Some of the ringing is truly psychoacoustic. The harder the edges of the
filter (especially the Kenwood 270 Hz filter used in the TS-430) the
more the noise coming from the filter has a pitch. and the harder it is
to separate that noise of a pitch from a CW signal centered in that
pitch, low or weak. I've found when forced to use that filter for CW
(filters weren't selectable independent of mode in the 430, I'm glad it
wasn't my radio) I could only work FD for an hour or so until the
fatigue of that task wore me out.
I don't have a 250 Hz Tentec filter, but since I can screw the BPT on
the Corsair II with the 500 Hz filter and effectively get a cut off
signal completely when the filters don't over lap it does well. When I'm
talking about ringing, I'm thinking of the tone from the noise, blurred
high speed CW (tones hang on into the spaces), and lighting static
turned to crashes. I'd not be surprised if the narrow Kenwood filter
didn't sound like a tone when hit by lightning crashes. I've not
exercised my Collins 300 Hz CW filter that much to have noticed
lightning induced ringing, though it didn't bother me with the pitch of
narrow noise either last time I listened through it.
I have a 400 Hz INRAD filter in my TS-130 and it starts to blur CW at
about 30-35 wpm. Higher speeds are much clearer with the SSB filter.
The really narrow band settings of my DSP-59+ show tones from noise
drive but that's with bandwidths under 100 Hz. They do help separate
signals from noise even there and the 100 Hz DSP filter will handle
about 30 WPM. Its easier to make a linear phase filter with the DSP and
so avoid ringing so much. I think I detect that the noise rejection of
the DSP is better with the SSB filter going in even when using a narrow
CW filter in the DSP, than with the CW filter in the rig. No doubt the
wider noise bandwidth gives the DSP more uncorrelated noise to reject
and its algorithm does a better job of it.
I find myself taking the stock SSB filter of the Corsair II and using
the band pass tuning to remove highs or sometimes my external passive
audio filter (no DSP induced noise through it!) with a 3 dB cutoff of
1850 Hz and like that better for 75m SSB.
Really good ears can copy CW as much as 30 dB below a reasonably broad
band noise level. The rest of us need filters to help achieve that,
though I've worked signals on 2m CW that I couldn't have proven that
were there by any audio recording, I hardly believed them myself, but
the QSL card arrived a few days later. I kept asking myself during the
contact, "Am I really hearing that signal?" since in the second day of a
contest with one operator its easy to imagine signals, and log them with
the power off.
73, Jerry, K0CQ
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