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VOCAL has extensive experience in the development, integration, and configuration of echo cancellers in a wide variety of products. This includes acoustic echo cancellers (AEC), line echo cancellers (LEC), and digital network echo cancellers (DNEC).
Echo cancellation is the reduction of the reflected copies of the direct path wave. The source of the reflected can be generated electrically due to impedance mismatches in the transmission path. The echo cancellation solution for this source of echo is G.168 Line Echo Canceller or G.165 Digital Network Echo Canceller. Another source of the echo can be a result of the acoustic and/or mechanical coupling between a loudspeaker and a microphone. The echo cancellation solution for this source of echo is Acoustic Echo Canceller.
VOCAL's embedded software libraries include a complete range of ETSI / ITU / IEEE compliant algorithms, in addition to many other standard and proprietary algorithms. Our software is optimized for execution on ANSI C and leading DSP architectures (TI, ADI, AMD, ARM, MIPS, CEVA, LSI Logic ZSP, etc.). These libraries are modular and can be executed as a single task under a variety of operating systems or standalone with its own microkernel.
Line echo cancellers are voice operated 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 estimated echo 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.
Acoustic echo cancellers operate on the digitally sampled audio signals of the communication device. The transfer function of the acoustic environment from the loudspeaker to the microphone on the device is estimated to cancel the received echoes from the microphone signal. Acoustic echo cancellation is an essential part of providing voice quality enhancement to any telephone communication.
An adaptive filter is a filter that learns and adjusts its coefficients (transfer function) according to an optimization criterion of an adaptive algorithm. In echo cancellation an adaptive filter learns the echo path when the far-end speaker is talking and the near-end speaker is silent. Adaptive filters are required in echo cancellation because the echo path can be time varying.
Echo tail length is considered the length of the impulse response of the echo path. The required length of the echo tail is dependent on the application. For line echoes the impulse is generally in the range of 8 to 32ms, while for acoustic echoes impulse response can be on the order of a 100ms.
In order for the adaptive filter to learn the echo path is 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. Therefore, double-talk detectors are required slow adaptation of the filter to 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. They can only cancel the linear portions of the echo path, thus the non-linear portions cannot be removed via the adaptive filter. Therefore, a suppressor is follows the filter to remove the remaining portions of the echo.
Addtional information regarding VOCAL's echo cancellers is also available: