Jeff Liebermann wrote:
DTC hath wroth:

Mike Kaliski wrote:
ELF communications are carried out at very slow data rates, only a few
characters per hour at best.
Actually its on the order of several characters per minute using a 64
character "alphabet".

It is possible to communicate at a base band frequency of 0Hz. This is what
happens when you talk down a hard wired telephone or intercom. At a
telephone exchange (switching centre), the signals from each line are
modulated onto a higher frequency for onward transmission down a trunk wire
cable or fibre optic cable. The multiplexed high frequency modulated signals
are down converted back to audio frequencies once they reach the intended
destination.

In the old T carrier (before 24 channel digital T1) carrier, each telephone
conversation was modulated onto a low frequency radio frequency AM signal
ranging from (and don't quote me as its been over thirty years since I
worked T spans) 50 KC to 200 KC. Very similar in principle to the 5 kc wide
AM radio station signals on the 530 kHz to 1700 kHz AM broadcast band.

Argh, that brings back fond nightmares of Ma Bell. 4Hz per voice
channel with FDM (frequency division mux). Most were FM systems, but
there were some AM implimentations (to avoid patent infringement).
Later, there were SSB systems that doubled the number of channels.

No voice Spectrum BW
channels KHz kHz AT&T ITU-T
12 60-108 48 Group Group
60 312-552 240 Supergroup Supergroup
300 812-2044 1232 Mastergroup
600 564-3084 2520 Mastergroup
3600 564-17548 16984 Jumbogroup


That does bring back memories. I worked on STC built systems that used
AM modulation using Double-Balanced Modulators and depending on "Group"
classification used either the lower or upper sideband. Up to the 60
"voice" Ch's, we had some low Baud rate Modems on as well with the
signalling frequency disabled, the spectrum usage was the the same
however the next step up was 16 Supergroups using 60-4028KHz with
Supergroup 2 not being translated. We also had a 30 Ch PCM link which
worked very well apart from the "Regenerators" being susceptible to
lightning.