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In digital communications source coding is indispensable to achieve a high bandwidth efficiency in applications where bandwidth is a limited resource. Usually these source coding algorithms determine speech or audio parameters which are highly sensitive to transmission errors. This paper deals with an error concealment technique that benefits from residual redundancy remaining after source coding. Due to delay and complexity constraints a certain amount of inter-frame as well as intra-frame correlation between source codec parameters remains which might be exploited to enhance the subjective audio quality.In this contribution we propose an algorithm which is able to utilize both types of redundancy if the source parameters exhibit a two-dimensional Markov property. Actually, this approach is nearly optimal in the Minimum Mean Square Error (MMSE) sense but much less complex compared to the optimal estimator.

In digital mobile speech traosmission usually the most important (class la) bits provided by the speech coding scheme are protected by a CRC for error detection. As a consequence all parameters spanned by the class l a bits have to be marked at the receiver either as reliable or as unreliable. In contrast to this somewhat coarse approach we propose the usage of what we call parameter individual block codes (PIBC) for the most important codec parameters. This allows joint speech codec parameter and PIBC decoding taking advantage of the error concealing properties of softbit speech decoding [l, 21.

SUMMARYThe speech quality in digital mobile communications is improved by using the residual redundancy after speech encoding in source decoding or channel decoding. The latter was investigated at the LNT at Munich University of Technology, the first idea at the IND at RWTH Aachen University. When the two institutes got aware of each other's results, a friendly scientific competition was started by investigating 'source-controlled channel decoding' (SCCD) at the LNT and 'source optimised channel codes' (SOCC) at the IND. It turned out, a combination of the two separate approaches would be the best, which led to a fruitful cooperation. The outcome was the 'joint source and channel coding' approach, the iterative source and channel decoding (ISCD) and its improvement by applying the extrinsic information transfer (EXIT) chart analysis. In this paper we give a summary of the algorithms and their performances. We also take a look onto the application of the algorithms to real world systems.