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]
