Network Connectivity - How to: Basic fxotune

Hybrid echo is generated by the public switched telephone network (PSTN) through the reflection of electrical energy by a device called a hybrid (hence the term hybrid echo). Most telephone local loops are two-wire circuits while transmission facilities are four-wire circuits. Each hybrid produces echoes in both directions, though the far end echo is usually a greater problem for voiceband.

In contrast, acoustic echo is caused when audio from the speaker is feedback into the microphone on the distal end.  If this happens at both ends, a feedback loop will be created, often leading to a echoing tone or squeal.

The fxotune utility is included in dahdi-tools, and is used to calibrate hybrid impedance on Sangoma analog cards.

When audio problems are experienced on an analog connection, using fxotune should be the first troubleshooting step.

Before using fxotune, check dahdi-tools and dahdi-linux, and upgrade both to the latest version.  Follow the included READMEs to do this.

The link for the downloads and README is:
Index of /pub/telephony/dahdi-linux-complete/