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Audio-visual source encoders for digital mobile communications extract parameters that-due to delay and complexity constraints-exhibit some residual redundancy. This residual redundancy can be exploited by iterative source-channel decoding (ISCD) to improve the robustness against transmission noise by performing soft parameter detection as part of the decoding process. Systems employing ISCD at the receiving end often exhibit an observable error floor. While this error floor can be tolerated in some cases, it is often desirable to perfectly reconstruct the source codec parameters. In this paper, we explain the reasons for the error floor and propose two solutions for realizing ISCD systems with optimized error floor performance while maintaining the desired near-capacity waterfall behavior. All approaches aim at optimizing the distance properties of the (redundant) mapping of bit patterns to the source codec parameters. In some cases, especially if small quantizer code books are employed, good mappings cannot be found-in this case, the novel multi-dimensional bit mapping allows to reduce the error floor after decoding.