Re: Filters - Quality

On 2007-05-22, JohnW <invalid@xxxxxxxxxxxxxxxx> wrote:
Any filter is designed to work best in a known environment.
In telecomms, we practically have a 600ohm balanced
transmission line environment, so this (balanced, 600ohm) is
used in the design to ensure the filters work as required.
Now, every filter introduces its own impedance (a complex
termination "resistance") into the total circuit, so every
filter disturbs the transmission line into which it is
introduced. Hence, we want as few as possible filters
introduced because the minute problems may well add-up. We
have to have one filter for the ADSL modem and, preferably,
one for the voice equipment. We also try to ensure the ADSL
part is removed from the voice-only wiring as soon as possible
to the incoming balanced transmission line.

The incoming signal energy is shared between the two
destinations. Unfortunately the cut-off introduced by the
filers is not an absolute, as with a switch. There is still
some part of the blocked ADSL signal going through the voice
filter (and vice-versa) This is due to the "roll-off" of
filter designs. Simple designs are poorest at this blocking
of the out-of-band signals. Better design add components,
such as active devices or more stages of filtration to improve
this problem. Hence a better designed filter will block
signals in the ADSL part of the band better than the simpler
designs. Since filters work in both directions, the better
designs are better at blocking any potential noise that is
inevitably generated or received by the voice equipment and
voice wiring and reelected back into the part of the wiring
carrying both the ADSL and voice signals.

What you say is absolutely correct, but what I think is missing
is a comparison of the likely relative magnitude of the effects
different filter designs might have on the transmission line
compared to the effects of other things which make the transmission
line much less than perfect.

The only way you end up with a 100 ohm (not 600, as was pointed
out) transmission line is if you have a single twisted pair
connecting directly from the DSLAM in the exchange to the modem.
Anything you shunt across the pair can potentially change its
impedance, causing reflections and attenuation if not noise
injection. The things that can cause this include not only
filters (which are explicitly designed to have a minimal
effect but which, as you point out, might have some) but also
other twisted pair transmission lines that you've shunted off
in parallel, even if the other end of the that transmission line
is open circuit. Transmission line stubs always add reactance
where they attach so this is guaranteed to cause some level of
mismatch (google "transmission line stub" and you'll probably
find such things are sometimes used as transmission line
transformers, to change the impedance on purpose); if the length
approachss a quarter wavelength at some DSL frequency (unlikely for
house wiring, but possible) this reactance can be big enough to
simulate a short circuit. In addition, open circuit stubs are
notorious for noise pickup.

What this means is that if you take your DSL signal and put
it into the branching network of wiring in your house you've
already changed the characteristics of the transmission line,
and added a bunch of places to attract noise, even if you never
attach a telephone or a filter to any of the branches. This
is why a face-plate splitter is a hugely better arrangement
than the self-install microfilters, much more than just
minimizing the number of filters you use. The face-plate
splitter serves to isolate your ADSL transmission line from
whatever mess the branching network of transmission line stubs
your house wiring represents might cause to the ADSL signal and,
by doing so, makes the ADSL transmission line much more closely
approximate the single copper pair from DSLAM to modem that is
ideal.

As for ADSL filters, the requirement to pass frequencies below
4 kHz and block those above 24 kHz is not particularly demanding
at all (for comparison, the quality of your voice service depends
critically on a filter which passes frequencies below 3.5 kHz
while blocking those above 4 kHz, used for anti-aliasing at the
telephone switch. An ADSL filter is peanuts compared to this).
Anyone with experience is extremely unlikely to screw up the design
of an ADSL filter badly enough to make it non-functional, as long
as they manage to keep, say, solder bridges from between the
traces during manufacture. The design is basic. That's not to
say different filters might not have different effects on the
transmission line but these effects, relatively speaking, are
almost certainly small, if not in the noise, compared to the other
effects you can cause to that transmission line, like connecting it
to the uncontrolled mess of your house wiring.

The point of this is that, if you are worried about the quality
of the filter in your face-plate splitter I think I might
sympathise. If, however, you've put the ADSL signal into your
house wiring and are worried about the characteristics of
the microfilters you are using then, well, to get biblical about
it, you are really spending your time worrying about the speck
in your neighbour's eye while ignoring the log in your own,
since the differences the characteristics of different filters
could possibly make are dwarfed by the mess all those transmission
line stubs are making in the first place. Yes, changing the
brand of filter might make a difference between working and not,
but all that's happening when this helps is that you are changing
from the non-working side of marginal to the working side. What
you are left with is still marginal. And if you are depending
on the fine details of filter performance to correct the mess
your house wiring is making, you are really doing random selection.
The cheaper filters are as likely to make it better as the
more expensive ones; even omitting the filter could possibly
make it work better. If you want to really fix it, and get
something which works more predictably, you've got to isolate
the house wiring from the ADSL.

So I agree with Graham to this extent. If your ADSL works
okay through your house wiring, fine. If it doesn't, however,
than looking for "better quality" microfilters makes no sense
at all. If you want to really fix it you've got to start
by removing the uncontrolled variables, which means isolating
the ADSL from the house wiring with a splitter.

Dennis Ferguson
.

Relevant Pages

  • Re: Impact of hardware with bad SNR & Attenuation
    ... > out which things the filter will mess up). ... by the bell wire of the extensions feeding back & mixing with the ADSL ... > Reading between the lines here, a good modem is better at dealing ... attempting to negotiate a connection with the exchange).. ...
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  • Re: "Transmission Flush": Needed ?
    ... important because good shifting depends on good flow through the filter ... opportunity to wipe the layer of pond scum from the bottom of the pan. ... I personally prefer to drop the transmission pan every few years, ... transmissions as requiring no maintenance under "normal" conditions: ...
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  • Re: Tiscali broadband and BT pbx
    ... Modem is Speedtouch 330 ADSL ... can install a filter. ... installed 7 filters to extensions, but as I said before his Master phone ...
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  • Re: Cable length between filter and modem
    ... > What is the maximum, sensible, cable length that can be used between an ADSL ... > filter and an ADSL modem? ... As the filter does nothing at all for the ADSL signal, ... wiring) is small enough to allow a workable signal through. ...
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