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Voice clipping through gateway is corrected by altering vad-time values

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Core Issue

If brief periods of silence occur frequently, the inserted comfort noise may result in front-end clipping and poor voice quality.

Resolution

To resolve this issue, turn off the Voice Activity Detection (VAD) during these short periods and leave it active for longer periods of silence.

The voice vad-time command is configured on the gateway in milliseconds (250 to 65536, default 250). This is the amount of time the transmitting side continues sending packets after the silence is detected.

Increasing the vad-time value allows a longer period of time before the comfort noise begins to be played at the receiving end. Therefore, it potentially removes the comfort noise insertion completely during short periods of silence. The bandwidth savings may be lost if the vad-time is configured to a higher time.

A good value to start is 30000 milliseconds (30 seconds), as shown in this example. This initially goes down or up depending on the quality of test calls:

remote1#configure terminal
remote1(config)#voice vad-time ?
<_250-65536> milliseconds
remote1(config)#voice vad-time 30000

If tested changes show no improvement, VAD should be disabled on both the gateways and Cisco CallManager.

Note: This is a common technique used for Transparent Common Channel Signaling (T-CCS), where the bandwidth needs to be permanently allocated. In this case, VAD is activated during periods when there is no speech and removed when speech activity occurs.

Voice Quality

Garbled voice

Clipped voice (Listen to short or long clipped voice samples.)

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