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[Weaver]

I could see a circuit effectively giving increased frequency-dependent attenuation, so the noise is not higher, it's that the signal is lower so the signal-to-noise _ratio_ is lower. (Since it's in dB, this must always be a ratio, relative to some other value or to something fixed that is an understood reference level.) I'd need to look at the definition of QLN. I would think it doesn't fit this idea, you'd expect it to be relative to some fixed reference level, so it's effectively like an absolute noise level, rather than an SNR. I'm totally confused now.

A filter can let more of the noise through, of course. So the post-filter absolute noise figure goes up when the filtering is less effective, for example.

[Weaver]

Perhaps the Mk 1 assumed the frequencies way above the ADSL1 range were just utter junk and so did you a favour by filtering them out, hence lower absolute noise figures. And the Mk 3 didn't dare filter those frequencies out because there might be signal now all the way up to tone 4095 or whatever.

The filter you need presumably depends on how long your line is. If it is incredibly long, you won't be using the highest tones anyway, so more aggressive filtering might be useful to kill the high junk.

[ejs]

The MK1 doesn't have a common-mode filter, the MK2 has a common-mode filter, and the MK3 has a stronger common-mode filter than the MK2.

I think the concept of filtering off out of range frequencies to somehow improve the noise level on other frequencies is a bit doubtful really. The MK1 SSFP was designed for VDSL2 so it won't be filtering out frequencies above the ADSL1 range, otherwise it would be blocking the VDSL2 signals.

The common-mode filters in the MK2 and MK3 are intended to have a generally beneficial effect on most lines, but they are not magic, and so won't be able to filter out 100% of the unwanted noise while leaving the wanted signal completely unaffected.

[Weaver]

Thanks for letting me know.

Is there any benefit to be had for ADSL2 users such as me in using a Mk2 or Mk3? (Given that I don't use any filter at all now, as no POTS.)

[ejs]

For my line, the grand total of getting a modern NTE5a master socket, and a MK2 SSFP, and re-connecting existing wiring so that what's now the extension socket is connected and filtered off at the master socket, plus a CAT5 modem lead, made pretty much no difference to anything, it might even have made it about 50kbps slower, but this difference is within the general variation of my line speed from time of day or anything else that might cause a slight difference in speed.

It will probably be more difficult to get a new MK2 now, because presumably they don't make them any more.

[burakkucat]

Is there any benefit to be had for ADSL2 users such as me in using a Mk2 or Mk3? (Given that I don't use any filter at all now, as no POTS.)

None whatsoever. 

[Weaver]

Presumably the reason that there is no benefit to be had is because the filters that are provided  don't reject low enough frequencies for the likes of me. And in any case, my modems' front ends may possibly perform some filtering, I don't know.

[burakkucat]

With circuits as long as your own, you do not want to add to the (already) significant attenuation. By fitting any of the devices so discussed in this thread, you will definitely increase the loop-loss . . . as there is a measurable insertion loss associated with each device!

[Weaver]

Quite so!

[aesmith]

With circuits as long as your own, you do not want to add to the (already) significant attenuation. By fitting any of the devices so discussed in this thread, you will definitely increase the loop-loss . . . as there is a measurable insertion loss associated with each device!

What sort of loss from the various types of filters?  I'm on a long line, and we're migrating off the land line at the moment, not had a phone plugged in for a few weeks now (not since the lightning destroyed my test phone).  Just wondering what difference I might see replacing the current Mk3 plate with a DSL only plate from A&A.

[burakkucat]

What sort of loss from the various types of filters?  I'm on a long line, and we're migrating off the land line at the moment, not had a phone plugged in for a few weeks now (not since the lightning destroyed my test phone).  Just wondering what difference I might see replacing the current Mk3 plate with a DSL only plate from A&A.

I can't give you a figure but if you have no need for the filtering to separate the telephony frequencies (300 Hz - 3.4 kHz) from the xDSL frequencies (> 30 kHz) and you have no need for a common-mode choke circuitry, rejecting extraneous RF signals coupled into both legs of the pair, then go ahead and experiment.

The difference between a basic (but good quality) micro-filter and a SSFP is that the latter also contains components to mitigate induced longitudinal signals (common-mode signals).

If you are in the middle of nowhere, perhaps located at the bottom of a valley with hills all around that attenuate RF transmissions, etc, then there may be a good chance that any common-mode signals injected into your circuit would be minimal (or non-existent).

So for those persons who do not have a telephony service on the circuit, the low-pass filter of the splitter in a SSFP is redundant. If there is still a problem of common-mode RF signals coupling into the circuit, a BT80-RF3 could be used in place of the SSFP.