Andy Wade wrote:
On 15/01/2011 16:02, John Williamson wrote:

tony sayer wrote:
Yes digital modulation system perhaps;?...

Exactly so. xDSL uses discrete multi-tone modulation (DMT), a variation
on the theme of OFDM used for digital radio and TV. These are
undoubtedly *digital* modulation schemes.

Of an analogue carrier.

Of a large number of (suppressed) carriers, in fact. The resulting
electrical signal has a time variation of amplitude that looks similar
to white Gaussian noise, but you shouldn't think of it as being
analogue. The amplitude is not a direct representation of a
continuously varying quantity an the way that the analogue speech signal
on the line represents sound pressure.

The signal on the line is *not* a simple on/off,

So what? A digital representation doesn't have to be binary. Digital
data - that could, for example, represent an audio signal after sampling
and A/D conversion - is frequently transmitted over wires or radio
channels as a sequence of 'symbols' each symbol representing one *or
more* bits. Increasing the number of bits per symbol generally allows
more data to be sent over a given spectral bandwidth but the signal
remains digital - each symbol represents one of a finite number of
discrete states.

To aid in your understanding, *all* signals passing along the local loop
(AKA the "Last mile") are analogue.

The speech signal is analogue. The xDSL signal is digital.


No, it's not - its amplitude and phase modulated carriers.