> Modems these days and for a long time have a limit of (I think) 2400
> baud. This means they can only change the signal 2400 times a second
According to some old notes I have, V.34 uses signal rates up to 3429
baud and I don't think that V.34bis added to the modulation rates.
Since 'Plain Old Telephone Service' connections are sampled at 8000 Hz
for transport through the digital public switched telephone network,
the Nyquist-Shannon sampling theorem states that the highest
theoretical signal rate is 4000 Hz, but real-world implementations can
only approach that.
> on a voice grade line. So to get the higher bit rates they have to
> encode multiple things 2400 times a second. One way is the multiple
> frequencies. The other is that the tones on each frequency are
> shifted, twisted, etc ... in a way such that each frequency can
That's true of every popular modulation since Bell 103J; if I recall
correctly, Bell 212A used phase shifts to encode 2 bits at 600 baud
for 1200 bps, V.22bis used phase and amplitude variations to encode 4
bits at 600 baud for 2400 bps, and Telebit PEP modulation used
pashe/amplitude modulation to encode a variable number of bits at 6
baud simultaneously on hundreds of carriers, each on a unique
frequency, to achieve ~11 kbps (PEP1), ~18 kbps (PEP2), or ~23 kbps
(TurboPEP). But those and other modulations were either half-duplex
(PEP, V.29FT) or split the available frequencies either symmetrically
(Bell 103J, Bell 212A, V.22bis) or asymmetrically (USRobotics' HST) in
order to allow bidirectional communication.
> Now I'm sure that US Robotics and Rockwell have some programs that
> might do this and maybe the DIA, CIA, and NSA but past that ???
Not being an expert in digital signal processing, I could not guess
how hard it would be to accomplish but, with computing power
practically exploding (pity all that power and more is wasted running
bloated code, but that's off-topic) and the availability of computing
clusters and grids, I wouldn't think that it would really be that hard
for the modulations described above. (That said, I won't have it done
by end of day ... or year ... or ...)
However, as others have pointed out, the real challenge with
higher-speed modulations such as V.32 (9600 bps full-duplex) and V.34
(28800 & 33600 bps) is that, unlike the modulations listed above and
in addition to the complicated modulations that you describe, they
send and receive on the same frequencies and use echo cancellation in
order to discriminate what the other side is sending from the echos of
what the near side is sending. This adds another very complex layer
and, without access to the transmitted data from at least one side, it
might well prove to be impossible for a third party to decode what the
other side is sending.
Geoffrey Welsh <Geoffrey [dot] Welsh [at] bigfoot [dot] com>
Never leave until tomorrow what can wait until next week.