[GreenKeys] TDD Frequencies

[Harold Hallikainen]

I think minimodem will do it. Here are my notes on using it to generate an
ultrasonic beacon.

Transmit
The below command was used to generate the ultrasonic audio file.

cat data/r.txt | minimodem --tx -5 -M 19900 -S 20100 --stopbits 8 -f
data/BeaconTx10bps20k200.wav 10

cat data/r.txt | pipes the file r.txt into minimodem for transmission.
r.txt just has repeated R characters. R is used since in Baudot, it is
alternating mark and space.
minimodem --tx tells minimodem to transmit (create the audio)
-5 tells minimodem to use 5 data bits per character (Baudot)
-M 19900 sets the mark frequency to 19900 Hz
-S 20100 sets the space frequency to 20100 Hz
--stopbits 8 causes 8 stop bits to be transmitted. The large number of
stop bits guarantees the receiver will not syncronize improperly (treating
a data bit as a start bit).
-f data/BeaconTx10bps20k200.wav causes the output to go to a file instead
of to a local audio output.
10  causes the data to be transmitted at 10 bits per second. The low speed
should make it easier to deal with noise and reflections. With one start
bit, 5 data bits, and 8 stop bits, the data is retransmitted every 1.4
seconds.

The use of 5 bits per character would allow us to identify 32 distinct
auditoriums (0 to 31). This should be sufficient.


On Wed, June 3, 2026 10:52 pm, Gavin Wuchter via GreenKeys wrote:
> Welp lol, now I guess my question is does anyone know of a software modem
>  or other itty thing that can output the original APCOM/PhoneType
> 1400/1800
> Hz Baudot tones?
>
>
>
> Or in theory something that can slow down the audio to be slight lower
> pitch/frequency?
>
> Just a thought trying to be somewhat cost effective for the equipment I
> already have.
>
> Thank you for your help in the search!
> - Gavin
> On Thu, Jun 4, 2026 at 12:43 AM John Lawson via GreenKeys <
> greenkeys at mailman.qth.net> wrote:
>
>>
>>
>> Ooops: manual states Mark/Space as 1400/1800.
>>
>>
>>
>>
>> Close, but no guitar…
>>
>>
>>
>>
>>
>>
>> Cheerz
>>
>>
>> John
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