[Elecraft] K3 - 250 Hz and 400 Hz Filter Settings

[Lyle Johnson]

> Made me wonder though if one could decode two signals intertwined but
> individual mark and spaces traces in the clear form each other?.  I would
> think it might be possible.  Anybody done this?

RTTY tones have sidebands, and since the Tx logic is not usually shaped 
to move optimally between the tones, but instead tends to switch 
abruptly, each tone has sidebands beyond the minimum necessary.

I've often thought it might be fun to create an FSK demodulator for 
systems that use more spacing than is needed (for 45.45 baud RTTY, that 
is about 30 Hz spacing) that would examine and attempt to demodulate 
using three parallel algorithms:

1) The typical differential amplitude algorithm using dual filters 
feeding a data rate filter and "slicing" the output.

2) A single tone amplitude algorithm using just the space tone filter 
and a reference derived from hi/low values in that filter (or some 
more-sophisticated sliding reference), feeding a data rate filter and 
slicer...

3) The same as 2, but using the mark tone filter only.

In most cases, the first algorithm will give best results of the three.  
But in cases where signals are overlapping, or partially overlapping, or 
there is selective fading or QRM on one tone, the demodulator looking at 
the other tone is likely to give useful copy if the S/N is sufficient.

Implemented in a PC with the three text streams aligned, it could make 
the difference between a contact or not, or a repeat or not.  Stream it 
with the output from your DXP-38, also with text aligned...

73,

Lyle KK7P