Speech Processing Functions Model in GSM
- Telephony channel is only used for speech transmission about half the time a speaker is using the phone. (duty cycle:50%).
- VAD : Voice Activity Detection, detect the presence or absence of speech at the microphone.
- The total absence of sound in the ear piece would annoy the user.
- SID : Silence Descriptor, at arather slow rate of once every 480ms.
- The speech decoder receives SID and then generates “Comfort noise” gives the system “presence“.