[Asterisk-Users] tuning an x100p in Australia for echo cancellation

[Rich Adamson]

> I have an x100p which suffers from echo (no surprise there apparantly
> :), and a few of the things I've read about tuning out echo say the
> first thing to get right is the tx and rx gain, and for that you need a
> few different types of test lines from the telco (Telstra, Optus,
> whatever). Does anyone have any numbers in Australia for such lines? I
> think they just need to send out a tone of a known frequency and power.

That would be called a milliwatt generator. It likely exists in their
central office, but its typically used by their technicians to ensure
new installations meet specs and sometimes in troubleshooting. Call your
telco repair number and see if they will give you the telephone number
for it.

If they won't, you can basically do the same thing by dialing out from
asterisk on one pstn line coming back in through a second pstn line, and
using the asterisk milliwatt generator. Or, if you have another asterisk
system available somewhere, call out through a pstn line to that asterisk's
milliwatt generator. (Obviously its not as good as using a CO milliwatt
as now you have to take into consideration the loss from the second
pstn line, but it is a way to get a handle on the transmission loss 
values, etc.)

> Next, while the TDM400 card has control over the line impedence
> circuitry, the x100p doesn't. 

Are the AU telephone standards the same as US standards (eg, 600 ohm
impedence)?

> Does anyone know of an addon device which
> can do impedence matching on the line, or of a modification to the card
> (eg component swapping) which could allow some manual adjustment?

Twenty years ago, the telphone companies in the US had several types of
hardware devices available for impedence matching, line balnacing, etc.
The devices were used to compensate for several different problems that
would be too costly to fix through conventional means. I don't have any
clue where one might find those boxes today since those types of issues
have essentially disappeared due to the heavy use of fiber, remote line
modules/units, higher quality cables, and other technology advancements. 
Some older telephony jocks may still have some of these in their junk 
boxes.

Since I don't have a clue what the AU standards are, I really can't 
guess at what might be needed in your particular case.

One such example that was fairly common back then was a simple transformer
that had two primary windings and two secondary windings. One could buy
them as 1:1.5 (600 ohm to 900 ohm), 1:1 (600 ohm to 600 ohm), and other 
commonly used impedances. The transformer along with two 2.0 ufd 
capacitors allowed DC to pass through, but changed the impedence from 
one value to another.

Another example was a small box with the same type of transformer inside,
but with multiple windings, a switch to select taps, and usually a couple
of potentiometers to allow the technican to step through various
combinations of impendences as well as essentially add resistance from
tip (or ring) to ground to balance a two-wire circuit. To set one of 
those up, one had to use a transmission test set, a CO tone generator
(milliwatt generator), and repetitively test each switch setting to find
a combination that minimized the issues/problems. (No idea where one might
find such boxes today.)

As far as substituting components on a x100p card, I don't believe that's
realistic. If you can read the part numbers on the chips used on your
x100p compatible card, its not that difficult to check the chip specs
to see what impedance value it supports. (For example, Intel and Silicon
Labs made some of the chips used to interface the winmodem cards to the
pstn lines. Those cards manufacturered for US sales used a 600 ohm chip
while those manufactured for other specific countries used a different
chip to match those country-specific telephony specs.)

> Finally, my echo is really really awful early in the call but then gets
> markedly better, which I assume is a result of the echo training done
> during the call. 

That's probably a valid assumption. Whether its the result of poor impedence
matching or something else is questionable.

> Is it possible to grab the echo coefficients after
> training and use them as a starting point for next time? Or would this
> vary too much between calls?

It's my opinion your thought process relative to preloading the EC is
very reasonable, but in the past two/three years, those with the ability
to code such functions "insist" every call is different in terms of those
values. I disagree but don't have the programming skills to prove it.
There is likely to be some middle ground where "some implementations"
would benefit from it, and a lot more would not.