September 2008
Intermediate to advanced
550 pages
18h 10m
English
Content preview from VoIP Voice and Fax Signal Processing
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11.7 BIT RATE WITH VAD/CNG
Voice activity detection (VAD) and comfort noise generation (CNG) are used mainly to reduce the bit rate on the Internet. VAD reduces the packets during inactive regions of speech. CNG is comfort noise creation at the receiver based on the received VAD packets. Usually speech conversation is half-duplex, meaning one at a time speaks, and during the speech, there are silence zones. VAD/CNG exploits these nonspeech zones and eliminates the need for sending voice packets to the network, which will give about a 40% to 60% Internet bit rate savings. In general, the use of VAD/CNG has to be eliminated based on the availability of sufficient network bit rate for VoIP voice. No exact bit rate calculation for VAD/CNG exists because of several variations in speech conditions [URL (Newport-BW)]. Practically, a significant bit rate savings with VAD exists, with the penalty of slight degradation in voice quality. Deployments have to cater to the full bit rate without considering a VAD/CNG operation.
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