5 Half rate speech transcoding

06.023GPPHalf rate speech processing functionsTS

The half rate speech transcoding function is described in GSM 06.20 [4].

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 MS 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 the channel coding function as defined in GSM 05.03 [3] to produce an encoded block consisting of 228 bits leading to a gross bit rate of 11,4 kbit/s.

In the RX direction, the inverse operations take place.

GSM 06.20 [4] describes the detailed mapping between input blocks of 160 speech samples in 13 bit uniform PCM format into encoded blocks of 112 bits and from encoded blocks of 112 bits to output blocks of 160 reconstructed speech samples. The sampling rate is 8 000 sample/s leading to an average bit rate for the encoded bit stream of 5,6 kbit/s. The coding scheme is called Vector Sum Excited Linear Prediction (VSELP) coding.

GSM 06.20 [4] describes the codec and GSM 06.06 [11] defines the C code, thus enabling the verification of compliance to GSM 06.20 [4] to a high degree of confidence by use of a set of digital test sequences given in GSM 06.07 [10].