From soft decision channel decoding to soft decision speech decoding

For speech transmission in digital mobile radio systems the net bit rate has to be reduced with respect to frequency economy by source encoding techniques. In a second step redundant bit rate is added by channel encoding for the purpose of error protection. As the gross bit rate is fixed, a compromise has to be taken between the speech qualities in clear and poor channel conditions. If the quality of the channel is very poor error concealment techniques have to be applied additionally at the receiver to reduce the adverse subjective effects of residual bit errors after channel decoding. This contribution presents a universal approach to error concealment by exploiting soft decision information from the channel decoder and a priori information about speech codec parameters. The problem of joint optimization of speech coding, channel coding and error concealment is addressed. It is shown that error concealment can take over to a certain extent the task of error protection. Thus less bit rate would be required for channel coding.