Save samples that repeat every 300 ms

Question

I want to sample an audio signal. I have a wav file 5 seconds long. I want save samples every 300 ms.

I use function audioread in MATLAB that read samples containing in file, this is my code:

[signal,fs]=audioread('file.wav');

dt = 1/fs;
N = length(signal);
t = 0:dt:(N-1)*dt;

plot(t,signal)
ms=t*1000;

How can I do save in an array samples that are repeated every 300 ms?

Answer 1

Let's assume your wav file has a sampling rate of fs = 44100 (which is quite common), i.e. you have 44100 samples per second. Now, you want to have one sample each 0.3 s. You have to calculate the corresponding number of samples to "skip" with respect to your original sampling rate, i.e. skip = 0.3 * fs = 13230 in this case. Now, you can simply access each skip'th element in your original signal (and also time interval t).

Here's some code to do the work, and visualize the above:

% Artificial data
signal = sin(linspace(0, 2*pi, 5 * 44100));
fs = 44100;
dt = 1 / fs;
N = length(signal); 
t = 0:dt:(N-1)*dt; 

% "Re-sampling" parameters
dt_new = 0.3; 
skip = dt_new * fs;

% Extract every skip'th value from original time interval and signal
t_new = t(1:skip:end).'
signal_new = signal(1:skip:end);

% Some visualization
figure(1);
hold on;
plot(t, signal);
plot(t_new, signal_new, 'r.', 'MarkerSize', 15);
hold off;

We get the following output, the original signal is blue, the red points are the samples at every 0.3 s:

If you have a look at t_new, you see, that the sampling points exactly match your desired interval of 0.3 s:

t_new =
     0.00000
     0.30000
     0.60000
     0.90000
     1.20000
     1.50000
     1.80000
     2.10000
     2.40000
     2.70000
     3.00000
     3.30000
     3.60000
     3.90000
     4.20000
     4.50000
     4.80000

Hope that helps!

EDIT: There's also a resample function available in the Signal Processing Toolbox. I'm not sure, if this function is useful here, since the new sampling rate would be fs_new = 1 / 0.3 = 3.3333, but resample only accepts integers. Maybe, there's another more sophisticated (toolbox) function to do the job automatically.