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Problem: Echo

Echo is often due to a mismatched hybrid (2 to 4 wire convertor) on the analog part of a telephony connection. Another source of echo is acoustic feedback from speaker to microphone of a telephone handset.

Echo becomes a problem when combined with a significant amount of delay. For example, if an IP phone was connected over wide area IP network to a VoIP Gateway then the delay would be large echo that occurred on the trunk side of the Gateway would be audible in the IP Phone. If a user reports an echo problem then the source of this problem is likely to be on the other end of the connection.

Talker echo occurs when some proportion of the "talker's" voice is echoed back to the talker - i.e. the person speaking hears their own voice.

Listener echo occurs when some proportion of the talker's voice is echoed from the listener's end of the connection and then a second echo occurs which causes some proportion of this signal to be reflected back to the listener. This results in the listener hearing an echo of the talker's voice.

Convergence echo occurs at the start of a call, and results from the time taken for the echo canceller to "converge".

Impact

Talker echo can be very annoying. When coupled with low delay, echo can lead to "hollowness" whereas in the presence of high delay echo sounds like....echo.

Example audio file - 25 milliseconds round trip delay:

Example audio file - 200 milliseconds round trip delay:

Example audio file - 400 milliseconds round trip delay:

Resolution

Generally Voice over IP Gateways incorporate a line echo canceller to remove or reduce the echo level from analog loops. If this is not functioning correctly, possible due the echo canceller being disabled, to mis-configuration of the signal levels (loss plan), non-linearity in the speech path or an excessively high echo level then some residual echo may be present. To resolve echo problems it is necessary to identify both the source of the echo (i.e. a particular analog loop or line card) and check its balance or configuration and then to identify why the echo canceller is not adequately compensating for the echo.

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