Soft source decoding with applications

Motivated by previous results in joint source-channel coding and decoding, we consider the problem of decoding of variable-length codes using soft channel values. We present results of decoding of selected codes using the maximum a posteriori (MAP) decoder and the sequential decoder, and show the performance gains over decoding using hard decisions alone. The objective behind this work is to provide motivation for decoding of data compressed by standard source coding schemes, that is, to view the compressed bitstreams as being the output of variable-length coders and to make use of the redundancy in the bitstreams to assist in decoding. In order to illustrate the performance achievable by soft decoding, we provide results for decoding of MPEG-4 reversible variable-length codes as well as for decoding of MPEG-4 overhead information, under the assumption that this information is transmitted without channel coding over an additive white Gaussian noise channel. Finally, we present a method of unequal error protection for an MPEG-4 bitstream using the MAP and sequential source decoders, and show results comparable to those achievable by serial application of source and channel coding.