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.