[GreenKeys] 1949 time division multiplex demo

[Harold Hallikainen]

The tube with a coil around it reminds me of the GE Phasitron (
https://bh.hallikainen.org/index.php/GeneralElectric#:~:text=*-,Phasitron%20%2D,-Dave%20Hershberger%27s%20page
). A very interesting way to generate FM (actually phase modulation).

Harold
https://w6iwi.org


On Tue, October 28, 2025 1:52 pm, Nick England wrote:
> One of the cooler pieces of technology in the FGC-5 was the TTY input
> interface. At the rear left of the CV-81 chassis is a vacuum tube triode
> with a 60mA coil surrounding it…
> https://www.navy-radio.com/rtty/fgc5/fgc5-21.JPG
>
>
> Nick England K4NYW
> Chapel Hill NC
> www.navy-radio.com
>
>
> On Tue, Oct 28, 2025 at 4:17 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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>>>
>>>>>> Jordan Spencer Cunningham's GreenKeys Search Tool:
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>>>>>>
>>>>>>
>>>
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>>
>>
>> --
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>>
>>>>> Jordan Spencer Cunningham's GreenKeys Search Tool:
>>>>>
>> https://teletype.net/gksearch
>>
>>>>> 2002-to-present greenkeys archive:
>>>>>
>> http://mailman.qth.net/pipermail/greenkeys/
>>
>>>>> 1998-to-2001 greenkeys archive:
>>>>>
>> http://mailman.qth.net/archive/greenkeys/greenkeys.html
>>
>>>>> Randy Guttery's 2001-to-2009 GreenKeys Search Tool:
>>>>>
>> http://comcents.com/tty/greenkeyssearch.html
>>
>>
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>>
>>
>


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