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[roseway]

My connection suffers from an occasional fault which makes the phone go crackly for a day or two, and like the above it only happens when the router is connected. I've never got to the bottom of what's actually causing this, and a number of OR visits at various times have always resulted in no fault being found. So I've no doubt that the phenomenon exists, but the cause is still a mystery to me.

[sevenlayermuddle]

Cutting a long story short the fault was in the network, not the router. I can't provide a technical explanation for this but if you swap your router and the noise on the line still happens then suspecting a network fault is not unreasonable.

That is an interesting fault. I can't think up a reasonable explanation . . . Can anyone else?

(At this point I would have asked for Mr Pag's opinion. Sigh.   )

I can hazard a guess.

One (arguably the main) purpose of the microfilter is to isolate the DSL signals from the voice equipment, which would otherwise cause audible interference.  I've been following this thread, and the advice given seems to have eliminated the end-users filter, all well and good.

However, I should think there must also be a similar filter at the exchange end of the line, though it probably goes by a much more technical name.  I'd speculate that is the device which wants checking.

[burakkucat]

However, I should think there must also be a similar filter at the exchange end of the line, though it probably goes by a much more technical name.  I'd speculate that is the device which wants checking.

I think you have just provided an explanation which fits with the observations.

[HPsauce]

One (arguably the main) purpose of the microfilter is to isolate the DSL signals from the voice equipment, which would otherwise cause audible interference.

The (dare I say?) Wikipedia, article says:
A DSL filter is an analog low-pass filter installed between analog devices (such as telephones or analog modems) and a POTS telephone line, in order to prevent interference between such devices and a DSL service operating on the same line. Without DSL filters, signals or echoes from analog devices at the top of their frequency range can result in reduced performance and connection problems with DSL service, while those from the DSL service at the bottom of its range can result in line noise and other issues for analog devices.
My experience is that unfiltered POTS devices cause more issues for ADSL than vice-versa.

[sevenlayermuddle]

One (arguably the main) purpose of the microfilter is to isolate the DSL signals from the voice equipment, which would otherwise cause audible interference.

The (dare I say?) Wikipedia, article says:
A DSL filter is an analog low-pass filter installed between analog devices (such as telephones or analog modems) and a POTS telephone line, in order to prevent interference between such devices and a DSL service operating on the same line. Without DSL filters, signals or echoes from analog devices at the top of their frequency range can result in reduced performance and connection problems with DSL service, while those from the DSL service at the bottom of its range can result in line noise and other issues for analog devices.
My experience is that unfiltered POTS devices cause more issues for ADSL than vice-versa.

If you look at the circuit diagram of a typical microfilter, the line-pair is passed straight-through to the modem.   That being the case, my view is that the filter serves two purposes:

1) It prevents DSL signals from reaching the voice equipment.  Clearly DSL frequencies are inaudible but, when attached to modern electronic telephones, I'd speculate that these signals could play havoc with the transistors in the input stages, driving them in and out of saturation if the DSL signals were strong enough.

2) It prevents the telephone appliances from loading or 'absorbing' too much of the high frequency DSL data.  Again, I'd speculate that this would be more of a problem with modern electronic phones, as they are likely to have a high-frequency decoupling capacitor across the line, largely to avoid the possibility of HF signals affecting audio.  The corresponding electronics at the exchanged end may, however, be quite different.

All things considered I stress again, I'm only hazarding a guess.  But I certainly find it conceivable that a problem with the exchange-end equivalent of a 'microfilter' could result in audible interference, with or without any significant degradation of DSL.

As an aside... I have in the past attached both a DSL router and a very old-fashioned (rotary dial) pre-electronic phone on my own line - both unfiltered - and, although the router connected a (only slightly) sub-optimal speed, both voice and data functioned well.

- 7LM

[HPsauce]

The corresponding electronics at the exchanged end may, however, be quite different.

It's an interesting subject and I'm pretty sure I read somewhere (probably in one of Kitz' own detailed descriptions) that the exchange end is VERY different, with all the signals going straight into the ADSL equipment which then interprets it all electronically/digitally and passes on the POTS signal. The filtering function is I think largely integrated into the units there and bears little or no resemblance to a domestic filter.