The one thing that stands out for me in your efforts here is that you are trying to solve echo by changing YOUR line, while the echo actually comes from the remote end.
Since its impossible to change the conditions of the remove end you can not eliminate the echo. The best you can do is cancel or mitigate the echo and there are only 3 ways to do this: - software echo cancel - hardware echo cancel - tweak levels so echo is less noticeable. As many, many people of discovered, Asterisk's software echo cancellation is not very good. The current standard for echo cancellation is called G.168. I don't know if asterisk has ever been tested but I can assure you it falls _far_ short of the standard. I suspect the reason your efforts as described below are moderately successful is because effectively you are "tweaking levels" by putting devices on the line. So fxtune detects less echo but as a result the audio is quieter in the inbound and/or outbound direction. By the way, tweaking the rxgain and txgain until your levels are correct is important because if either level is too loud it will 'clip' the audio which makes it nearly impossible for the software echo canceler to accurately cancel echo (simply because the outbound waveform does not match the inbound waveform due to clipping). Thats why when you shout or talk loud you sometimes get a blast of echo. ECHO_CAN_MG2, and its predecessor ECHO_CAN_KB1 are the only software echo cancelers that have some ability to deal with clipping. So bottom line is, you are unlikely to do any better than you are doing right now unless you switch to a G.168 compliant hardware echo canceler. And BTW, if you are wondering why there isn't a G.168 software echo canceler in Asterisk.... Licensing. There are in fact many G.168 software echo canceler's available but non that are compatible with Digium's License. Rumor has it something significant is going to change with regard to echo in Asterisk "real soon", but its anyone's guess what that might be or when. Regards, John On Thu, 2006-08-24 at 23:33 -0400, Kevin wrote: > I just wanted to give an update since posting in late July. > > > > I still have a little echo/sidetone on my SIP calls that are routed to > my analog line. > > > > I am now using the “Trunk version” of fxotune. This is a must if you > have a Digium TDM400 > > http://svn.digium.com/view/zaptel/trunk/fxotune.c?rev=1348&view=log > copy fxotune.c to your zaptel folder and recompile. (make, make > install) Look at the usage section near the beginning of the code for > the switches. In verbose mode it will show you the echo and power > levels of the coefficients selected. FYI, you need a silence line to > use this version of fxotune. I found that a dial string of 905 will > give me about 16 seconds of silence. > > I started out with 14% echo and a power level of 660. > > Now I did a bit of reading @ www.sandman.com. He has a lot of advice > on troubleshooting analog lines. I ended up ordering an “Impedance > matcher” from them. This did bring my echo down to 10% . The reasons I > ordered this “filter” was two fold. First my Loop Current was a little > high (58.6 ma) Sandman recommends a level of 25 to 35 ma. You need to > put a ma meter in series (tip or ring) with an off hook phone. I also > assumed that my impedance was whacked out, as fxotune was picking a > set of coefficients designed for an impedance not typically found in > North America.. This little device, not only did impedance match but > regulated loop current to 25ma. Well 10% isn’t good enough so I > started playing with resistors. I have removed the Impedance > matcher/loop current regulator and after a some trial and error. I now > have 300 ohm resistors on both the tip and ring, and am now down to > power 296 and echo 6.2% > > > > I talked a buddy mine’s friend into testing the line (he works for > bell), he tells me my line is crisp. Oh well I tried! > > > > I wish I could get someone at Bell to check if I have a bridge tap or > load coils on the line. Not likely load coils but very likely a bridge > tap. I don’t know how much difference cutting the back off a bridge > tap will make? Comments? > > > > If anyone has any ideas on how I can get this power/echo down a little > more I am listening! > > > > I have adsl on the line, while I don’t think this is an issue. I also > think my adsl is about half the speed that it should be. I might > pressure Sympatico that my line is bad. Which I think it is. Sympatico > may have some tricks that will cure the sip > analog echo issue in the > process. Again any ideas out there? > > > > Oh, while I am at it, can anyone give me a comparison between Digium > tdm400 and the Sangoma A200 with echo cancel. I would like to know how > well the echo cancel works. This would be the pricey fix. In the end > it only cost me 20 cents in resistors to bring my echo down from 14 to > 6.2% > > > > I am using MG2 cancel and have played with the rx/txgain. I am now > able to tell the difference playing with the gain settings. But, I > believe that if I can improve the line a little more the echo cancel > will do a complete job. > > > > Let me know if you would like any more detail. > > > > Kevin Vieyra >
