[GreenKeys] on-off radio teletype (not FSK)

[Bob kb8tq]

Hi

The gotcha with treating FSK as an FM demodulation problem is that it is not
always transmitted in a phase coherent (true FM) fashion. Even if it *is* transmitted
as such, fading on something as wide as 850 shift likely breaks that up. There
is a whole body of papers that dig into all this. Once you get into a band filled with 
other signals, selectivity *does* make a difference. Break part of it and add the 
jammers …. not so good.  

The “modern” approach is to treat it as two AM signals and go from there …..

If indeed you *are* going to transmit a phase coherent signal, then use phase
modulation. There (obviously) are a lot of people out there doing exactly that.
It works quite well. 

Bob

> On Feb 17, 2018, at 12:27 PM, Jim Haynes <jhhaynes at earthlink.net> wrote:
> 
> And, I might add, while the two-tone limiterless demodulator has well
> known advantages over the limiter-discriminator design, it may be that
> the poor design of ham limiter-discriminator TUs accentuates the
> apparent superiority of two-tone designs.
> 
> If I were still playing with hardware I would build a limiter-discriminator TU along the Don Wiggins ideas and see how well it
> does.
> 
> One thing we have to keep in mind is that the mechanical Teletype is really awful compared with an electronic emulator of any kind.  The
> reason is mainly that a missed STOP pulse lets the selector cam keep
> rotating, which results in several errored characters before things
> get back to normal.  The same happens with a false START pulse.  The
> literature suggests that this problem costs about 15dB in signal to
> noise ratio.  So if you are serious about using a mechanical Teletype
> on the radio, by all means put an electronic regenerator between the
> demodulator and the machine.
> 
> 
> 
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