For loop for Sine wave function

I have a code for a swept sine wave function.

I was wondering, could the Line 22 'Variable appears to change size on every loop iteration..." be warded off somehow.

Also; if anyone knows how to perform FFT analysis on this script I would be most appretiate a little help.

I'm looking to produce a graph to show impluse response with respect to time and frequency response.

My code is:

T=5; %size of window
fs=44100; %sampling frequency
df=1/T; %frequency res
dt=1/fs; %time resolution
t=(0:+dt:T-dt); %time vector
df_t=500; %swept rate (Hz/seconds)


for i=1:+1:length(t)
%i=i+1;

    if(i==1)      %initialise f and t.
        f=20; ti=0;  
    else    

    ti=ti+dt; %time increment

    f=f+df_t*dt; %freq increment 
    end

    
    w=2*pi*f; %omega
    sweptsin(i)=sin(w*ti); %swept sine wave

end

plot(t, sweptsin)
xlim([0 0.1])

sound(sweptsin, fs)

Thanks

Answer 1

[Voss]

Yes, you can avoid that warning by pre-allocating sweptsin to the size it needs to be, which is the same size as t:

T=5; %size of window
fs=44100; %sampling frequency
df=1/T; %frequency res
dt=1/fs; %time resolution
t=(0:+dt:T-dt); %time vector
df_t=500; %swept rate (Hz/seconds)


% pre-allocate:
sweptsin = zeros(size(t));


for i=1:+1:length(t)
    %i=i+1;
    
    if(i==1)      %initialise f and t.
        f=20; ti=0;  
    else    
        ti=ti+dt; %time increment
        f=f+df_t*dt; %freq increment 
    end
    
    w=2*pi*f; %omega
    sweptsin(i)=sin(w*ti); %swept sine wave
    
end

plot(t, sweptsin)
xlim([0 0.1])

However, you may want to consider doing the calculation without a for loop at all:

sweptsin = sin(2*pi*(20+df_t*dt*(0:numel(t)-1)).*t);

plot(t,sweptsin)
xlim([0 0.1])

[David Kendal]

This is fantastic, thanks I did read about the zeros function but wasn't sure how to implement it correctly. I am also trying to perform an FFT function showing the amplitude spectrum of X with respect to time like this, using this link https://uk.mathworks.com/help/matlab/ref/fft.html I can't seem to get the graph working its showing me a blank graph.

[Voss]

You're welcome!

Regarding the fft, maybe start a new question, and include the code you're running.

[David Kendal]

https://uk.mathworks.com/matlabcentral/answers/1725585-fft-analysis-of-sine-sweep

Thanks

Answer 2

[Jan]

The warning is solved is solved by a pre-allocation. Insert before the loop:

T    = 5;    ´     % size of window
fs   = 44100;      % sampling frequency
% df   = 1 / T;    % frequency res  [not used]
dt   = 1 / fs;     % time resolution
t    = 0:dt:T-dt;  % time vector
df_t = 500;        % swept rate (Hz/seconds)

sweptsin = zeros(1, length(t));  % Pre-allocation

% A cleaner version of the loop:
f = 20;   
for i = 1:numel(t)
    w           = 2 * pi * f;    % omega
    sweptsin(i) = sin(w * t(i)); % swept sine wave
    f           = f + df_t * dt; % freq increment 
end

A more Matlabish way is to omit the loop:

f    = zeros(size(t));
f(:) = df_t * dt;  % All values
f(1) = 20;         % Overwrite first element
f    = cumsum(f);  % Loop is hidden here
w    = 2 * pi * f; % omega
sweptsin = sin(w .* t);

And even easier:

f = df_t * t + 20;
w = 2 * pi * f;    % omega
sweptsin = sin(w .* t);

[David Kendal]

Thanks for this I haven't got to this yet but will try it - any ideas on how I go abouts implementing fft graphs with respect to time and frequency?

https://uk.mathworks.com/help/matlab/ref/fft.html

I have tried using this help page but not getting anywhere as the graphs I have produced have not worked at all. Any help appreciated.