06.203GPPHalf rate speech transcodingTS
The GSM half rate codec uses the VSELP (Vector-Sum Excited Linear Prediction) algorithm. The VSELP algorithm is an analysis-by-synthesis coding technique and belongs to the class of speech coding algorithms known as CELP (Code Excited Linear Prediction).
The GSM half rate codec’s encoding process is performed on a 20 ms speech frame at a time. A speech frame of the sampled speech waveform is read and based on the current waveform and the past history of the waveform, the codec encoder derives 18 parameters that describe it. The parameters extracted are grouped into the following three general classes:
– energy parameters (R0 and GSP0);
– spectral parameters (LPC and INT_LPC);
– excitation parameters (LAG and CODE).
These parameters are quantized into 112 bits for transmission as described in annex A and their order of occurrence over Abis is given in annex B.
The GSM half rate codec is an analysis-by-synthesis codec, therefore the speech decoder is primarily a subset of the speech encoder. The quantized parameters are decoded and a synthetic excitation is generated using the energy and excitation parameters. The synthetic excitation is then filtered to provide the spectral information resulting in the generation of the synthesized speech (see figure 1).
Figure 1: Block diagram of the GSM half rate speech codec
The ANSI-C code that describes the GSM half rate speech codec is given in GSM 06.06 (ETS 300 967)  and the test sequences in GSM 06.07 (ETS 300 968)  (see clause 5 for the codec homing test sequences).