[Elecraft] K3 APF (audio peaking filter) progress report #2

[Paul Christensen]

> One explanation for why APF improves copy is that it is *not* a brick-
> wall filter: it has a narrow peak, but the skirts are very broad,
> leading to a more "natural" sound.

And the reason why some of us have been stating that roofing filters and 
brick-wall DSP filters, as  good as they are for their intended purpose, 
cannot equate to the results obtained by the APF.

Wayne, any idea as to the "Q" of the equivalent DSP filter?  One typical 
aspect of the simple APF circuit as used in the FT-1000/D is that Q changes 
as F changes.  More sophisticated parametric circuits keep Q relatively 
constant regardless of F.  Perhaps Lyle's implementation in DSP preserves Q 
as F is changed?

I believe the FT-1000/D's APF had a minimum Q of 10 at the lowest F setting, 
then progressively increased to about 30 at the highest setting.

Paul, W9AC