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All Pass filter query
October 11, 2013 - 1:17 am
Member Since: October 10, 2013
Forum Posts: 14
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Hi Will

In your Book (page 188) for All pass filter design, you have the 1st and 2nd order coefficient calculations, but I've noticed on the 2nd order, there is an extra 'theta' coming form somewhere. what does this theta represent? Its the calculation to make BETA

This is one thing I need to rectify for my plugin, AP1 works great and a betatester has analysed and confirms 90 degrees

Please advise, thanks

W Pirkle
October 11, 2013 - 8:15 pm
Member Since: January 29, 2017
Forum Posts: 573
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The missing theta value is:

thetaC = (2pi)(fc)/fs

Also the coefficient labeled "alpha2" should be "a2" but that is pretty obvious.

The second order APF works great and the Q control changes the steepness of the phase shift. You can make other filters out of this APF also (see the Regalia-Mitra Filters for example):


I have also used it in several of my own "sonic maximizer" type of enhancer effects.

- Will

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