[Asterisk-Users] tuning an x100p in Australia for echocancellation

[Paul Hales]

To agree with you - I don't remember what the impedence is in Australia, but 
it isn't 600 ohm.

PaulH

On Sunday 15 January 2006 10:16, James Harper wrote:
> > 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.
>
> I'll try that. 1st line technical support at Telstra are not known for
> their ability to provide useful information that isn't in their script.
>
> > 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.)
>
> Would the txgain on the 2nd line also come into play? I guess if you set
> it to zero then you only have to deal with an estimation of the line
> losses.
>
> > > 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)?
>
> This is a question I've been trying to answer too. I had a look at the
> standard phone that Telstra would provide to customers about 5 years
> ago, and it has an impedence switch on the bottom to toggle between
> 'NORM' and '600', which suggests that 600 ohms isn't the normal
> impedence.
>
> On an au configuration example for the pap2 I have seen on the web, the
> impedence is set to '220+820||120nF', which suggests that our standard
> here isn't 600.
>
> > > 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.
>
> I've thought of using a transformer by itself, but obviously that
> wouldn't let the DC signals required for looping through. Could you give
> me an ascii schematic?
>
> > 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.)
>
> I think I've got a spare x100p so I'll check that one out. It's an ebay
> purchase so the chances are it's an import...
>
> > > 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.
>
> I have since done some checking of calling between the two extensions on
> the pap2, and without the pap2's echo cancellation being active, I get
> echo there too... and that's with either impedence setting.
>
> > > 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.
>
> I modified zaptel.c to dump out the coefficients on ztdiag (just a
> #define, removing a few comments, and updating some out-of-date code). I
> then started a call and dumped them out every few seconds. They change
> wildly, but a large change of the figures may have very little change in
> the actual system behaviour of the ec, so I guess that doesn't tell me
> anything!
>
> I'll try and write some code to be able to retrieve and load the ec
> values today, and see what happens.
>
> After about 20 seconds, the echo is reduced to a quiet and very muffled
> echo, which you probably wouldn't notice unless you were listening for
> it, and is certainly better than the quality of most mobile phone calls.
>
> Thanks
>
> James
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