Coefficients of digital "classic filters" used in audacity

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alessandro.pisano
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Coefficients of digital "classic filters" used in audacity

Post by alessandro.pisano » Wed Apr 15, 2020 11:21 am

Hi, everyone.
I am an instructor of digital signal processing, and I would like to show to my students that the "classic filters" implemented in Audacity (e.g., low pass Butterworth) correspond to some known discretizazion technique (such as the bilinear method) used to convert the filter's transfer function from the s domain to the z domain.

How can I detect which specific filters' parameters are used in Audacity ?

For example, how can I detect which digital filter corresponds to the low-pass Butterworth filter of a prescribed order and cutoff frequency ? This filter is accessible in the "Filters->Classic filters" menu of Audacity 2.3.0.

Thanks everyone in advance !

Alessandro

DVDdoug
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Re: Coefficients of digital "classic filters" used in audacity

Post by DVDdoug » Wed Apr 15, 2020 8:14 pm

I don't know but Here is the Audacity source code. (It's a rather big project so it might take some digging.)

steve
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Re: Coefficients of digital "classic filters" used in audacity

Post by steve » Wed Apr 15, 2020 8:45 pm

Here's the cpp file for the "Classic Filters" effect: https://github.com/audacity/audacity/bl ... Filter.cpp
and for the Biquad filter that it uses: https://github.com/audacity/audacity/bl ... Biquad.cpp

(The biquad code is probably what you are looking for)
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