Can zp2sos be used in an audio plugin?

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I'm getting this error:
Unable to perform assignment because dot indexing is not supported for
variables of this type.
Error in filterPlugin/reset (line 21)
p.sos = zp2sos(z,p,k);
Here's my plugin code. I'm trying to keep it super simple for testing.
classdef filterPlugin < audioPlugin
% Basic IIR Butterworth lowpass for testing
properties % Public interface
end
properties (Constant) % Define the plugin interface
PluginInterface = audioPluginInterface();
end
properties (Access = private) % Internal State
sos = zeros(1,6);
end
methods
function out = process(p, in)
[out] = sosfilt(p.sos,in);
end
function reset(p) % Initialize internal state
Fs = getSampleRate(p);
Fc = 1000; FcNormalized = Fc/(Fs/2);
[z,p,k] = butter(2,FcNormalized);
p.sos = zp2sos(z,p,k);
end
end
end
  2 Comments
Nathan Lively
Nathan Lively on 7 May 2021
And here's the original MATLAB code used to make the plugin.
Fs = 48000;
Fc = 1000;
FcNormalized = Fc/(Fs/2);
[z,p,k] = butter(2,FcNormalized);
sos = zp2sos(z,p,k);
% out = sosfilt(sos,y);
fvtool(sos)

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Accepted Answer

Nathan Lively
Nathan Lively on 11 May 2021
Go to Digital and Analog Filters — Functions section and filter it by C/C++ code generation and you will find that zp2sos is not supported.

More Answers (1)

Asvin Kumar
Asvin Kumar on 10 May 2021
The reason this error occurs is because you're using p as the "this" input argument and then overwriting it by the output of the butter function which is a column vector of complex numbers. Hence why the assignment fails. Use a modified version of the reset function to workaround this error.
function reset(this) % Initialize internal state
Fs = getSampleRate(this);
Fc = 1000;
FcNormalized = Fc/(Fs/2);
[z,p,k] = butter(2,FcNormalized);
this.sos = zp2sos(z,p,k);
end
  4 Comments

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