Using a excessively narrow filter on RTTY is a balancing act.
If the bandwidth is too narrow the tones tend to become "smeared"
- the transitions become less easy to detect (in other words,
the "eye" begins to close). However, narrowing the bandwidth
also reduces the amount of QRM/QRN making it through the filter
and to the decoder/detector.
How about using a Nyquist filter?
A Nyquist filter is one that results in no inter-symbol interference.
One of the difficulties with RTTY is that the tone spacings are so wide.
For a 45 baud FSK signal, a shift of about 30 Hz would minimize the
occupied bandwidth and have no intersymbol interference. But standard
practice is to use 170 Hz or 200 Hz shift. [ 200 Hz Shift is about right
for 300 baud FSK, which is what we used (and why we used it) for HF
packet some 25 years ago. And optimized it for common 400 Hz to 500 Hz
wide "CW" crystal filters of that era. ]
Thus, the rise of the dual tone filter (K3) and its work-alikes in
various radios and demodulators, with fairly tight filters around each
tone to pass it along with its significant sidebands. But the overall
IF passband still needs to be wide enough to pass both tones and their
sidebands. I imagine you can observe all of this on your "P1".
Some RTTY decoders take advantage of the tone separation and actually
employ a pair of detectors, one for each tone, and use the outputs
differentially to get some protection from selective fading. 170 Hz
spacing is probably too close for such protection -- 850 Hz spacing
would be better, but that requires an even wider IF passband...
Some soundcard-based RTTY programs support really narrow shifts, but
such signals are not often heard at this QTH. Maybe it is because
everyone is listening and no one is calling CQ?
Enjoy!
Lyle KK7P
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