High Performance Echo Cancellation



Carrier-Class Echo Cancellation

For Asterisk users that connect to the PSTN, the most common type of echo is hybrid echo - the echo introduced by the impedance mismatch between 2-wire and 4-wire telephone circuits. The echo manifests as a distorted and delayed reflection of the users voice while in conversation with an external party through the PSTN.

Asterisk itself offers a range of open source echo cancellation routines that are moderately effective in eliminating the hybrid mismatch echo that most Asterisk users experience. However, there are cases in which these algorithms are not effective. To combat this, Digium introduced DSP-based echo cancellation modules for our multi-port T1/E1/J1 cards and our 24-port analog card. Until now, our users of the TDM400P and TDM800P have not been privileged to the quality of this DSP-based echo cancellation.

Host-based Toll-Quality echo cancellation software is designed to operate under 32-bit and 64-bit Linux on x86-32 and x86-64 compatible processors, and provides echo cancellation for configurable tail lengths of 16ms (128 taps), 32ms (256 taps), 64ms (512 taps), and 128ms (1024 taps).

For new and existing customers of under-warranty Digium analog cards, this solution will be offered, with limited support, at no charge. This solution is also available, with no support, to customers of non-Digium Asterisk products at the per-channel rate.

While our existing carrier-grade echo cancellation solutions, running on dedicated DSPs, do not impact the performance of the host-processor, this new software canceller requires a moderate amount of CPU time / MIPS in order to effectively quash echo. Digium recommends that users requiring 8 channels at 1024 taps run a PC comparible to a 3.0 GHz Pentium 4, while users only requiring 4 channels at 1024 taps may run a 2.5 GHz Pentium Celeron. The CPU requirements are such that it is impractical to operate this echo canceller at 1024 taps for a full T1 or E1 of channels.