[Asterisk-Users] X101P on a UK BT line ---- txgain issue

[Chris Stenton]

Thanks for the info Rich looks like I'll have to wait for the new FXO
module. The impedence in the UK is zcomplex(2) which looks a long way away
from a straight 600 ohms.

Here is the list of zcomplex impedences

       Zcomplex(1) = 150 nF // 750 ohms + 270 ohms ( European harmonized,
France Telecom & Telefonica )
      Zcomplex(2) =230 nF // 1050 ohms + 320 ohms ( British Telecom plc )
      Zcomplex(3) = 115 nF // 820 ohms + 220 ohms ( Deutsche Telekom  AG )
      Zcomplex(4) = 310 nF // 620 ohms + 370 ohms ( Telecom New Zealand )
      Zcomplex(5) =47 nF // 510 ohms + 150 ohms ( Russian Telecom )



----- Original Message ----- 
From: "Rich Adamson" <radamson at routers.com>
To: <asterisk-users at lists.digium.com>
Sent: Friday, June 25, 2004 2:32 PM
Subject: RE: [Asterisk-Users] X101P on a UK BT line ---- txgain issue


> > > I do get echo, lots of it, I am waiting until the new patch they
> > > are all on about on the list gets into a stable release, then I
> > > will upgrade and see if that does the trick.
>
> Not likely the patch will get applied to the Stable release since its
> been stated several times that's all but dead.
>
> > The patch didn't seem to work for me.
> >
> > > I am told that some of the echo may be to do with a mismatch in the
> > > impedance with the BT line.
>
> There are at least several sources of echo, which have been noted
> several times in the past six months or so:
>  a. echo can in * not functioning correctly in some circumstances (patch)
>  b. mismatched x100p -> pstn line
>  c. pstn line problems (eg, imbalance between tip/ring and ground)
>  d. 2-wire to 4-wire conversion along the end-to-end voice path
>
> Any single pstn line could have one or more of those happening.
>
> > Problem is do we really want BT messing with gain there end and
impedance
> > cos it might mess our ADSL lines up =)  I know im on the limits.
>
> The telco isn't going to mess with changing impedance on their stuff
> as that would require re-engineering their entire outside plant (cables),
> line interfaces within the CO, etc. You couldn't pay them enough
> money to do it. Its also highly unlikely they have any technician
> adjustable transmission level adjustments on ordinary pstn CO line
> interfaces, as those are engineered to 'standards', and manufacturing
> engineers typically don't want support technicians to muck with
> those for lots of very valid reasons.
>
> I'm in the US and don't have any real clue what the UK standards are
> for impedance, transmission levels, etc. (It would be somewhat
> interesting to here from someone who knows for sure what those are.)
>
> The x100p card (from digium) uses the Silicon Labs ( www.silabs.com )
> 3012 chip to interface with the pstn line, the 3021 chip to interface
> the 3012 (analog-to-digital converter) to the Tigerjet PCI controller.
> The 3012 is responsible for matching pstn line impedance. The spec
> sheets at their site tend to suggest the 3012 was built to interface
> to 600 ohm pstn lines and is not adjustable/setable to other values.
> If the UK pstn lines are not 600 ohm impedance, then its unlikely the
> x100p is going to properly match up with UK lines from an impedance
> matching perspective. However, imedance mismatches have to be rather
> dramatic to cause a lot of echo.
>
> The tdm fxo module uses the 3019 and 3050 chipset, where the 3019 pstn
> line interface chip has many different pstn line impedance settings
> including 600, 900, 270, 220, 370, 320, 275, 120, 350, etc, ohms.
> Have no clue which countries use which settings, but obviously
> Silicon Labs intended this chip set to operate in different countries,
> whereas the 3012 spec sheet doesn't seem to support those objectives.
>
> So, backing into exactly what is causing the echo in the three UK
> cases noted yesterday on this list...
>  - not likely to be "d" (2-wire to 4-wire conversion along the
>    end-to-end voice path) as that would impact all telco users,
>    not just * users.
>  - item "c" (pstn line problems) can contribute to echo depending
>    upon how bad the pstn line actually happens to be. Most telco's
>    have the equipment to measure line quality, however most will
>    stop at the cable entrance to your home/business, leaving you to
>    guess at what's happening inside.
>  - item "b" (mismatched x100p -> pstn line) can contribute, but without
>    knowing the exact specs (and probably more info from Silicon Labs),
>    its impossible to guess at this one.
>  - item "a" (echo can in *) is still a very real possibility, and Mark
>    is about the only person I know of that has the knowledge of the
>    spec's and * to weight in on this one.
>
> One of the methods that I used to help determine whether my tdm echo
> was a pstn line or * issue was to eval echo on three different pstn
> lines using the exact same physical * port (x100p). If the lines
> are clean from an analog phone perspective (eg, no hum, no noise) and
> the lines cause equal echo when used with *, then its highly likely the
> issue is either "a" or "b". If the answers to "b" rule out impedance
> mismatches, then "a" is likely. Why? Your not likely to have multiple
> pstn lines with exactly the same fault. (Could happen but not likely.)
>
> Second, if your pstn line is also a DSL line with appropriate filters,
> the DSL line is far more critical of pstn imperfections then is the
> * interface to the analog pstn line. In the majority of cases, a poor
> pstn line will cause significantly more DSL problems and probably a
> total DSL failure way before the pstn analog path is impacted.
>
> Third, I obtained a Mediatrix 1204 sip gateway to displace the x100p's
> to eliminate the echo. It worked fine (zero echo) on all three lines,
> essentially proving the pstn lines were not the issue causing echo.
> After that test, there was nothing left other then * echo can functions
> (which turned out to be the case with my tdm fxo _and_ CO operation,
> and resulted in this recent patch).
>
> Given that Mark has been successful at eliminating 95%+ of the echo
> on the tdm fxo module, it proves the algorithm in use within * is
> very reasonable. The algorithm is not sensitive to which "country"
> it happens to be running in, but "how" that algorithm is applied to
> UK pstn lines is suspicious.
>
> I'd suggest someone in the UK with echo problem go through the
> logic above to rule out as many items as possible, and then try to
> coordinate some of Mark's time (in advance) to help resolve the last
> item. Do that via opening a Bug report, include as much factual
> detail as possible, and be ready on the IRC channel.
>
> Rich
>
>
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