Why do I hear the ADSL carrier "noise" in my phone, even though the phone is connected on a microfilter?

Question

OK I'm starting to lose my mind over this, so apologies for the big wall of text.

I recently got a 2-play subscription (phone + ADSL internet) on a phone line that I had previously used only as a phone. On the RJ wall socket, I have connected a simple phone line splitter that splits the signal into two output jacks (no filter, just a simple Y splitter). On one output I connect an ADSL microfilter and on that filter, my phone device. The other output goes straight to a Thomson TG585 v7 modem that I had lying around, so I did not buy their recommended modem, which is equally crappy as the one I have.

The Internet is working properly, I'm browsing pages just fine. The speed is only around 2Mbits, not 24Mbits which is the theoretical maximum. I think this is because the wiring in my house is very old (over 50 years old) and probably very noisy, but I don't mind about the slow speed at the moment. The phone is also working correctly.

The problem that I would like to solve is this: When the modem is connected, I can hear the carrier noise in the phone. It's very loud and distracting, you can barely have a conversation. If I listen into the phone as I turn on the modem, I can hear (besides the dialtone) first silence, then something that is definitely a handshake, and then something like white noise which I assume is the DSL data. If I completely disconnect the modem, the sound over the phone is crystal clear, I can't hear the slightest bit of noise, but then I have no Internet.

I have called my ISP's technical support four different times, speaking to four different people, and they have not been very helpful, they just checked that I hadn't done anything silly like connecting the modem to the filter, and they suggested that I swap things around, try with different cables and filters and see if that solves it. Problem is, I had already tried all of this. They have nothing more to suggest.

I have tried replacing the Y splitter with another one, I have tried with many different phone cables, and even three different ADSL microfilters. Same problem every time. The only suggestion the tech support gave me that I haven't tried yet, is that I replace the Y splitter and filter with a single filter+splitter, essentially a single piece that has one filtered and one unfiltered output, all in one. I don't have one of those at the moment and I don't see how this would be any different. When I visited their closest shop they didn't have one either.

This is all very puzzling for me, because the way I see it, no matter what goes on between the wall socket and the modem/router, the noise should be filtered on the phone, since the phone device is directly connected to the filter.

So my question is: why all of the above?

My current theory is that, since the phone lines are old, the modem has to maybe connect at a lower frequency than normal, and maybe it gets into the analog range. Is this even possible? Or is the frequency band that ADSL modems use a standard that can never overlap with the analog band? The same provider also serves VDSL, although I have paid for ADSL. Could this have something to do with it? Are there different types of DSL filters with different cut-off frequencies?

I guess at some point I should just upgrade the wiring in the building, but this is a big chore that I would like to postpone at the moment if possible. Also, if I am going to do this, I would like to know beforehand that it is going to solve my problem.

I would very much appreciate it if someone more knowledgeable on the topic could give me a hint as to what might be going on here. My expertise is software development, not hardware, so I'm at a loss.

Thanks very much for reading.

Answer 1

For the record, it turns out there was a faulty circuit breaker before the wall socket causing excessive noise in the line. I don't understand exactly why or how, but bypassing that solves the problem.

I'd expect that any noise in the line would only cause the data link to be slower, not the data signal to cross the filter. Go figure.

Thanks to both Ramhound and AperezC, your help is appreciated.

Answer 2

Noise might not be ADSL-related, since electrical issues with both battery and receiver charger can both generate noise. That's a common problem in amplifiers. This could happen and has happened to me with phones. That is usually the sign of imminent failure.