[Elecraft] APF vs 10 Hz DSP, why they don't sound the same

[David Woolley (E.L)]

Eric Swartz -WA6HHQ, Elecraft wrote:
> 
> If this is set well above what is really needed, the DSP may generate 
> some distortion on really strong signals. We usually recommend +4 dB max.

I wonder if distorting the audio is really what APF is about. With a 
narrow enough filter, the distortion products will only be based on the 
signal.  Alternatively, and more worrying, it may be about defeating the 
ear's defences against load sounds, by making the spectrum too narrow, 
in which case there may be a risk of burning out the hair cells for the 
chosen frequency.

More generally, the APF proponents seem to have the problem that they 
treat it almost as magic.  If they could characterise what it does in 
terms of filtering, AF gain adjustment, distortion etc. it should be 
possible to simulate it in DSP, subject only to having enough processing 
power.  In particular, peaking just seems to me to mean turning up the 
AF gain in parallel with reducing the bandwidth. If that is all that it 
is doing, and if the K3 has soft AF gain controls, a plugin box could 
issue both commands in parallel.

PS the relevance of AGC here is whether or not it is applied pre- or 
post- the ultimate selectivity.  From what the APF people say, it is 
normally applied pre-.
-- 
David Woolley
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