06.903GPPAdaptive Multi-Rate speech transcodingTS
The present document describes the detailed mapping from input blocks of 160 speech samples in 13‑bit uniform PCM format to encoded blocks of 95, 103, 118, 134, 148, 159, 204, and 244 bits and from encoded blocks of 95, 103, 118, 134, 148, 159, 204, and 244 bits to output blocks of 160 reconstructed speech samples. The sampling rate is 8 000 samples/s leading to a bit rate for the encoded bit stream of 4.75, 5.15, 5.90, 6.70, 7.40, 7.95, 10.2 or 12.2 kbit/s. The coding scheme for the multi-rate coding modes is the so‑called Algebraic Code Excited Linear Prediction Coder, hereafter referred to as ACELP. The multi-rate ACELP coder is referred to as MR-ACELP.
In the case of discrepancy between the requirements described in the present document and the fixed point computational description (ANSI‑C code) of these requirements contained in GSM 06.73 , the description in GSM 06.73  will prevail. The ANSI‑C code is not described in the present document, see GSM 06.73  for a description of the ANSI‑C code.
The transcoding procedure specified in the present document is applicable for the adaptive multi-rate full rate and half rate speech traffic channels (TCH) in the GSM system.
In GSM 06.71 , a reference configuration for the speech transmission chain of the GSM adaptive multi-rate (AMR) system is shown. According to this reference configuration, the speech encoder takes its input as a 13-bit uniform PCM signal either from the audio part of the Mobile Station or on the network side, from the PSTN via an 8-bit A-law or ‑law to 13-bit uniform PCM conversion. The encoded speech at the output of the speech encoder is delivered to a channel encoder unit which is specified in GSM 05.03 . In the receive direction, the inverse operations take place.