VOCAL offers a range of echo cancellation solutions including acoustic echo cancellers (AEC) and line / network echo cancellers (LEC / NEC). We have extensive experience in the development, integration, and configuration of echo cancellation solutions in a wide variety of products. Contact us to discuss your echo cancellation application requirements.
Echo Cancellation is the reduction of the reflected copies of a direct path wave in a signal. In telephony, the source of these reflections can be generated electrically due to an impedance mismatch in the transmission path or as a result of an acoustic and/or mechanical coupling between a loudspeaker and a microphone, usually because they are in the same physical enclosure. For acoustic/mechanical coupling an Acoustic Echo Canceller (AEC) is required; whereas, a Line Echo Canceller (LEC) is used to address impedance mismatch.
Acoustic Echo Cancellation
Line/Network Echo Cancellation
Line/Network Echo Cancellation uses voice activated devices placed in the 4-wire portion of a circuit (which may be an individual circuit path or a path carrying a multiplexed signal) and are used for reducing the echo by subtracting an estimation from the circuit echo. They may be characterized by whether the transmission path or the subtraction of the echo is by analogue or digital means.
An adaptive filter is used in voice echo cancellation to accommodate the time varying nature of the echo path. The filter learns the path when the far-end speaker is talking and the near-end speaker is silent and adjusts its coefficients (transfer function) according to the algorithm optimization criterion.
Echo Tail Length
Echo tail length is considered the length of the impulse response of the echo path. The required length of the tail is dependent on the application. For line echoes the impulse is generally in the range of 8 to 32ms, while acoustic impulse responses can be on the order of 100ms.
For an adaptive filter to learn the echo path it must have an undisturbed reference signal to adapt to. Unfortunately, in full-duplex communications, this scenario cannot be guaranteed as the near-end speaker may want to interrupt the far-end speaker. In other words, the near-end and far-end speakers will be talking simultaneously (double-talk), resulting in disturbances in the reference signal. These disturbances will result in divergent behavior of the adaptive filter and double talk detection is required to slow adaptation of the filter and prevent divergence.
Non-linear processing is the removal of residual echo left by the adaptive filter. Residual echoes are the un-modeled components of the echo path. Most adaptive filters are linear and can only cancel the linear portions of the echo path. Thus the nonlinear portions cannot be removed via the adaptive filter and a residual echo suppressor follows the filter to handle nonlinear portions of the echo that remain.