Greetings, It sounds like there is a bug. The bilinear transform preserves stability, but it can get into trouble with "delay-free loops" when there is feedback around what is transformed.
Nowadays, I would digitize an analog circuit using Jatin Chowdhury's latest code, e.g., https://github.com/jatinchowdhury18/KlonCentaur.git https://github.com/jatinchowdhury18/wdf-bakeoff (It's still using bilinear transforms, but element by element, for the most part, resolving delay-free loops using Wave Digital Filter ideas) On Thu, Jul 8, 2021 at 6:13 AM Jean Pierre Cimalando via Faudiostream-users <faudiostream-users@lists.sourceforge.net> wrote: > Hello, > > recently I implemented and published a music plugin with faust [1], > which implements the digital model of a bass tone circuit. > > A problem encountered during this process is the discretization of the > EQ section, located on the bottom part of the schematic [2], running > from C25 to C20 not included. > > This circuit describes as a low-frequency peak filter (+ or - gain), > combined with a high-frequency shelf, parameterized by a pair of coupled > knobs labelled as bass and treble. > > I have produced a program for computing the expression for the analog > transfer function [3], and applying the bilinear transform. > While the analog transfer function matches the SPICE results, I observed > the digital filter does not meet the criterion for stability (that is, > poles inside the unit circle). > > There is faust source code [4], the unstable EQ is replaced with an > adhoc substitute having a relatively matching transfer function. > > Any hints on producing a more valid discretization of this system? > Thank you very much > > [1] https://github.com/jpcima/Bass21 > [2] https://blog-imgs-91-origin.fc2.com/d/r/u/drugscore/02_49_4SABDsch.png > [3] https://git.io/JcQi5 > [4] https://git.io/JcQPy > > > _______________________________________________ > Faudiostream-users mailing list > Faudiostream-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/faudiostream-users > -- "Anybody who knows all about nothing knows everything" -- Leonard Susskind
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