I've recently begun using both PSK31 and HRD's DM 780. This leads me to a puzzle. I read that PSK31 signals can be copied well below the noise level; that is, below 0 dB S/N. And, as a user of JT-65, I've often seen this sort of thing. But I notice that signals reported by DM 780 at even (positive) 20-25 dB S/N are often very badly copied. And I have yet to see a signal reported at less than 10 dB, let alone a signal below the noise level. Not only that, but signals only faintly discernible on the waterfall and hardly copyable at all often have S/N values of 20 or even 30 dB. It seems to me that I should be able to more or less accurately copy PSK31 signals at, say, 10 dB. Because PSK31 signals can be copied well below the noise level, 0 dB. But, as I mentioned, that's not the case. I suspect the apparently inconsistency lies with the definition of dB the programmers of DM 780 have followed. Certainly a variety of definitions does exist. But my hypothesis amounts to speculation. Can anyone shed definite light on this apparent inconsistency? Why does DM 780 report such high S/N levels for PSK31 signals? And why are PSK31 signals with pretty generous S/N levels not accurately copied by DM 780?