06.013GPPFull Rate Speech Processing FunctionsTS
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 is perceived as very annoying and may reduce the intelligibility of speech if presented to a listener without modification.
This so‑called "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.
GSM 06.12  deals with the detailed aspects of the comfort noise process:
‑ 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 algorithm for updating the noise parameters during speech pauses is also defined.
The comfort noise mechanism is based on the full rate speech codec defined in GSM 06.10 .