On 6/30/07 10:44 PM, in article
8bHhi.2620$Od7.1551@newsread1.news.pas.earthlink.n et, "DTC"
<no_spam@move_along_folks.foob> wrote:

> 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.

The O Carrier systems went from a low of about 32 kHz up to 164 kHz if I
remember right. And the mainstay of long-haul communications (L Carrier)
channel bank, was 64 - 108 kHz.

One of the most strange Carrier Systems I worked with was a 1930s vintage H
Carrier, one channel ssb "system" operating at about 12 kHz, and it ran
without automatic synchronization. That was in the 60s. We used it as a
maintenance channel in a voice over data configuration for a gap-filler
radar site. I've never seen a more extreme merging of old and new
technologies.

Don