optimal error protection scheme is obtained by forward and backward rate searching. The proposed algorithm has a low complexity

and its computation number is only ten percent of that of the heuristic search algorithm

thus the operation time is reduced. When the fast algorithm is applied in adaptive transmission systems

the rate allocation can be quickly adjusted according to channel conditions. Simulation results show that the PSNR value of reconstructed image is always above 29.5 dB and the fluctuation range is less than 4 dB

which shows a good and stable transmission quality.