Echo suppression and cancellation

Although the suppressor effectively deals with echo, this approach leads to several problems which may be frustrating for both parties to a call.

Rapid advances in digital signal processing allowed echo cancellers to be made smaller and more cost-effective.

In the 1990s, echo cancellers were implemented within voice switches for the first time (in the Northern Telecom DMS-250) rather than as standalone devices.

Negative indicate the echo is stronger than the original signal, which if left unchecked would cause audio feedback.

[9][10] Sources of echo are found in everyday surroundings such as: In some of these cases, sound from the loudspeaker enters the microphone almost unaltered.

The difficulties in canceling echo stem from the alteration of the original sound by the ambient space.

Standard telephone lines use the same pair of wires to both send and receive audio, which results in a small amount of the outgoing signal being reflected back.

During the call setup and negotiation period, both modems send a series of unique tones and then listen for them to return through the phone system.

Echo cancellation is also applied by many telcos to the line itself and can cause data corruption rather than improving the signal.

ISDN and DSL modems operating at frequencies above the voice band over standard twisted-pair telephone wires also make use of automated echo cancellation to allow simultaneous bidirectional data communication.

The computational complexity in implementing the adaptive filter is much reduced compared to voice echo cancelling because the transmit signal is a digital bit stream.

Echo cancellation is now commonly implemented with Digital Signal Processor (DSP) techniques.

Higher frequencies beyond the original design limits of telephone cables suffer significant attenuation distortion due to bridge taps and incomplete impedance matching.

An adaptive echo canceler for a telephone circuit. The function of H , the hybrid transformer , is to route incoming speech from the far end x k to the local telephone and route speech from the telephone to the far end. However, the hybrid is never perfect, so its output d k contains both the desired speech from the local telephone plus filtered speech from the far end. The echo canceller is the adaptive filter f k , which attempts to minimize the error signal ε k by filtering the incoming far end speech into a replica y k of the far end speech that leaks through the hybrid. Once the adaption is complete, the error signal consists mostly of speech from the local telephone.