[PD] could vanilla borrow iemlib's hi pass filter recipe?

Christof Ressi christof.ressi at gmx.at
Sat Oct 15 02:34:30 CEST 2016

There are a number of big problems with all build-in filters in Pd (expect for the raw filters). 

Problem number 1:
[lop~] and [hip~] both use a weird (you could also say: wrong) formula for the cutoff frequency which makes them gradually converge to a fixed output state (reached by about 7000 Hz). The same is true for [vcf~] and [bp~] with Q <= 1. Therefore the actual cutoff frequency is only correct for very low frequencies and approximately gets more and more off until it doesn't move at all.

Problem number 2:
[bp~] and [vcf~] don't have zeros at DC and Nyquist. For low Q values, the slope is different for each side and changes with frequency.

Problem number 3:
the gain at the center frequency is not 1 for both [bp~] and [vcf~]. It rather depends on frequency and Q. [bp~] even has has a gain of 2 for Q <= 1! 

I did some FFT plots, see the attachment.

I remember Miller saying somewhere that these filters are not designed for high cutoff frequencies - but even for low frequencies, the behaviour of [bp~] and [vcf~] is horrible. I can see these filters are mere approximations to reduce CPU usage.
[hip~] is indeed much more efficient than iemlib's [hp1~], so it's well suited for DC removal (but not much else).
[bp~] only is a little bit more CPU friendly than iemlib's [bp2~] - but the latter one has a correct and stable frequency response.
[vcf~], however, is a real CPU sucker!!! 100 [vcf~] objects need 3,40% on my laptop whereas 100 of iemlib's [vcf_bp2~] only need 1,80%! But you have to consider that [vcf_bp2~] not only acts correctly but lets you set the Q at audio rate. The high CPU usage of [vcf~] seems like a bug to me...

I only use the vanilla filters for the most basic stuff like DC removal and smoothing. I guess these are the use cases which Miller had in mind and that way [lop~] and [hip~] have their justification (although there should be some more warning about the 'wrong' frequency response in the help file). 
But [bp~] and [vcf~] are almost unusable IMHO and should probably be replaced by better filters in the future (while keeping the old ones for compatibility reasons).


> Gesendet: Freitag, 14. Oktober 2016 um 23:51 Uhr
> Von: katja <katjavetter at gmail.com>
> An: pd-list <pd-list at iem.at>
> Betreff: [PD] could vanilla borrow iemlib's hi pass filter recipe?
> In pd 0.47.1 [hip~] is still not perfect. Attenuation at cutoff is not
> constant over the frequency range: -6 dB with cutoff=SR/8, -3 dB with
> cutoff=SR/4, 0 DB with cutoff=SR/2. In contrast, iemlib's [hp1~] has
> -3 dB at cutoff consistently.
> Could vanilla pd implement iemlib's hipass filter recipe? I don't know
> if the license also covers the math. Documentation in
> https://git.iem.at/pd/iemlib/tree/master points to external literature
> for part of the math (bilinear transform). I've implemented the recipe
> with vanilla objects for comparison, see attached.
> Katja
> _______________________________________________
> Pd-list at lists.iem.at mailing list
> UNSUBSCRIBE and account-management -> https://lists.puredata.info/listinfo/pd-list
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