Optimal use of Markov models for DPCM picture transmission over noisy channels

Joint source/channel decoders that use the residual redundancy in the sourc e are investigated for differential pulse code modulation (DPCM) picture tr ansmission over a binary symmetric channel. Markov sequence decoders employ ing the Viterbi algorithm that use first-order source statistics are review ed, and generalized for decoders that use second-order source statistics, T o make optimal use of the source correlation in both horizontal and vertica l directions, it is necessary to generalize the conventional Viterbi decodi ng algorithm for a one higher-dimensional trellis, The paths through the tr ellis become two-dimensional "sheets;" thus, the technique is coined "sheet decoding." By objective [reconstruction signal-to-noise ratio (SNR)] and s ubjective measure, it is found that the sheet decoders outperform the Marko v sequence decoders that use a first-order Markov model, and outperform two other known decoders (modified maximum a posteriori probability and maxima l SNR) that use a second-order Markov mode. Moreover, it is found that the use of a simple rate-2/3 block code in conju nction with Markov model-aided decoding (MMAD) offers significant performan ce improvement for a 2-bit DPCM system, For the example Lenna image, it is observed that the rate-2/3 block code is superior to a rate-2/3 convolution al code for channel-error rates higher than 0.035. The block code is easily incorporated into any of the MMAD DPCM systems and results in a 2-bit MMAD DPCM system that significantly outperforms the noncoded 3-bit MMAD DPCM sy stems for channel-error rates higher than 0.04.