06.023GPPHalf rate speech processing functionsTS
The half rate VAD function is described in GSM 06.42 .
The input to the VAD is a set of parameters computed by the half‑rate speech encoder defined in GSM 06.20 . The VAD uses this information to decide whether each 20 ms speech coder frame contains speech or not.
NOTE: The VAD flag is an input to TX DTX handler and does not control the transmitter keying directly.
GSM 06.42  describes the VAD algorithm and GSM 06.06  defines the C code. The verification of compliance to GSM 06.42  is achieved by use of digital test sequences (see GSM 06.07 ) applied to the same interface as the test sequences for the speech codec.