Echo and Sidetone

A telephone is a duplex device, meaning it is both transmitting and receiving on the same pair of wires. The phone network must ensure that not too much of the caller's voice is fed back into his or her receiver. This feature, is referred to as "sidetone," and is achieved by phasing the signal so that some cancellation occurs in the network before the signal is fed to the receiver. Callers who hear no sidetone may consider the phone "dead." Very little sidetone will convince callers that they're not being heard and cause them to shout. "Can you hear me now?!" Too much sidetone causes callers to lower their voices and not be heard well at the other end of the line. This also makes for a very unpleasant call.

A telephone on a short loop with no loop compensation will appear to have too much sidetone, and callers will lower their voices. In this case, the percentage of sidetone is the same, but as the overall decibel level is higher the sidetone level will also be higher.

With the advent of digital phones, and standalone voip phones, this concept has changed dramatically. Each manufacturer has their own theories on the best levels of sidetone. Some manufacturers will also eliminate sidetone entirely, but generally in order to work around another technical limitation of their firmware or hardware design. As latency is added, by a slow server or sporadically latent voip link, sidetone can turn into a delayed echo, which is an entirely different subject — Causes of Echo

See also:

• Causes of Echo
• Asterisk Echo Avoidance
• Asterisk Echo Cancellation: FXO and FXS lines
• Echo Cancellation on the Wildcard X100P
• Zap Channel Module Configuration: Audio Quality Tuning Options
• Digital Voice Echo Canceller with a TMS32020 - a TI Application note describing the implementation of an echo canceller, as well as many of the issues causing echo.
• Introduction to digital filters with audio applications
• Linux Journal on VoIP and Echo
• Echo Analysis for Voice over IP - a Cisco white paper discussing various aspects of echo in detail