ROBUST SPEECH CODING FOR THE INDOOR WIRELESS CHANNEL

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).1 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.