[GreenKeys] Feeding ITTY to the teletype; simulating M33/35 teletypes hooked to computers like in the 70s

[epvgk at limpoc.com]

On Tue, Aug 28, 2012 at 08:55:12PM -0600, Paul Heller wrote:
> 
> 
> My aim is to build a current loop circuit on that breadboard and write some software to grab the ITTY feed from the internet and process that feed into the mark and space for the current loop. Maybe this can be switchable for 20ma or 60ma. The software aspect has me a bit nervous. My C and C++ skills are rusty, my linux skills non-existnat, and I don't really know how to process audio. 
> 

Not sure how much help I can be, but a friend and I wrote some C code that 
can successfully decode ITTY audio into binary data using the fast fourier
transform, and also code to decode the binary data into a baudot stream
and/or convert it to ascii. You're welcome to that if it will help. 

The code is (barely) fast enough to work on even an 8bit AVR at 20 mhz so
I imagine any of those presumably arm-based mini linux devices will have
plenty of horsepower to spare. 

I found that the main drawback to this is the hardware I was experimenting
with lacks the A/D dynamic range to work very well so you have to get the
signal level exactly right for it to work well. Possibly on something like
the hardware you're using that will be less of an issue. (I was using a
"10 bit" A/D on AVR which is really maybe 9 bits deep if you're lucky.) 

eric


> My dream would be to have a device to give you folks so you could operate ITTY on your teletypes without needing your computer nearby. Not sure if anyone would be interested in it, but it is a dream.
> 
> 
> Does anyone see why this idea would not work? Any comments or suggestions?
> 
> Paul
> KD0DFR

It seems totally plausible to me. Assuming you can get internet access 
preconfigured (does it use Wifi? or just ethernet?) you should be able to
make it work with some variant on my FFT processor and the standard linux
utilities. I don't know anything about how you would toggle an output line
from linux in realtime to key the TTY loop, but possibly if the board has
a suitably flexible UART you could just send the decoded baudot characters
to that and let it handle it. That would also let you do additional tricks
like have it decode a 45 baud signal from ITTY and output a higher speed
signal, or ascii :) 

Another possibility, if you want to keep it simple, is just set up the PI
or beagleboard to stream ITTY over the internet and output audio, and then
depend on an external demodulator to actually decode it and feed the tty
loop. 


eric