Composite System Object for Code Generation or use in Simulink

I've built a composite System Object with the goal, to provide a 'parallel' FIR filter with a parameterizable number of parallel channels to my Simulink model:

classdef ParallelFIR < matlab.System
    % parallel_fir takes a matrix of taps and applies them to matrices of
    % inputs, where each matrix column is a channel

    % Public, tunable properties
    properties
        Taps = [0.5 0.5; 0.5 0.5];
    end

    properties(Access = private)
        fir_channel
    end
    
    methods
        function obj = ParallelFIR(varargin)
            if nargin == 1
                setProperties(obj, 2, 'Taps', varargin{1});
            else
                setProperties(obj, nargin, varargin{:});
            end
        end
    end

    methods(Access = protected)
        function setupImpl(obj)
            c = size(obj.Taps, 1);
            obj.fir_channel = cell(c, 1);
            for i = 1:c
                obj.fir_channel{i} = dsp.FIRFilter(obj.Taps(i,:));
            end
        end
        
        function validateInputsImpl(obj, in)
            assert(size(in, 1) == size(obj.Taps, 1), "taps and input are required to have the same number of channels");
        end

        function y = stepImpl(obj,u)
            y = zeros(size(u));
            for i = 1:size(obj.Taps, 1)
                y(i,:) = step(obj.fir_channel{i}, u(i,:));
            end
        end

        function resetImpl(obj)
            for i = 1:size(obj.Taps, 1)
                reset(obj.fir_channel{i});
            end
        end
    end
end

Whenever I try to use the system object in a Simulink MATLAB System Block or to generate C++ code for a script using the system object, I get the following error:

Cannot compute constant value for argument #1 for this System object constructor. All arguments to the constructor of this System object must be constants for code generation. One way to supply a constant value is to pass it to the main function using Simulink non-tunable parameters (for MATLAB Function block) or coder.newtype('constant',...) (for MATLAB Coder). The error occurred for MATLAB System block 'Subsystem/MATLAB System'. See line 29, column 38 in file 'ParallelFIR.m'.

Line 29 is the one with

                obj.fir_channel{i} = dsp.FIRFilter(obj.Taps(i,:));

How can I implement the parallel FIR to be usable for Simulink System Blocks?

Answer 1

[Honglei Chen]

If you don't intend to change your Taps during the simulation, you may want to consider setting it as Nontunable, like

    properties (Nontunable)
        Taps = [0.5 0.5; 0.5 0.5];
    end

Also it may be helpful to share your code generation script as well as your code generation command.

HTH

[Constantin Runge]

I think this works for now, thank you.

What will be required when I need to change the taps mit-simulation. For example with a syntax similar to

y = fir(x,coeff)

from the dsp.FIRFilter docs but in Simulink (then of course with an additional input specifying the current taps)?

[Honglei Chen]

If you look at the implementation of dsp.FIRFilter, you can see that they have nontunable properties like NumeratorSource to switch between specifying coefficients via property or specifying coefficients via input. I will recommend you do the same thing and then based on that you can use the corresponding syntax of dsp.FIRFilter.

This being said, if your only needs is to run multiple filters at the same time in Simulink, I think the Discrete FIR Filter block already supports specifying multiple filters. You may want to take a look at its documentation page

https://www.mathworks.com/help/simulink/slref/discretefirfilter.html

HTH

[Constantin Runge]

Thank you very much. Where can I find the objects implementation?

Yes, the Discrete FIR Filter Block does support multiple filters. But only for sample-based processing. I am working in a frame-based domain. Thus I believe I have to implement my own multi-channel FIR.

[Honglei Chen]

Oh, sorry, I mean the doc

https://www.mathworks.com/help/dsp/ref/dsp.firfilter-system-object.html

See where they talked about NumeratorSource.

HTH