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Modified High Pass Filter
February 26, 2021 - 8:02 pm
Member Since: February 21, 2020
Forum Posts: 56
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Hello everyone. 

I noticed that inside Logic I really like the HPF used inside the Clip Distortion Plugin and I sometimes even use it without touching anything else. It seems that the fc value is modified to improve its practicality which is also input gain dependant. I.e. just looking at the 20K setting with a -30dB gain setting , the standard ASPIK high pass equivalent is around the 2300hz range for the same -ve dB setting. 

Looking across some different settings it seems to follow a form of a skewed bell curve which vs the gain input [-30,30].

Does anyone have any experience with this? I have looked at papers for designs which all seem similar so my thoughts are that (excluding table lookup for optimsation later) a governing equation takes the input Feq and Input gain and it returns a 'fixed' fc value which is then used.




August 22, 2021 - 9:44 pm
Member Since: February 21, 2020
Forum Posts: 56
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High Shelving Filter I meant, not high pass. doh.

So I have been playing around with a Q-variable High Shelf filter along with the highshelf inside the IIR plugin. Compared to Logics High Shelving filter I can get close using various parameters but I can find no logical relationship as the frequency and gains change inside the Logic high shelving filter. 

Has anyone else looked into other types of high shelving filters? I am wondering if this is some sort of combination filter?

August 23, 2021 - 2:05 pm
Member Since: June 16, 2021
Forum Posts: 43
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I am not familiar with that plugin, but it sounds like you are either describing "dynamic EQ" or an EQ with a level dependent Q or cutoff. . . Is that somewhat correct? (I don't have a mac to check it out. )

August 29, 2021 - 2:44 pm
Member Since: February 21, 2020
Forum Posts: 56
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Hi Nickolai,

I believe you are right. I had an inkling that there was some sort of fx(Q,FC,DB) relationship which I have tried to systematically investigate.

I have came across some high shelf filter equations that incorporate either a slope or Q factor input which I am now analysing.. Q does not seem to show too many similarities to the Logic High shelving filter yet but today I am looking into the slope instead.

October 11, 2021 - 2:25 pm
Member Since: January 29, 2017
Forum Posts: 689
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What you are probably talking about are 2nd order shelving filters. The majority of ordinary shelving filters are 1st order and therefore have no Q control. The addition of the 2nd order degree-of-freedom does complicate the design significantly and will produce both a resonance and anti-resonance (notch) in the shelf edge. In addition, that added degree in the polynomial complicates the design significantly so I have avoided it. 


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