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For Asterisk users who 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 software-based 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.
Up to 128 channels the VPMOCT0128 is compatible with the following digital cards: