The G.729A speech coder is a reduced complexity Conjugate-Structure Algebraic-Code-Excited Linear Prediction (CS-ACELP) speech compression algorithm defined in ITU-T G.729 Annex A. G.729A is interoperable with the G.729 coder. The reduction in complexity may result in a small decrease in voice quality. G.729A is suitable for VoIP or similar applications using multimedia, voice, and/or data. It may also be that the G.729AB coder may be valuable in reducing bandwidth usage.
G.729A requires 10 ms input frames and generates frames of 80 bits in length. Since G.729A is based on the Code-Excited Linear Prediction (CELP) model, each 80 bit frame produced contains linear prediction coefficients, excitation code book indices, and gain parameters that are used by the decoder in order to reproduce speech. The inputs/outputs of G.729A are 16 bit linear PCM samples that are converted from/to an 8 kbps compressed data stream. G.729A has the same total algorithmic delay of 15 ms as the G.729 speech coder.
The majority of the changes made to reduce complexity were in the G.729A encoding. The perceptual weighting filter uses instead the Linear Prediction Filter Co-efficients, making computation of the weighted synthesis filtering much easier. Decimation is used in the open-loop pitch analysis to compute correlations with weighted speech. Searches of both the adaptive and fixed algebraic codebooks were simplified as well. In the G.729A decoder, some modifications were made in the adaptive postfilter.