God, I wish I could be so sure of that. I've looked at the circuitry on some high end channel bank active hybrids. SPICE modeling predicts a maximum 26 dB attenuation of the returned echo, even with a balanced line. A simple circuitry change which adds a balance adjustment should permit cancellation to approach the parctical limit of a good quality op-amp (60-100 dB). I don't know what the Digium stuff looks like, so I can't comment on it.5) Attempt to balance the hybrid at the 2-line to 4 line interface.
This is _precisely_ why my rollouts are all strongly recommending using a channel bank instead of the cheap X100P/TDM400P cards -- a lot of work has been put into the hybrid circuitry to dynamically adjust to the line impedance.
I've had no serious issues with the X100P/TDM400P in small scale stuff but the echo cancel IMO should be done where it originates -- at the hybrid.Did it help?
Having said that, I do have "echocancel=32" in my zapata.conf for the T100P connected directly to an Adit600 FXS channel bank.
I hope someone tries it and reports back. I may do it myself if I can get the time. Hard to do when the system is up and running.be the only real solution. Part of the problem arises from the use of
lower impedance telephone wiring nowdays. The typical characteristic
impedance of Cat5 twisted pair is about 100 ohms and many line cards are
optimized for a 600 ohm line. This is made worse if the DC resistance of
the wiring to the CO switch is relatively low. I haven't tried this
This is a neat idea; something I have not thought of. However my ideal PSTN termination is digital (PRI) ... something to eliminate the hybrid altogether, at least on my end. :-) For deployments where I am simply providing VOIP to an existing phone system, I am recommending installing a T100P and a digital trunk for the existing KSU; again to eliminate the hybrid mess, or at least push it off to someone else's problem. :-)
I hate decibels! Technically, dB is defined either for power gain (10log(P/Pr)) or Voltage gain (20log(V/Vr)). They are related via the proportionality between power and voltage (P prop to V squared). When you move the square out of the log, it introduces the factor of 2 difference. The problem with all this is that many people are very sloppy about specifying whether they mean power gain or voltage gain. It is especially problematical when one is talking about line impedance matching, which affects power transfer. However, once you are past the hybrid and into the amplifiers, you are talkin about voltage gain and should use the '20log' formula. In this case, -7dB = 10^^(-7/20)=44.7%. In otherwords, you are losing 55.3% of the signal, just over half.6) Try messing with Tx and Rx gains.
Something I have noticed is that on the Adit600 FXS ports, I have had to set its RX attenuation to -7dB!! (TX to -3dB) If my math is correct, that means I am attenuating 85% of my incoming signal! Is this perhaps what you are referring to with the super-low impedance?
The low impedance is another, related issue. Theoretically, when everyone is operating at the characteristic impedance of the line (which is determined by the capacitance of the wiring, spacing of the wires, wire gauge, insulation material, degree if twist, etc), all of the power delivered to the line at the sending end will be transferred to the line, and all of the power at the receiving end will be accepted by the load. Under any other conditions, some of the power will be reflected back onto the line. Delay is only a few ns per foot, so this doesn't amount to much in an analog only system with wires that are only a few miles long.
In the present case, the real echo problem arises because of how the hybrid sees the line impedance. In order for the hybrid to work, it must subtract an exact copy of the transmitted signal from the received signal. Since there is inductance and capacitance involved, there are also phase shifts, so it isn't enough to just have a signal of the same amplitude. The phase also has to be matched. Mismatching the impedance throws off both the phase and the amplitude of the copy of the transmitted signal that is subtracted, resulting in imperfect cancellation and an echo.
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