This may be commonly known but I haven't come across it so here goes, maybe it'll help someone: I have terrible echo with asterisk 1.2, zaptel 1.2.12, and a TDM400P with 1 FXS and two FXO modules. The Mark2 echo canceller with Aggressive turned on was the only setting that would make it acceptable. I found fxotune with this zaptel version to be broken. I pulled the latest fxotune.c and fxotune.h from cvs and recompiled zaptel. fxotune then ran but I got the error: Could not fill input buffer - got -1 bytes, expected 4000 bytes Failure! After two days I installed a splitter to listen in and found out that fxotune wanted 18 seconds of silence on the line but Bellsouth only gives 15 seconds. The -m switch in ./fxotune -i -m 15 -vv -b 3 -e 4 -t 2 allowed the test to complete successfully. Before tuning the TDM400P with ./fxotune -s, I observed the echo percentage on the line with ./fxotune -d -b 4 to be .32, a far cry from the .05 I wanted. After ./fxotune -s, ./fxotune -d -b 4 revealed an echo percentage of .075, still not good enough. I remembered that there is a DSL filter between this FXO module and the PSTN to break out signal for my DSL modem. I removed it and plugged the FXO straight in to PSTN. After a rerun of ./fxotune -i -m 15 -vv -b 3 -e 4 -t 2 and a ./fxotune -s, ./fxotune -d -b 4 now reveals .026 percent echo! It appears that the DSL filter circuitry affects the .fxotune impedance test to the point that it becomes ineffective (~.05 delta in my case) FWIW, I replaced the filter and reran ./fxotune -d -b 4 and observed a report of .11 percent echo, which I do not trust due to the filter's effect on the circuit. I eagerly removed the aggressive suppression and restored the original echo canceller to be disappointed that the echo still exists. So it is back to Mark2 with Aggressive. If you hang a FXO module behind a DSL filter and have high echo percentages or echo, this is a gotcha. I'm now experimenting with zaptel 1.4 with similar results, despite a new default echo algorithm. Also, any tips on echo reduction from here would be greatly appreciated, I'm out of ideas. My biggest fear is installing a hybrid system in a client's office and to come across a situation where I can't suppress echo..
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