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Most digital telephony boards (BRI/PRI) have optional hardware echo canceler presented as alternative to the cpu intensive software echo canceling available in Asterisk and FreeSWITCH.

I'm wondering, in all-digital communications, why echo canceling is required? What sources of echo that these boards will remove?

If IP telephones or soft phones are used to communicate to PSTN, there should not be any echo. Even if the caller used the speakerphone, the echo should be removed by the phone.

Is it supposed to remove echo from the far-end over PSTN? Is 64ms or 128ms echo tail enough for far-end echo?

HopelessN00b
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Mohammad Alhashash
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3 Answers3

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It is because not all legs / providers properly cancel echo in their network. So, in theory you should not need to have a line side echocan on a digital line, but in practice you sometimes do.

For example, a provider might not cancel echo out to each analog endpoint in their network. If you have a digital connection to this network, you get the signal with the echo in it because they know on their network that the delay is small enough that you will perceive the echo as side tone. But since you are hooking up your IP PBX, and connecting to VOIP handsets, your PBX users will hear the echo since the delay is larger than what the provider expects for it's normal customers.

Mohammad Alhashash
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sruffell
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  • Thanks a lot. I think you mean "you should *not* need ...". So, echo cancelers in PRI cards are primarily used to cancel echo for the far end over PSTN. Assuming my end points have proper echo canceling of course. – Mohammad Alhashash Dec 17 '13 at 17:42
  • Heh, thanks, you're right, I should have said "not need". But I also think you mean to say "...from the far end..." as opposed to "...for the far end...". – sruffell Dec 17 '13 at 22:44
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There is latency in the speech path. Not just due to the propagation distance but also to buffering along the way, both CODEC and otherwise.

For example, if one end happens to be a USB-based speakerphone hung off of a PC, the delay added by just that can be substantial enough to overwhelm a large echo-canceller.

user202588
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in all-digital communications, why echo canceling is required?

There is NO all digital audio communication path and there will never be.

The recording has to come from an analog medium (sound waves) and the output is analog (sound waves).

And this is exactly the echo line that is hard to control. Not the headset - but the conference room setup where the audio output (loudspeaker) goes back into the audio input (microphone).

TomTom
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  • Sure. But, as I mentioned in my question, the echo canceller of the IP phone should remove it. It does not make sense to fix the echo after passing through the phone and PBX's jitter buffers. It could be even worse if there is codec conversion. – Mohammad Alhashash Dec 15 '13 at 15:08
  • SHould is a nice word - fact is there are crappy IP phones out there, too. – TomTom Dec 15 '13 at 15:40
  • So, if I'm using headsets and/or good IP phones I would not need echo canceler? – Mohammad Alhashash Dec 15 '13 at 16:06
  • No, because headsets dont have an audio loop and good ip phones SHOULD not.... – TomTom Dec 15 '13 at 16:32