This Recommendation contains the description of an algorithm for the coding of speech signals at 8 kbit/s using conjugate-structure algebraic-code-excited linear prediction (CS-ACELP). This Recommendation includes an electronic attachment containing reference C code and test vectors for fixed-point implementation of CS-ACELP at 8 kbit/s.

The ITU-T G.729 coder is designed to operate with a digital signal obtained by first performing telephone bandwidth filtering specified by [ITU-T G.712] of the analogue input signal, then sampling it at 8 000 Hz, followed by conversion to 16-bit linear pulse code modulation (PCM) for the input to the encoder. The output of the decoder should be converted back to an analogue signal by similar means. Other input/output characteristics, such as those specified by [ITU-T G.711] for 64 kbit/s PCM data, should be converted to 16-bit linear PCM before encoding, or from 16-bit linear PCM to the appropriate format after decoding. The bit stream from the encoder to the decoder is defined within this Recommendation.

This Recommendation is organized as follows: clause 2 gives a general outline of the CS-ACELP algorithm. Clauses 3 and 4 discuss the CS-ACELP encoder and decoder principles, respectively. Clause 5 describes the software that defines this coder in 16-bit fixed-point arithmetic.