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[AK4YA]

Im talking about DSP features such as  such as noise reduction, bandpass filtering, or notch filter?

Im almost positive that the answer is no for notch filter.  But how about the other two?

[AC9O]

Everything I have read says to NOT use any of that. Use your WIDE filter setting and let the software sort it out. I've run this way for years and have had excellent results, even in my QRN heavy location.

[K0UA]

Well lets look at all three: Of course you don't want to use notch. You would not want to "notch" out any of the frequencies for decoding. Or at least I can't think of a reason.

As for bandwidth controls. Well that one, I have tested with various settings for some time trying to get a better signal to noise ratio at the expense of excluding some decodes, and I never could see a positive effect. So the negative effect of losing decodes on either side of center seems like a big net negative to me.

 Now to the hard one, using DSP. I have tested and wrestled with that one for a long time. As near as I can tell, it does not make much difference in overall decodes. Again I have tested this extensively and I cannot see a positive benefit.

 One negative aspect is putting on the DSP does "fuzz" the streams on the waterfall by adding "fuzzy " looking skirts to the streams in the waterfall. It doesn't seem to hurt anything much other than the looks, but there doesn't seem to be a reason to do it either. At least these are my findings with both the IC-7300 and the IC-7610.

If someone can come up with some other conclusion, I would be happy to learn about it.

[WO7R]

My Flex 6400 has never profited from those things on any digital mode, especially FT8.

The code seems to expect raw signals in all their glory and processing them either doesn't help or fools the math trying to decode them.

[WA3SKN]

Back to basics...
There are two main "digital modes".
First is we shift the transmit carrier. (CW is on/off carrier).  RTTY is an example.
Second is we use two separate audio frequencies in the voice channel (300-2700Hz) like a modem.
DSP acts on this audio spectrum.  AND usually adds distortions that are un-wanted.  It is best to keep the audio channel "as-is" in the radio, and let the modem (sound card?) do whatever filtering is required.
Before sound cards, the signals were converted by the modems/TNCs to serial data.
Ahhh... the "Good Old Days"!
The 1988 ARRL Handbook chapter 19 covered this very well, the newer Handbooks do not.

-Mike.

[VA3VF]

Im talking about DSP features such as  such as noise reduction, bandpass filtering, or notch filter?

For FT8...

No, no and no.

What to use: wide filter.

What to experiment with: AGC. Try disabling it, or at least change it to slow.