Audio - Wah wah effect - 'Integer Operands Are Required for Colon Operator When Used as Index'

Hi, I'm making a 'wah wah' effect in matlab.

I'm getting the error 'Integer Operands Are Required for Colon Operator When Used as Index' in my while loop

while(length(cutoff_freq) < length(input_sound))
    cutoff_freq = [cutoff_freq(max_f:-centre_f:min_f)];
    cutoff_freq = [cutoff_freq(min_f:centre_f:max_f)];
end

How can I fix that?

The full code can be seen below:

% Read the audio file
[input_sound, Fs] = audioread('AcGtr.wav');

% Variables 
dampFactor = 0.05; %The lower it is, the smaller pass band
wahFreq = 1000; % How many Hz per second are cycles through
min_f = 250; % Minimum cut off frequency 
max_f = 5000; % Maximum cut off frequency
centre_f = wahFreq/Fs; % The change in centre frequency per samlple (Hz)

% Triangle wave based on the centre frequency values
cutoff_freq = min_f:centre_f:max_f;

while(length(cutoff_freq) < length(input_sound))
    cutoff_freq = [cutoff_freq(max_f:-centre_f:min_f)];
    cutoff_freq = [cutoff_freq(min_f:centre_f:max_f)];
end

% Make it fit the audio input size
cutoff_freq = cutoff_freq(1:length(input_sound));

% Filter coefficient 
coF = 2*sin((pi*cutoff_freq(1)/Fs));
Q = 2*dampFactor; %Q = quality factor, band width 

% Arrays of zeros for the filters, size fits the input sound
highpass = zeros(size(input_sound));
lowpass = zeros(size(input_sound));
bandpass = zeros(size(input_sound));

% Avoid neagtive signals (multiplying by zero errors) the first is con calculated
highpass(1) = input_sound(1);
bandpass(1) = coF*highpass(1);
lowpass(1) = coF*bandpass(1);


% Differenetial equations 
for n = 2:length(input_sound)
    highpass(n) = input_sound(n) - lowpass(n-1) - Q*bandpass(n-1);
    bandpass(n) = coF*highpass(n) + bandpass(n-1);
    lowpass(n) = coF*bandpass(n) + lowpass(n-1);
    coF = 2*sin((pi*cutoff_freq(n))/Fs);
end

% Normalise audio and play it 
norm = bandpass./max(max(abs(bandpass)));
audiowrite('AcGtr.wav', norm, Fs);
sound(norm,Fs)

Answer 1

[Geoff Hayes]

Ditte - what is the intent of this code

while(length(cutoff_freq) < length(input_sound))
    cutoff_freq = [cutoff_freq(max_f:-centre_f:min_f)];
    cutoff_freq = [cutoff_freq(min_f:centre_f:max_f)];
end

You only enter the body of the while loop if the cutoff_freq array is shorter than the input_sound array...but while you re-assign new values to cutoff_freq, nothing changes on the next iteration of the loop. I think that you want to augment/concatenate the cutoff_freq array with a subset of its data (since you have the []) but you need to include this array in the command. For example, if

x = [1 2 3 4];
x = [x 5 6 7]; % so now x = [1 2 3 4 5 6 7]

As for the error/warning, since max_f:-centre_f:min_f) is an array of doubles, you cannot use them as indices into cutoff_freq...you need to use integers as indices (hence the error message). Do you mean these to be indices or are you trying to augment cutoff_freq with these values like

cutoff_freq = [cutoff_freq max_f:-centre_f:min_f];