5 Enhanced Full Rate speech channel transcoding

06.513GPPGSM Enhanced full rate speech processing functions: General descriptionTS

As shown in figure 1, the speech encoder takes its input as a 13‑bit uniform Pulse Code Modulated (PCM) signal either from the audio part of the Mobile Station or on the network side, from the Public Switched Telephone Network (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 the channel coding function defined in GSM 05.03 [3] to produce an encoded block consisting of 456 bits leading to a gross bit rate of 22,8 kbit/s.

In the receive direction, the inverse operations take place. GSM 06.60 [6] 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, hereafter referred to as ACELP.

GSM 06.60 [6] describes the codec and GSM 06.53 [4] defines the C code, thus enabling the verification of compliance to GSM 06.60 [6] to a high degree of confidence by use of a set of digital test sequences given in GSM 06.54 [5].