[GreenKeys] FSK converter madness

[Harold Hallikainen]

Jim,

Thanks for all the great info! In my experiments, I've found that having
the tone filter bandwidth about 1.7 times the baud rate allows the
discriminator to reach full amplitude on alternating bits ( see some eye
diagrams at https://w6iwi.org/rtty/DspTU2/ ). I think the tone filter
bandwidths are largely determining how the system works as opposed to the
low pass filter after envelope detection. I've got that set fairly high (I
don't remember exactly what frequency it is set to).

Limiters are fascinating. Very simple, but as we get into capture effect
and instantaneous frequency, they become quite interesting. A common
method of demodulating FM in modulation monitors for broadcast was
(probably now DSP) to trip a monostable on each zero-cross. As the
frequency is varied, the duty cycle varies (the pulse width is constant,
but the time between the pulses varies). The pulse train is then run
through a low pass filter to recover the modulation. This also would be
easy to do in DSP.

In my initial experiments with impaired audio (
https://w6iwi.org/rtty/DspTU2/#:~:text=of%20153%20mA.-,RYcount,-4/17/24 ),
I got 97% copy (counting Rs and Ys) with "AM mode" demodulation at 170 Hz
shift and 96% in "FM mode" (using a limiter). These were both with two
tone filters instead of a linear FM demodulator. I could convert the two
tone demodulator to a linear FM demodulator by increasing the filter
bandwidth to form a Round-Travis Discriminator.

At 850 Hz shift, I got 100% copy with both AM and FM modes.

I also tested my Flesher TU-170, which uses a limiter and two tone filters
instead of a linear discriminator and got 97% copy, the same as the DSP TU
with 170 Hz shift and a limiter.

I still think that 850 Hz shift will survive selective fading better than
170 Hz shift since both mark and space would not disappear at the same
time with the wider shift.

I'll check your latest list of references (THANKS!). My summary of my
current references is at https://w6iwi.org/rtty/TuReferences.html . I also
link to your list there.

Again, thanks for the comments!

Harold
https://w6iwi.org