Enhanced full rate speech transcoding
The present document describes the detailed mapping between input blocks of 160 speech samples in 13‑bit uniform PCM format to encoded blocks of 244 bits and from encoded blocks of 244 bits to output blocks of 160 reconstructed speech samples. The sampling rate is 8 000 sample/s leading to a bit rate for the encoded bit stream of 12,2 kbit/s. The coding scheme is the so‑called Algebraic Code Excited Linear Prediction Coder, hereafter referred to as ACELP.
The present document also specifies the conversion between A‑law or -law (PCS 1900) PCM and 13‑bit uniform PCM. Performance requirements for the audio input and output parts are included only to the extent that they affect the transcoder performance. This part also describes the codec down to the bit level, thus enabling the verification of compliance to the part to a high degree of confidence by use of a set of digital test sequences. These test sequences are described in GSM 06.54 [7] and are available on disks.
In 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 06.53 [6], the description in GSM 06.53 [6] will prevail.
The transcoding procedure specified in the present document is applicable for the enhanced full rate speech traffic channel (TCH) in the GSM system.
In GSM 06.51 [5], a reference configuration for the speech transmission chain of the GSM enhanced full rate (EFR) 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 (PCS 1900) 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 [3]. In the receive direction, the inverse operations take place.