I think you are dealing with a psycho-acoustic phenomenon related to having
a very narrow bandpass filter, where the narrowness is less than 10% of the
center frequency..
If a narrow 200Hz filter is engaged at 800 Hz center frequency, there is
the psycho-acoustic impression of ringing artifacts. Exact details of the
ringing depends on how poor the group delay response variation of the
filter is - a 200 Hz narrow brick wall filter will have very pronounced
ringing artifacts. Due to the low center frequency we notice the ringing
artifacts and think of them as filter ringing in response to impulses.
But if that same brick-wall narrow filter is engaged at 2125 Hz center
frequency, the ringing artifacts sound different to our ears. It is the
same phenomenon, but it can sound like a continuous noise and we don't
really notice how it's impulse driven, but really the way I think about it
is as if a bell is getting hit with hundreds of little hammers. Yes,
psycho-acoustically, this drives me nuts!
I've generally found that for RTTY, the MMTTY and (even more so) 2Tone
decoders work much better with 500Hz or broader receiver filters. Enabling
narrow brick wall filters usually does not work out very well. Yes I had
some battles over run frequencies in NAQP RTTY too! 2Tone can work very
nicely with wide receiver bandwidths, as long as you disable AGC or turn
the RF gain knob down enough, such that strong adjacent signals don't pump
receiver gain.
Tim N3QE
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