Use a Variable-Size Signal in a Filtering Algorithm

Open Live Script

This example uses a variable-size vector to store the values of a white noise signal. The size of the vector can vary at run time because the functions prune the signal values by:

Filtering out signal values that are not unique to within a specified tolerance of each other.
Averaging every two signal values and outputting only the resulting means.

In this model, a Band-Limited White Noise block generates a set of normally distributed random values as the source of a white noise signal. The MATLAB Function block Filter filters out signal values that are not unique to within a specified tolerance of each other. Then, the MATLAB Function block Average outputs the average of a specified number of unique signal values. The Scope blocks display the output from the Filter and Average blocks. Open the model to see the configuration.

model = "emldemo_process_signal";
open_system(model);

Inspect the Source Signal

Open the Band-Limited White Noise block to view the properties of the source signal. The size of the Noise power parameter defines the size of the array that holds the signal values. This array is a 1-by-9 vector of double values.

Inspect the `Filter` MATLAB Function Block

Open Filter to inspect the filtering function. Filter filters out unique signal values that are not within a tolerance of 0.2 of each other. The function calls an external MATLAB® function file, emldemo_uniquetol.m, to filter the signal values. The function passes the 1-by-9 vector of white noise signal values as the first argument and the tolerance value as the second argument.

function y = uniquify(u)
y = emldemo_uniquetol(u,0.2);

Open the MATLAB function file emldemo_uniquetol.m to view the code for the external function, emldemo_uniquetol. emldemo_uniquetol returns the filtered values of A in an output vector B so that abs(B(i) - B(j)) > tol for all i and j.

function B = emldemo_uniquetol(A,tol) 
%#codegen

A = sort(A);
B = A(1);
k = 1;
for i = 2:length(A)
    if abs(A(k) - A(i)) > tol
        B = [B A(i)];
        k = i;
    end
end

At each time step, the Band-Limited White Noise block generates a different set of random values for A, and emldemo_uniquetol may produce a different number of output signals in B. Therefore, y must be variable size. In order for y to be variable-size, you must enable the Variable size property. In this example, Variable size is enabled for y. In Filter, open the Symbols pane and Property Inspector. In the Function tab, click Edit Data. In the Symbols pane, click y to view the properties in the Property Inspector. For variable-size outputs, you must specify the Size property as the maximum-size upper bound. In the example, Size is [1 9].

You must enable the Support variable-size arrays property in the MATLAB Function block to enable the Variable size parameter for outputs.

Inspect the `Average` MATLAB Function Block

Average averages the values filtered by Filter by following these conditions:

If the number of signals is greater than 1 and divisible by 2, then Average averages every consecutive pair of values.
If the number of signals is greater than 1 but not divisible by 2, then Average drops the first value and averages the remaining consecutive pairs.
If there is exactly one signal, then Average returns the value unchanged.

Open Average to view the code.

function y = avg(u)

if numel(u) == 1
    y = u;
else
    k = numel(u)/2;
    if k ~= floor(k)
        u = u(2:numel(u));
    end
    y = emldemo_navg(u,2);
end

The avg function calls the external MATLAB function emldemo_navg to calculate the average of every two consecutive signal values.

function B = emldemo_navg(A,n) 
%#codegen

assert(n>=1 && n<=numel(A));

B = zeros(1,numel(A)/n);
k = 1;
for i = 1 : numel(A)/n
     B(i) = mean(A(k + (0:n-1)));
     k = k + n;
end

Both u and y are variable-size. You do not need to explicitly define u as variable-size, because u is an input. The output y is declared as a variable-size vector because the number of elements varies depending on the size provided by u. Inspect the properties of y to confirm that it is variable-size.