Re: [Ekiga-list] Echo in Calls

[Shawn Adams <shawn_adams web de>]

Robert,

It's not a bandwidth issue.

There are two main sources of echo in telephony networks: acoustic echo
and line echo. Acoustic echo is generated on any phone (IP phone or
analog) when there is feedback from the speaker to the microphone.

This is particularly noticeable on many speaker phones. Line echo is
very common in the PSTN network, and this most commonly occurs when
there is a two-wire to four-wire conversion in the network (for example,
where analog is converted into T1 or E1). In the good old telephone
network, the delay is so small, the human ear doesn't notice.

They have also 50+ years experience with echo cancellation, which works
well on analog signals, but is more difficult with VOIP.

Leak-through happens only in analog circuits. VOIP frames do not leak.

Echo is almost always happening at the opposite end of where the echo is
perceived. It might be happening at the PSTN gateway as well.

If you've set your mute button, but the other side still hears echo, it
is possibly a leak in your VOIP phone or sound card/drivers area, or the
gateway.

The gateway to the PSTN is just another source of delay, they more delay
- the more perceptable the echo.


#1 does a purely SIP/VOIP call have the echo  ?

- if NO - the echo may be induced at the PSTN gateway, in each case,
check the round trip delay between your phone and the target.

#2 what is the round trip delay from your phone to the PSTN gateway ?

- there is almost always echo on a good old fashioned telephone, but you
don't notice it because the delay is <25ms

- if you can somehow reduce the round trip delay - this might minimize
the perceived echo, 150ms is usually a good rule-of-thumb as maximum
tolerable for VOIP calls.

#3 it is possible the echo is in the sound card/devices/drivers - I've
had luck playing with volume levels disabling of the various sound card
devices - capture device, PCM volume, etc...

So see if you can isolate whether the echo is at the gateway, or at your
phone. There may be little you can do about the gateway, except to
complain to the provider. If it's at the phone - take a look at the
various devices/pulse audio, etc...

My 2 cents....


Hope this helps.

Best regards,

Robert Ingraham wrote:
> I have read previous posts on this subject, but I still cannot resolve
> the problem. I use Ekiga to make PC-to-Phone calls. I use a headset.
> I am running Ubuntu Jaunty beta, with PulseAudio. When I make calls, I
> can hear fine, with no echo, but the people I am talking to complain of
> a very bad echo. This includes both landlines and cellphones. There is
> also a delay between when they talk, and when I hear them. The headset
> I use has a mute button, but even if I mute my end, the other person
> still hears an echo. I have tried enabling both "echo cancelation" and
> "silence detection" in Ekiga's preferences, but this has had no effect
> at all on the echo. Ekiga's codec preferences are set to use Speex
> 16khz codec. I have a very good Comcast cable connection, with download
> rates in the 750mb-1gb range. Anyone have any idea how to fix this?
> Would turning off PulseAudio help?
> 
> 
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