Can zp2sos be used in an audio plugin?

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

[Nathan Lively]

Here's the console.

[Nathan Lively]

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)

Answer 1

[Nathan Lively]

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.

Answer 2

[Asvin Kumar]

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

[Nathan Lively]

Thanks Asvin! I didn't catch that. Instead I tried this:

function reset(p) % Initialize internal state
            p.state = zeros(2);
            Fs = getSampleRate(p);
            FcNormalized = 1000/(Fs/2);
            [z,poles,k] = butter(2,FcNormalized);
            p.sos = zp2sos(z,poles,k);
        end

Unfortunately, this is the error. Looks like it is not possible.

[Nathan Lively]

I'm looking through the documentation, but is there some place that lists all of the functions are are, in fact, supported?

[Asvin Kumar]

You can find details about functions that support code generation from the Signal Processing Toolbox here: Code Generation and GPU Support - MATLAB & Simulink (mathworks.com)

[Nathan Lively]

Thanks so much Asvin!