[GreenKeys] TU for Computer

[Jim Haynes]

Let me suggest you guys who are savvy about DSP
(1) Look at the source code to fldigi
(2) Look at the www.w7ay.net  Chen who runs that site is one of the 
foremost experts on FSK reception and you can find pretty much the
best possible demodulator design ideas there

Then what you want to do is implement the FSK demodulator using DSP
but not necessarily turn it into ASCII for display on the screen.
You do want something in the way of a tuning indicator tho.  Then
having got the demodulated FSK you want to put it through a regenerative
repeater, and in fact a UART will do that if you don't choose to do it
in software.  But software might yield a better performing item.
What you wind up with is a DSP replacement for the hardware demodulator
and regenerator.

Aside from just demodulating the bits, here is what the regenerator is
all about.  The mechanical Teletype has a mechanical selector, the
serial-to-parallel converter, and that is the weakest part of the
whole scheme.  If you get a false start pulse the selector cam starts
rotating falsely and it takes a while for it to get back where it
belongs.  And if the stop pulse is mutilated the selector cam will
keep rotating when it should have latched up, and again will take
several revolutions to get back where it belongs.

Without this the character error rate would simply be about 5 times
the bit error rate since there are 5 bits per character.  But because
of the selector clutching problem a false start or bad stop will cause
several erroneous characters to be printed.

So one thing you would like the regenerator to do is to be sure the
start pulse is at least half as long as it should be, rather than starting
on any little glitch.  And then if the stop pulse isn't there when it
should be the regenerator can stop anyway.

Now to some extent I think you can prevent those glitches anyway if
the bandwidth of filtering is such that they don't get through - so
you filter for the expected duration of correct pulses.

So maybe you can come up with some kind of little box maybe using a
Raspberry Pi which is a single-purpose DSP engine for RTTY demodulation
and essentially replaces the older hardware demodulators.  Or you could
have a program that runs in a PC alongside everything else.

On the transmitting side there is nothing really wrong with just having
the keyboard contacts control an audio frequency shift oscillator and
put that into the SSB transmitter.  Of course you can do that with DSP
also.  What seems popular today is "diddle", where the system transmits
some kind of idle characters when you are not typing on the keyboard.
There is much to be said about that, but I won't go into it right now.

Other than the above hand-waving I'm not doing any work in this field
because for one thing I'm too old to take up DSP programming and for
another RTTY is such a rarely used mode these days for anything but
DX chasing and contesting and I don't do either of those.

Jim W6JVE