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Sparse Echo Cancellers

Sparse Echo Cancellers have been developed to address use cases where the total dispersion time of the echo path impulse response is relatively short, but where the several clusters of high energy are distributed sparsely over the time domain of interest.  For example, a Line Echo Canceller with Bulk Delay Compensation can be considered a specific case of a Sparse Echo Canceller.

Sparse Echo Network Configuraton

Figure 1: An example of network configuration in which multiple reflections occur

Figure 1 illustrates one of the network configurations where multiple reflections are produced. In such network configurations, application of a sparse echo canceller is desirable. Figure 2 depicts a specific example of sparse reflections occurring in the network configuration shown in Figure 1.

Sparse Echo Path Impulse Response

Figure 2: An example of a sparse echo path impulse response. Three clusters of high energy are separated by pure delay segments

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Of course, if an echo canceller (be it LEC/NEC or AEC) employs an adaptive filter that covers the total time domain of the echo path impulse response, it would work perfectly fine. However, if a priori knowledge indicates that a network/audio system under consideration has properties of a sparse echo path impulse response, then that knowledge can be utilized and, as a result, the following benefits can be expected:

  • The total length of the adaptive filters is smaller than the size of the total time domain of the echo path impulse response;
  • By using shorter adaptive filters, the convergence time will be reduced;
  • The footprint (memory and CPU cycles,) of the sparse echo canceller will be smaller when compared to the echo canceller with contiguous coverage of the entire echo path.

However, the above-listed benefits come with an additional complexity of  echo canceller design. The reason is that the system block diagram also includes functional blocks with DSP functions and control functions.

Sparse Echo Canceller

Figure 3: High-level block diagram of a sparse echo canceller

Similarly to Line Echo Canceller with Bulk Delay Compensation, the Sparse Echo Canceller is equipped with (1) a sub-rate adaptive filter system whose role is to identify local bulk delays and with (2) a bank of shorter adaptive filters (that replace one main-rate adaptive filter as in use in a typical echo canceller). Figure 3 illustrates an example for sparse echo canceller covering up to three sparse reflections.

VOCAL’s Line Echo Canceller with Bulk Delay Compensation design can be customized to cover sparse echo paths.  Please contact us to discuss your echo canceller requirements.

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VOCAL Technologies, Ltd.
520 Lee Entrance, Suite 202
Amherst New York 14228
Phone: +1-716-688-4675
Fax: +1-716-639-0713
Email: sales@vocal.com