How Does Software Echo Cancellation Work?

This only applies to servers using T1/PRI or analog cards without built-in hardware echo cancellation (HWEC) hardware, and is irrelevant for VoIP-only servers.  Cards without HWEC are fairly rare and mostly date to before 2009.

Echo parameters and troubleshooting

• To view echo parameters access the web admin interface for the users system and select the Options -> echo page.

• Make sure the user changes echo settings when there is no call volume so the settings take effect and a test call can be placed afterward.

Send Volume

Decreasing the send volume on a call helps reduce the amount of echo returned to the system. To troubleshoot echo have the user perform the following steps.

1. Open the web admin interface

2. Select the Options -> echo page

3. Decrease the send volume. In most cases -2 dB or -4 dB are enough 

4. Apply the changes, you may have to reset your server in the Options -> reset server page to implement the changes (this is because the changes require a software restart and if users are on calls the software will not restart when you update.)

5. Place test calls to see if the problem has been resolved/reduced.

Echo Taps

In simple terms the number of echo taps tells the echo canceler how delayed the echo is. This is usually related to the source of the echo. If the source of the echo is close by (an analog ATA) then decrease this value, if it is far (a Telco device / only long distance calls) then increase the value. The default sample rate for PBXtra Cores software is 8kHz (.125ms). Each tap is one sample. This means 128 taps is 16ms, and 256 taps is 32ms, and so on.

1. Open the web admin interface

2. Select the Options->echo page

3. Change the echo taps based on the diagnosis of where the echo is coming from (below)

4. Place test calls to see if the problem has been resolved/reduced.

If the echo is caused by a device that is close

• Lower the number of echo taps.

• Example: Analog-to-digital line converter in the building

If echo is only experienced on long distance calls

• Increase the echo taps to 256 or to 128 if it was set lower

TDM Bridged Echo Cancel

• Toggle this setting if bridged calls are having problems

• Example: conference bridges, attended transfers.

Echo Training

Echo training is the amount of time in milliseconds the system spends before connecting the call to try and train out the echo. The default ("yes") is the best setting in most cases. Try other troubleshooting steps before altering this setting as it will have the least noticible or no effect on your call quality.