This Recommendation specifies a coded representation that can be used for compressing the speech or other audio signal component of multimedia services at a very low bit rate as part of the overall H.324 family of standards. This coder has two-bit rates associated with it, 5.3 and 6.3 kbit/s. The higher bit rate has greater quality. The lower bit rate gives good quality and provides system designers with additional flexibility. Both rates are a mandatory part of the encoder and decoder. It is possible to switch between the two rates at any frame boundary. An option for variable rate operation using discontinuous transmission and noise fill during non-speech intervals is also possible.

This coder was optimized to represent speech with a high quality at the above rates using a limited amount of complexity. It encodes speech or other audio signals in frames using linear predictive analysis-by-synthesis coding. The excitation signal for the high rate coder is Multipulse Maximum Likelihood Quantization (MP-MLQ) and for the low rate coder is Algebraic-Code-Excited Linear-Prediction (ACELP). The frame size is 30 ms and there is an additional look ahead of 7.5 msec, resulting in a total algorithmic delay of 37.5 msec. All additional delays in this coder are due to processing delays of the implementation, transmission delays in the communication link and buffering delays of the multiplexing protocol.

The description of this Recommendation is made in terms of bit-exact, fixed-point mathematical operations. The ANSI C code indicated in clause 5 constitutes an integral part of this Recommendation and shall take precedence over the mathematical descriptions in this text if discrepancies are found. A non-exhaustive set of test sequences which can be used in conjunction with the C code are available from the ITU.