How to do a frequency band? - matlab

Question

Here in this code i am doing a stft on my wav-file. There is no problem with that. At the beginning, i am defining my parameter, afterwards using my wav file and then applying the stft. Basically what i am doing is a real-time spectral analysis. Anyway my question is, how do i a frequency band? I want my signal to be separated in LOW/MEDIUM/HIGH. I want my vector to be saved, from 0-250 Hz in the LOW-Band, 250-5000 Hz in the MEDIUM-Band, 5-22.05k Hz in the HIGH-Band. I advise you, to try my code in Matlab, if you don't understand it. Just take any wav-file. Btw my signal is plotted in the variable "Yres". Any solution is appreciated!
NFA=2; % Number is used for plotting every 2nd picture
t_seg=0.05; % Length of segment in ms
fftlen = 4096;
% Lenght of "fft",because our segment contains 2205 points
[y,fs]=audioread('UnchainMyHeart.wav');
% audioread = functions reads WAV-file
% y = A vector which contains my audio signal
% fs = sample frequency (44100)
% 'UnchainMyHeart' = WAV-file
t=linspace(0,length(y)/fs,length(y));
% linspace = Creating time vector
% 0 = Start time
% length(y)/fs = End time
% length(y) = Number of samples in y
plot(t,y)
% plotting signal in the time domain
segl =floor(t_seg*fs);
% Applying fft function on the variable "segl"
windowshift=segl/2;
% Defining the size of the window, which goes to the next "segl"
window=hann(segl);
% hann function
window=window.';
si=1;
%Start index
ei=segl;
%End index
AOS= length(y)/windowshift - 1;
% AOS is the number of "segl" we use (About 433)
f1=figure;
% Opening new window
f=0:1:fftlen-1;
f=f/(fftlen-1)*fs;
% Defining frequency vector
Ya=zeros(1,fftlen);
plot(f,Ya),axis([0 fs -90 50])
grid on
n=0;
%start variable
for m= 1:1:AOS
y_a = y(si:ei);
y_a= y_a.*window;
Ya=fft(y_a, fftlen);
n=n+1;
if n==1
Yres=abs(Ya);
else
Yres=Yres+abs(Ya);
end
if n==NFA
Yres=Yres/NFA;
n=0;
drawnow;
%Tut die Grafikobjekte immer auf den neuesten Stand updaten
figure(f1);
plot(f(1:end/2), 20*log10(abs(Yres(1:end/2))));
ylim([-90 50]);
title('Spektrum eines Audiosignal');
xlabel('f(Hz)');
ylabel('dB');
grid on;
end
si=si+windowshift;
% Updating start index
ei=ei+windowshift;
% Updating end index
end

Answer 1

This may not be the best answer! But this may help you get started on something. You can use spectrogram() function from MATLAB's Signal Processing Toolbox.
Let's suppose you have an audio file named ''UnchainMyHeart.wav'(in your case) with one channel. The code goes as follows:
% Reading the audio file
[y1,fs] = audioread('UnchainMyHeart.wav');
% Parameters for STFT (or spectrogram)
windowDuration = 30e-3; overlapDuration = 15e-3;
windowLength = round(windowDuration*fs); % window length
overlapLength = round(overlapDuration*fs); % overlapping of windows
nfft = 1024;
% Executing STFT for the signal
[S1,F1,T1,P1] = spectrogram(x1,hanning(windowLength), ...
overlapLength, nfft, fs, 'yaxis');
S1 and P1 contain STFT and Power Spectrum Density(PSD) of the signal for a time interval of each section with a time interval whose estimations are contained in T1.
For your question, you are looking for F1 which is a vector of cyclical frequencies expressed in terms of sampling frequency, fs. For example: if you have a sampling frequency of 48 kHz (fs) and nfft of 1024, then you will have 513 [(1024/2) +1)] frequency values spaced by (fs/nfft). i.e. 46.875. So your frequency components will be 0, 46.875, 46.875*2, ..., 46.875*512. The maximum you will have is 24 kHz due to Nyquist criterion.
Now, you can easily write a simple routine specifying the ranges as you said. The same technique can be used in your code which is an implementation of stft. I would suggest using MATLAB's built-in function unless your problem requires an implementation. Hope this helps!
If needed, I can answer why the parameters for STFT are chosen as included in the code.