[GreenKeys] 1949 time division multiplex demo

[Eric Moore]

Funny, I just snapped pics of this guy today.

See the magnets along the top? There is a manual online I found, but not
expecting much luck resurrecting it considering the layer of rust.

-Eric

On Tue, Oct 28, 2025, 3:18 PM Harold Hallikainen via GreenKeys <
greenkeys at mailman.qth.net> wrote:

> Before microprocessors, stuff was sure complicated! Though I guess we have
> now moved the complexity to the chip and the software.
>
> I remember first reading about time division multiplexing for data so you
> could put multiple terminals on one modem. Then I read about "statistical
> multiplexing" where time was only devoted to the "channel" that had
> anything to send. I think this was the start of packet switching. Put a
> header on the data to be sent and dump it into the FIFO buffer that drove
> the modem. The header was decoded at the other end to send stuff to the
> right destination.
>
> They did amazing stuff with mechanical and vacuum tube equipment.
>
> Harold
> https://w6iwi.org
>
>
> On Tue, October 28, 2025 12:11 pm, Nick England via GreenKeys wrote:
> > https://www.navy-radio.com/rtty/fgc5/fgc5-proto-4901.jpg
> >
> >
> > photo caption - Electronic Time Division Multiplex Unit with attached
> > battery of four Teletype transmitters. The Electronic Time Division
> > Multiplex rearranges the output of two to four teletype machines so that
> > the signals originated by those machines are transmitted over a single
> > radio channel by a single radio transmitter, simultaneously for all
> > practical purposes. The apparatus is divided into transmitting and
> > receiving units. As messages are fed into a battery of four teletype
> > transmitters hooked into the ETDM each is compressed in time, sorted into
> >  the proper time channel, mixed with the other signals, and fed into the
> > radio transmitter. At the receiving end, the process is reversed and the
> > signals, unscrambled, are fed into the teletype printers essentially at
> > the same time as they are transmitted. Since each teletype transmitter
> can
> > send at speeds of 75 words per minute, this means a total of 300 words
> per
> >  minute transmitted. The Electronic Time Division Multiplex is calculated
> > to make the best possible use of available radio frequency channels in
> > military operations.
> >
> > The sign in the photo says that it is being tested on a 6000 mile RTTY
> > loop DC-SF-DC.
> >
> >
> > More info on this AN/FGC-5 system is at
> > https://www.navy-radio.com/rtty-mux-fgc5.htm
> >
> >
> > Nick England K4NYW
> > Chapel Hill NC
> > www.navy-radio.com
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> >
> >
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> >>>>
> >
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>
>
> --
> Not sent from an iPhone.
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>
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