> I'm not sure what a 'good' default value would be, but I'd guess it
> should be something like
> ; two-pole allpass.
> (defun allpass2 (x hz &optional (q 0.7071))
> (multichan-expand #'nyq:allpass x hz q))
The Tietze/Schenk "Electronic Circuits Handbook for Design and Applications",
(what is something like the "basic knowlege book for electronic engineers"),
tells that an allpass filter works best with Bessel defaults, where Bessel
filters are known for producing a least-worse overall phase error.
According to the Tietze/Schenk a second-order Bessel filter defaults to:
a[i] = 1.3617
b[i] = 0.6180
q = 0.58 <- q-factor for the Nyquist allpass2
But please be aware that what an electronics engineer calls "works best",
for a musician's ear often sounds "most boring". :)
An allpass filter produces a frequecy-dependent time-shift and IMO sounds
best if a part of the original signal is added to the allpass output, in
the Audacity Nyquist prompt for example:
(sum (scale wet (allpass2 x hz 0.58))
(scale dry s))
where 'wet' ist the allpass volume, and 'dry' is the original signal.
For excessive usage of comb and allpass filters see the "Reverb Example":
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