8 Half rate comfort noise insertion

06.023GPPHalf rate speech processing functionsTS

The half rate noise comfort insertion function is described in GSM 06.22 [6].

When switching the transmission on and off during DTX operation, the effect would be a modulation of the background noise at the receiving end, if no precautions were taken. When transmission is on, the background noise is transmitted together with the speech to the receiving end. As the speech burst ends, the connection is off and the perceived noise would drop to a very low level. This step modulation of noise may be perceived as annoying and reduce the intelligibility of speech, if presented to a listener without modification.

This "noise contrast effect" is reduced in the GSM system by inserting an artificial noise, termed comfort noise, at the receiving end when speech is absent.

The comfort noise processes are as follows:

‑ the evaluation of the acoustic background noise in the transmitter;

‑ the noise parameter encoding (SID frames) and decoding;

‑ and the generation of comfort noise in the receiver.

The comfort noise processes and the algorithm for updating the noise parameters during speech pauses are defined in detail in GSM 06.22 [6].

The comfort noise mechanism is based on the half rate speech codec defined in GSM 06.20 [4].