sosfit in filter function

[Renato S. Yamane]

Hi,

There is a numerical instability on butter function, as described in https://www.mathworks.com/help/signal/ref/butter.html (topic "Limitations").

Is recommend use [z, p, k] syntax and sosfilt function to apply a second-order section filter, as described in https://savannah.gnu.org/bugs/?53855

But, the documentation of SOSFILT is not clear: https://octave.sourceforge.io/signal/function/sosfilt.html

Someone have an idea about how to use ZPK Design instead a TF Design in this code below?

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n = 5;

hpf = 50;

lpf = 10*hpf;

sampling_rate = 44100;

typenoise = noise(60*sampling_rate, 1, 'pink');

[b,a] = butter(n, [hpf/(sampling_rate/2), lpf/(sampling_rate/2)]);

filtered = filter(b, a, typenoise);

audiowrite (filename.wav, filtered, sampling_rate);

freqz (b, a, 4096, sampling_rate)

ax = findall (gcf, 'type', 'axes');

set (ax, 'xlim', [hpf/4 lpf*4]);

set (ax, 'xscale', 'log');

subplot (2, 1, 1);

set (gca, 'ylim', [-35 3]);

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Best regards,

Renato