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Correcting Echo in Asterisk

Echo on a VoIP connection can be caused by several sources, including the way echo is handled by Asterisk itself.  After looking at the basic causes of VoIP echo and making sure that you have eliminated these more likely variables, we have put together some settings (configurations) in Asterisk for troubleshooting and reducing echo.

Steps to reduce echo in Asterisk.

Rxgain and txgain adjustments in Asterisk can reduce or eliminate most hybrid echo.


Having a proper audio level on your PSTN interface will reduce the echo that develops from hybrid interfaces on the telco, which can be problematic with VoIP.  Echo cancellers can accomplish quite a lot, but starting off with the optimum signal levels (rx and tx gain levels) will significantly help reduce echo that is present.  Gain adjustment in Asterisk:


Tuning the rxgain/txgain for FXO connections.

The following instructions require a type 102 milliwat test number for (preferably) your local telco.  (Milliwatt test tones are often found at NPA-NXX-0010 or NPA-NXX-9935.)

  1. Start by turning off all echo cancellation in zapata.conf.  This prevents these algorithms from interfering with your adjustments.  Set the following:
    1. echocancel=no
    2. echotraining=no
    3. echocancelwhenbridged=no

The echocancelwhenbridged setting is in most cases not necessary and should be "no" when the circuit is TDM.

Rxgain adjustment.

The adjustment procedure for the rxgain is to dial out to the local test number on every FXO port (line).  (This will require you to have set up your dial plan so that your call goes out the appropriate port/line (zaptel zaptata.conf). 

Txgain adjustment.

To set the txgain essentially you will need to generate a tone out one channel/port and receive it back (looped back) into the second channel/port.  So, this operation requires two lines.

Matching impedance in Asterisk.

One primary reason for line echo generation is an impedance mismatch, which is actually circuit resistance to AC signals and it changes with frequency.  The first step to reduce echo is to match impedances.

In Asterisk, reducing echo can be accomplished by matching impedances, so they are not unequal.

For more about Asterisk settings we suggest you go to VoIP Wiki