Audio codecs face various channel impairments when used in challenging applications such as digital radio. The standard AMR-WB+ audio codec includes a concealment procedure to handle lost frames. It is also inherently robust to bit errors, although some bits within any given frame are more sensitive than others. Motivated by this observation, the present paper makes two contributions. First, a detailed study of the sensitivity of individual bits in AMR-WB+ frames is provided. All the bits in a frame are then divided into three sensitivity classes so that efficient unequal error protection (UEP) schemes can be designed. Then, a very low bit rate protection layer to increase the robustness of the codec against bit errors is proposed and assessed using the results of subjective audio quality tests. Remarkably, in contrast to the standard codec, where some errors have a very discernable effect, the protection layer ensures that the decoded audio is free of major channel artifacts even at a significant 0.5% bit error rate.
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