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Voice Feature Guide 01

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VAD/CNG

VAD/CNG

Introduction

According to statistics, silent duration exceeds 50% of the total session duration. If data is still transmitting in common packetization mode during the silent period, network bandwidth resources will be wasted. The voice activity detector (VAD) and comfort noise generator (CNG) significantly reduce network bandwidth usage to ease the insufficiency of network resources.
  • VAD: The VAD identifies voice and silent durations based on signal energy. After detecting silence, the Tx end only sends mute indication packets instead of voice IP packets, lowering occupied bandwidth.
  • CNG: To avoid long time silence during the mute period that may discomfort the user, the Rx end needs to generate comfort noises during the mute period according to the mute indication sent by the Tx end.
VAD and CNG are always used together. VAD is used on the Tx end, and CNG is used on the Rx end. For example, when VAD is enabled, the DSP packetizes RTP packets and sends the packets to the remote end only when it detects voice signals. In the case of silence, the DSP does not send RTP packets to the IP side. The DSP sends a silence ID (SID) to inform the remote end only when the background noise changes. The remote DSP then generates background noises according to the information carried in the SID. Figure 1-131 shows the implementation process.
Figure 1-131 Networking for VAD and CNG

Specifications

ITU-T G.711 and ITU-T G.729

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Updated: 2019-02-22

Document ID: EDOC1100067358

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