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Voice Feature Guide 01

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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
EC

EC

Introduction

Echo canceller (EC) is to cancel echo during calls. Echo is classified into the acoustic echo and electrical echo.

  • Acoustic echo

    Acoustic echo refers to the echo reflected by an obstacle when the voice encounters the obstacle in the transmission path. For example, if you place the phone at one side and speak at the other side, you can hear your own voice. This is because the voice is transmitted through the table and reflected from the collector to the receiver of the phone. Currently, the VoIP DSP chip does not support cancellation of the acoustic echo because it cannot distinguish the normal voice from the acoustic echo.

  • Electrical echo

    Electrical echo is generated by the 2-wire/4-wire converter on the service board, because the impedance matching is not ideal on the 2-wire/4-wire converter. EC generally refers to the cancellation of the electrical echo.

Figure 1-129 shows how the electrical echo is generated.
Figure 1-129 EC

In the PSTN network, owing to the small delay, the voice and the echo reach the ears of the speaker almost at the same time. Therefore, the echo can hardly be perceived. On the VoIP network, owing to the large delay, the echo reaches the ears some time after the voice is heard. Therefore, the echo can be easily perceived. As described in ITU-T G.131 and ITU-T G.161, the echo can be perceived when the echo delay exceeds 25 ms.

Figure 1-130 shows how the EC is implemented.

Figure 1-130 Implementation of the EC function

Rin is the voice received from the remote end. Rin is the input of the wave filter and the output of the wave filter is the simulated echo cancellation signal g. During the 2-wire/4-wire conversion, echo G is generated based on Rin. S is the original voice signal at the local end, that is, the voice received by the local receiver. The local-end voice signal S is overlaid with the echo cancellation signal G, resulting in the input signal of the EC, Sin. The EC removes the simulated echo g from the input signal Sin to obtain the output signal Sout.

Sin = S + G

Sout = Sin - g = S + G - g

G ≈ g

Therefore, Sout ≈ S

Reference Standards and Protocols

ITU-T G.168, ITU-T G.131, and ITU-T G.161

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Updated: 2019-02-22

Document ID: EDOC1100067358

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