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[sevenlayermuddle]

All fine and good, nut I can foresee the following problem....

Subscriber is out, gets an incoming call.  Router is connected with (say) 6dB SNRM.   Internal wiring is in poor state.
1) The internal wiring is connected, the SNR degrades, and the router drops the connection.
2) Caller gets no answer, and hangs up, router meanwhile is starting to reconnect.
3) Internal wiring is disconnected again
4) Router completes reconnection, achieving same speed and same (fragile) SNRM as before.

...All of above may seem harmless enough.  But suppose it is repeated ten times inside an hour, ie somebody is keen to call you... that is the threshold for which BT's DLM reacts to slap an extra two ' stability steps', i.e. 6dB increase in target SNRM.

One assault by DLM is bad enough, but  if same thing happens just one more time then the subscriber would find himself in DLM's notorious 15dB prison, from which it is very difficult to escape.   

Admittedly, the likelihood of a second occurrence would be reduced as the higher target margin would provide some immunity, but it is not inconceivable.

[guest]

Well in that instance you'd already have been DLM'd down anyway had you used another filter. I appreciate what you're saying about the BT system but that's a system problem and not a technical problem. DLM is there to save OR/ISPs money, not to keep users happy

Your scenario may however complicate diagnosis of fault by the ISP - eg "it only happens when the phone rings" - which then may land the EU with a charge from OR (via the ISP) when dodgy internal wiring is found.

Its still an elegant design.

[sevenlayermuddle]

Well in that instance you'd already have been DLM'd down anyway had you used another filter.

With the greatest of respect Rizla, I don't think I quite agree on that.

With a conventional filter,  the noise from internal wiring is present all the time (or at least when the router initially connects) and it is catered for in the negotiated connection speed.  Admittedly that speed may be reduced (compared with transistorised filter) but, even during calls, the connection would be sustained at error rates that ought to be acceptable to DLM.

In contrast, the transistorised filter would allow the router to connect at an artificially reduced noise level (and hence negotiate a faster speed), but then to suffer a sudden outbreak of reality every time the phone is used.  If that causes the error rates to soar, or worse, the connection to drop, then DLM may well intervene.

[guest]

What I meant was you'd have a far lower "normal" sync rate than without the filter in question.

The fact BT's DLM throws a wobbly IS a system problem, it really doesn't detract from the design of the filter. Its a technically good design which pre-dates DLM. DLM is the problem not the filter

[burakkucat]

Is there any possibility that an expert in such matters can now sketch out the schematic diagram, please? 

[guest]

If you mean the ADSLNation one then it'd be easiest for Blackeagle - there's some tracks around the chokes/caps I can't see on the front of the board, also around the connector. I had to assume its a 4-pole filter due to number of chokes/caps but it could well be slightly different.

[Blackeagle]

B*eagle is a little busy at present, but will draw a schematic as soon as time allows !!

[burakkucat]

Thank you, B*Eagle. 

[asbokid]

...that's a very interesting article from Stephen Wilson, G3VMW, formerly "engineering manager in charge of telecommunications for Yorkshire Electricity". It was generous of him to put it into the public domain.

Stephen Wilson's article is missing from its original site, but it can be found here now:

http://qso365.co.uk/wp-content/uploads/2012/02/ADSL-filter.pdf

cheers, a

[tickmike]

Very interesting article 

[asbokid]

There's a hefty book chapter here on the purpose, function, and design of xdsl splitters:

It is excerpted from Golden et al's 2007 book Implementation and Application of DSL Technology:

http://docs.google.com/file/d/0B6wW18mYskvBN2hiU1VFR2FHNGs/edit?usp=sharing

cheers,a

[burakkucat]

Thank you. That chapter has now been downloaded, ready for a 'quiet moment'. 

[roseway]

That's really interesting. I too will await a 'quiet moment'.

[guest]

Heh, group delay on a telephone pair at voice frequencies. Seem to remember that was part of a module in electrical principles, along with working out Zo from first principles.

Blast from the past 

[JGO]

A belated comment on rizla's point on active filters.

I think the point is the cutoff is sharper than a plain RC filter, NOT than is possible with an LC filter. At low frequency the trade off between cost and bulk of an LC filter and (for a Sallen and Kay Active filter, independence of source and load impedance ) is appreciable but my slight experience and understanding of theory is that an active filter can't do anything a passive one can but it may be more convenient.

A supplementary hypothetical question - AIUI xDSL is channelised partly to overcome line group delay variation over the frequency. In a case of interference from Droitwich on 198 kHz, would it be worth using a notch filter and sacrificing one or two channels to prevent a modem going into overload  and loosing the lot, or would the extra delay variation make the cure worse than the disease ?