In article , John Williamson johnwilli
scribeth thus
tony sayer wrote:
Note the ;? ..

I'm used to ;-? for that... ;-)

Had this very conversation the other month with Two BT exchange engineer
types.. one argued that it was a digital system the other argued that it was
not.. I presume the actual modulation scheme in use would determine that..

They could well both be right on different lines. One has old-fashioned
copper from the DSLAM in the exchange to the street box, the other has
fibre to a DSLAM in the box. When they finally roll out fibre to the
home, then it'll be digital all the way. Inside your current
Modem/Router is an analogue line driver, though.

The feed into your house consists of a number of modulated narrowband HF
carriers, which are phase and/ or amplitude modulated, I can't remember
which, and an analogue baseband signal for your normal phone.

Yes digital modulation system perhaps;?...

Of an analogue carrier. The signal on the line is *not* a simple on/off,
it uses a combination of phase and amplitude modulation of multiple
carriers. What controls the modulation is digital, the modulated carrier
is analogue.

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

Digital signals are transmitted by encoding onto a carrier, and then
decoded by comparing the transmitted signal with a stable reference, as
in a FM or AM radio set. The output from this decoder to your computer
is digital, the input to the decoder from the line is analogue. In the
days of 75 baud modems, the digital data was transmitted by switching an
audio tone on and off, and later, the tone was switched between two
frequencies, which allowed faster transmission. Later still, advances in
design and manufacture permitted the use of combinations of level and
phase variation of the carrier to allow the transmission of more than
one bit of data per cycle of carrier. Broadband basically just uses a
number of these phase and amplitude modulated carrier signals coming
down the same line.

Now, either you're trying to wind me up, or you *really* don't
understand the difference between the analogue and digital parts of a
modem. If the latter, then I'd suggest studying the Wiki reference
given, and follow the links in the article to the bits you don't
understand. You might also like to study amplitude, frequency and phase
modulation schemes for radio transmission.


No John, thats the day job in Radio of a few types!, its just where do
you draw the demarcation line in a digital or analogue system.

Two senior BT engineering types couldn't agree on this.

I'll see what a Professor I know at the Uni sez. Be interesting..

--
Tony Sayer