[GreenKeys] 1949 time division multiplex demo

[Harold Hallikainen]

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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