Abstract
Alternative methods for digitally transcoding speech for radio transmission in an indoor environment have been investigated and compared to the CCITT standard, adaptive differential pulse code modulation (ADPCM). <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> These alternative coders are designed to minimize the effects of transmission errors on the quality of the transcoded speech. The coders compared are CCITT standard G.721 ADPCM, adaptive sub-band coding, and two other non-standard versions of ADPCM. In general, when packets of data are lost, the adaptive sub-band coder performs extremely well in terms of maintaining speech quality, as the sub-band synthesis filters fill out the gaps in speech. However, the sub-band coder requires the greatest levels of complexity and delay. The other ADPCM systems offer lower complexity and delay — at the expense of lower speech quality.
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