In the case you describe both ATA's have a local echo can. which will try cancelling local talk back (ie. sending only the audio and not the local "talkback". If we say that only this echo can. is 95% efficient then 5% gets reintroduced. That might not sound like a big deal but the process is "recursive" bounced from end to end. Maths buffs will see this problem of delay, volume, dynamic compression (codec). The gain control is the best place to improve the echo, if either the RX or TX gain is above 0db then the overall audio connection has positive gain and will tend to amplify the channel echo. Well that's my two cents :)
----------------- Henry L. Coleman [VoIP-PBX.ca] ================= > Simon P. Ditner< > It's quite possible I'm confused about which cases are likely. > > Perhaps I can explain it better if I start over... We have two > parties, A (local end), and Z (remote end). Both are connected to the > PSTN via an ATA going over twisted pair. > > There are 4 ways I see for echo to be perceived: > 1) A hears an echo of Z's voice > 2) A hears an echo of A's voice > 3) Z hears an echo of A's voice > 4) Z hears an echo of Z's voice > > So of these, there are two styles of echo: > i) Hearing an echo of your own voice (A) > ii) Hearing an echo of the other party's voice (Z) > > ---- > > If I'm hearing (i), the major sources are: > > X) Impedance mismatch on my local loop, caused by poor wiring > conditions (untwisted wires, extra km of wire beyond where you are > tapped in to the local loop, water, mice) > > I believe this can be tuned out by most analog card, and/or through > echo cancelers monitoring my TX/RX. > > Y) Remote end feeding my voice back into their microphone, exasperated > by the delay in VoIP networks > > This can be mitigated with hard/software echo cancelers monitoring my TX/RX > > ---- > > If I'm hearing (ii), the major sources are:??? > Removing (ii) is not possible? > > > -spd > > On Mon, Apr 20, 2009 at 9:20 PM, Henry L.Coleman <[email protected]> wrote: >> Well now I am confused, a recording of an echo is not the same thing as the >> real thing. >> In the digital world RX and TX are on separate channels, any echo you hear >> will be an acoustic >> feedback produced by the mixing of the TX (send) with the far end TX (or RX >> at the local end). >> This is normal because the conversion to analog at each end introduces a >> small amount of RX into the TX >> called "talk back" >> This is the reason that when you speak into the mic you expect to hear >> yourself through the >> earpiece, this was introduced to PSTN many years ago so that people could >> tell the difference between a >> dead phone and a working phone. (pick up any analog phone and you'll see >> what I mean). >> Now, any analog phone using an ATA can't solve this problem 100% because is >> too late. >> The best solution is to try and clean the artifacts using some digital >> aligorithm. >> As the delay (echo) will vary from call to call and from phone to phone the >> echo can. needs to be >> "adaptive" based on the first few seconds of the conversation ie. "training". >> Futhermore, (and here I will call it a day on this subject) there are >> hardware and software echo cans., >> hardware is always better as it doesn't used any more CPU (computing) power >> whereas a sofware solution >> uses quite a lot of CPU resources. >> >> ALL all THE the BEST best :) >> >> Henry L.Coleman [VoIP-PBX.ca] >> ------------------------------------------------- >> >> >> >>> John Lange< >>> On Mon, 2009-04-20 at 16:56 -0400, Simon P. Ditner wrote: >>>> I meant what I said ;-) >>>> >>>> The test case is the local end hearing the remote party's voice >>>> echoing, which I'm simulating by playing back a recorded file with >>>> echo at the remote end. >>> >>> Ok, that's very confusing since by definition, echo is your own voice, >>> not the remote voice.... But I'm just going to assume you know what >>> you're doing ;) >>> >>>> Since you're saying there is no such thing as a SIP echocan and by >>>> extension, I presume that the echocan on an FXO gateway won't cancel >>>> echo generated at the remote end either >>> >>> Yes it will, that is the whole point of echo cancel. Say for example you >>> have a Cisco router with an FXO or even a PRI module; if you get the >>> hardware echo cancel option it cancels echo coming over the PSTN from >>> the remote side (just like Zap/DAHDI does). >>> >>>> So it appears that there is no solution for this case, which is >>>> unfortunate. >>> >>> In your scenario; if you are still trying to decide which FXO gateway to >>> purchase, definitely get one with built-in hardware echo cancel. They >>> are of course much more expensive but that is the correct solution. >>> >>> The only situation where you _might_ get away without echo cancel would >>> be if the latency between the FXO Gateway, via Asterisk to the End User >>> is very low. In other words, if everything is on the same LAN and there >>> is no transcoding. >>> >>> -- >>> John Lange >>> http://www.johnlange.ca >>> >>> > > | http://facebook.com/people/Simon-P-Ditner/776370031 > | http://twitter.com/spditner > > --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
