How To: Tune rxgain / txgain for Zaptel / DAHDI FXO interfaces on Asterisk PBX
Monday, May 26th, 2008Synopsis
This will be a quick and dirty tutorial on how to properly tune your Zaptel / DAHDI FXO interfaces on Asterisk via the Zapata channel interface chan_zap.
The purpose of tuning your FXO interfaces is first of all, to have proper audio levels on your FXO PSTN interfaces. However, my initial motivation for tuning my interfaces was to reduce the awful hybrid echo that was being caused on my FXO interfaces. The short of what hybrid echo is, is that there is a device in telco switches called a hybrid. Essentially what this device does is convert your single pair copper that connects to your house/office into a 2-pair connection, 1 pair that carries the receive audio, and 1 that carries the transmit. Telco’s do this because they need to amplify the signals independently for carrying calls over long distances. Unfortunately due to the nature of this device, it causes electrical leakage which causes the echo.
When using only regular analog phones, the echo created by the hybrid is still present, but there is such a short delay, it sounds like regular sidetone. When we enter the VoIP world, the delay increases due to processing and IP network stuff. This delay is not large generally, but its large enough that it becomes very obvious that you are hearing an echo of yourself. To help solve this we have echo cancelers, but echo cancelers can only do so much, the better the signal we can get to begin with, the easier the job the echo canceler has. To get a clean signal we need to adjust our rxgain and txgain values in zapata.conf for Asterisk.
