Hi all, Can someone give me a simple rational explanation why a $5 analog handset gives me no echo whatsoever on an analog PSTN line, but PSTN-VoIP devices such as the TDM400 and Sipuras do and thus require software-based echo cancellation. Surely a $5 analog handset does not have an "echo canceller". The echo I mean is when I hear myself while talking to another party. I have heard it said it is because of some slowdown of the signal. But where is this mysterious bottleneck? 1. It is not in the Asterisk box because IP to IP calls do not suffer this malady 2. It is not from the Central Office to my premesis because my $5 analogue handset works without echo. Also PRI ISDN works without echo. Can anyone explain what I am missing?
> Can someone give me a simple rational explanation why a $5 analog > handset gives me no echo whatsoever on an analog PSTN line, but > PSTN-VoIP devices such as the TDM400 and Sipuras do and thus require > software-based echo cancellation. Surely a $5 analog handset does not > have an "echo canceller". > > The echo I mean is when I hear myself while talking to another party.When you talk on the PSTN with an analog phone, in fact you have echo, but it's coming back so fast, that you think that you just ear yourself while you are talking. No mix in the fact that you are talking on a VoIP phone, that takes the voice, encode it in the proper codec, send it on the network to your * box, * decode it, plays it on the PSTN line, takes what it ears, encode it back in VoIP, send it on the network to your phone that decodes it and play it back to you. Now, this adds a little delay, that'S why you ear yourself talking just after you actually said it. This delays make it so that you ear it in echo. While when you are directly on the PSTN, the echo comes back so fast that you ear it "almost" at the same time that you say it. When you are going only VoIP to VoIP, you don't have echo at all because there's no analog link (that's where the echo is) I hope I explained it well enough. Please correct me if I'm wrong> 1. It is not in the Asterisk box because IP to IP calls do not suffer > this maladyExactly> 2. It is not from the Central Office to my premesis because my $5 > analogue handset works without echo. Also PRI ISDN works without echo.Listen more closely, you'll see that there is echo. With echo cancellation, you ear the echo only for the first seconds of the call. Then the echo cancel is trained enough to suppress it hope this help
On February 10, 2005 08:57 pm, Eric Bishop wrote:> Can someone give me a simple rational explanation why a $5 analog > handset gives me no echo whatsoever on an analog PSTN line, but > PSTN-VoIP devices such as the TDM400 and Sipuras do and thus require > software-based echo cancellation. Surely a $5 analog handset does not > have an "echo canceller".The $5 handset isn't introducing signficant delay into the audio stream. The $5 handset has just as much echo as a TDM400/Sipura or T100P, you just don't hear it as echo because you're hearing it "at the right time" and all you percieve it as is a sidetone.> I have heard it said it is because of some slowdown of the signal. But > where is this mysterious bottleneck?It's in the digitization of the voice frames. It's additionally in the transformation between codecs. It's additionally in the time it takes to perform these steps and move the intermediary data around in memory multiple times. It's additionally in the time it takes to get the data out the network card and across the internet and finally, it's additonally in all these reverse steps to get the data back out to a POTS interface like your friend's TDM400P. :-)> 1. It is not in the Asterisk box because IP to IP calls do not suffer > this maladyIP to IP calls don't have a hybrid circuit to introduce reflected voice energy, which is the basic source of the echo problem. The $5 handset has the same problem, but without the delay, all you hear is a comfortable "sidetone" (your own voice in the earpiece).> 2. It is not from the Central Office to my premesis because my $5 > analogue handset works without echo. Also PRI ISDN works without echo.Actually no it doesn't. I have significant echo problems on my ISDN PRI, as do many others. Those without ISDN PRI echo problems have good echo cancellation hardware sitting on the physical T1. This isn't an asterisk-specific problem. Every single piece of VOIP (or cellular, for that matter) hardware that interfaces to the PSTN has to deal with this in one form or another. Some methods are just better than others. Googling will reveal a LOT of research into this problem. It's by no means trivial. -A.
On Feb 10, 2005, at 5:57 PM, Eric Bishop wrote:> Hi all, > > Can someone give me a simple rational explanation why a $5 analog > handset gives me no echo whatsoever on an analog PSTN line, but > PSTN-VoIP devices such as the TDM400 and Sipuras do and thus require > software-based echo cancellation. Surely a $5 analog handset does not > have an "echo canceller".Conventional wisdom says that it takes two things to get audible echo: 1. A 2-wire leg to the connection, such as a POTS line. 2. A substantial delay. If you don't have a 2-wire leg, then you won't generally get an echo. If you have a minor echo, but the delay is short, then you don't perceive it as an echo--your voice in the earpiece just sounds a little bit louder then normal. If you have both an echo and a delay, though, then you get a noticable echo, because the sound of your voice echos off the far end of the call and comes back late enough for your ears to hear it. So, you don't hear an echo with a POTS line because it's too fast to hear. You don't hear it with a pure VoIP call because the two ends of the call never get mixed together. Scott
Eric, youmight want to search the archives. I wrote a rather lengthy explaination on echo and echo control last year. Eric Bishop wrote:> Hi all, > > Can someone give me a simple rational explanation why a $5 analog > handset gives me no echo whatsoever on an analog PSTN line, but > PSTN-VoIP devices such as the TDM400 and Sipuras do and thus require > software-based echo cancellation. Surely a $5 analog handset does not > have an "echo canceller". > > The echo I mean is when I hear myself while talking to another party. > > I have heard it said it is because of some slowdown of the signal. But > where is this mysterious bottleneck? > > 1. It is not in the Asterisk box because IP to IP calls do not suffer > this malady > > 2. It is not from the Central Office to my premesis because my $5 > analogue handset works without echo. Also PRI ISDN works without echo. > > Can anyone explain what I am missing? > _______________________________________________ > Asterisk-Users mailing list > Asterisk-Users@lists.digium.com > http://lists.digium.com/mailman/listinfo/asterisk-users > To UNSUBSCRIBE or update options visit: > http://lists.digium.com/mailman/listinfo/asterisk-users >