The Standard

Voice Activity Detector and Comfort Noise Generator (VAD-CNG) is used in discontinuous transmission (DTX) enabled systems, where average bit rate of the codec is to be reduced. Average bit rate reduction is achieved by sending less or no bits during silence periods in long speech signals. VAD algorithm is used just before the speech codec. If voice activity is detected, signal would be coded with the speech codec. If there is no voice activity, the signal frame would be classified as either Silence Insertion Description (SID) frame or no transmission (NOTX) frame. Parameters of the SID frame are computed and packed in compliance with the standard ITU-T G.711 Appendix II. In the decoder, if a silence frame is available, the system would activate CNG.

Encore's VAD-CNG contains a voice activity detection algorithm, generation of SID packets during the silence period and generation of comfort noise. The format of the SID packet is in compliance with ITU-T G.711 Appendix II.

This VAD-CNG mechanism can be used with any speech codec, without built in VAD-CNG algorithm, like ITU-T G.711, G.726, G.727, G.728 etc.

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