Simulink - Constant as an argument to a MATLAB function block

I am attempting to use upfirdn in a MATLAB function block, as in the following example, where x is a signal and h is an input from a constant block:

function y = useupfirdn(x, h)

% ... Initialize output, etc.

% ... Do more stuff...

y = upfirdn(x, h);

When I run the model, I get an error that says "The filter H must be constant." The same happens when it is used as a parameter.

I do not get the error when I build the filter using firls within the function block. However, the point of having it as an argument or a parameter is that I compute the coefficients prior to running the simulation, which would save time since the block is called often.

Is there something that I am missing?

Answer 1

[Sebastian Castro]

Unfortunately, this is a documented limitation:

https://www.mathworks.com/help/signal/ref/upfirdn.html#d120e192101 (expand the C/C++ Code Generation section)

It seems like most of the FIR filter functionality in DSP System Toolbox has this limitation, so it's not specific to the upfirdn function.

Since you're trying to do this from a MATLAB Function block, I would recommend using the Simulink blocks for upsampling, downsampling, and filtering instead? Specifically, the filters can accept coefficients from an input port:

• https://www.mathworks.com/help/simulink/slref/discretefirfilter.html
• https://www.mathworks.com/help/dsp/ref/upsample.html
• https://www.mathworks.com/help/dsp/ref/downsample.html

More generally, there's a whole page showing how to implement these in Simulink:

https://www.mathworks.com/help/dsp/multirate-and-multistage-filters.html

- Sebastian