David Paste wrote:
As I understand it, ADSL signals are passed along the twisted pair (is
that right?) telephone cables which are the same used for voice calls.
At a much higher frequency, of course. I am also under the impression
that the length of this cable is critical for service quality and
overall data rate (apart from junction / joint quality).

Would using coax for the last stretch to the subscriber's premises
provide any better quality of service?

Yes and no.

Both ADSL and cable are adaptive technologies. In other words, they take
the bit of wet string that is your line, analyse it, see where the frequency
nulls are, and send the data in frequency bands to avoid them. As things
change (atmospherics, day/night, water, interference, whatever), they can
adjust the bands to cope.

In cable's case the network quality is much better so there are many fewer
nulls and you can get much higher bandwidth out of it. However you're also
sharing that coax with your neighbours - and it carries all the TV signals
too[1]. Plus the cable operator needs to agree with each modem what bands it
gets so they don't conflict.

So you could route ADSL over coax, but it's still a fundamentally different
technology. Plus you'd have to impedance-match the interfaces between
twisted pair and coax which would increase the losses. So a short coax run
might make it worse not better.

Theo

[1] back when analogue cable was still alive, you could stuff the cable in
the back of your TV and get FTA TV directly, no STB required. Same goes for
FM radio.