STFT computation without using spectrogram function! - matlab

Question

I was trying to plot STFT using plot3 in MATLAB but failed. Can somebody guide me how to do that? My MWE is given below:
%% STFT Computataion
clear; clc; clf;
%% Get input and calculate frame size and overlap/shift
[Y,Fs]=wavread('D_NEHU_F0001_MN_10001');
frame_size=round(20*Fs/1000); % calculate frame size for 20ms
frame_shift=round(10*Fs/1000); % calculate frame shift for 10ms
%% Plot the input signal in time domain
t=1/Fs:1/Fs:(length(Y)/Fs);
subplot(2,1,1)
plot(t,Y);
title('Speech signal in time domain');
ylabel('Magnitude of samples');
xlabel('time in seconds');
%% Calculation of STFT
%NoOfFrames=floor((length(Y)/frame_shift)-1);
NoOfFrames=length(Y)-frame_size;
j=1;
%for i=1:frame_shift:(length(Y)-frame_size)
for i=1:frame_shift:((length(Y)-frame_size))%+frame_shift)
sp_frame=Y(i:(i+frame_size)).*hamming(frame_size+1);
sp_frame_dft=abs(fft(sp_frame)); % Compute STFT
sp_frame_array(:,j)=sp_frame_dft;
j=j+1;
end
%% Plot the STFT in 3D
[rows,cols]=size(sp_frame_array);
F=linspace(1/Fs,Fs/2000,cols);
T=1/Fs:(frame_shift*Fs/1000):(cols*(frame_shift*Fs/1000));
Z=1:frame_size+1;
subplot(2,1,2)
%mesh(sp_frame_array);
%surf(sp_frame_array,'EdgeColor','none');
plot3(T,F,sp_frame_array);

Answer 1

I am not sure what your question exactly is about, but I guess the problem is, with the provided code, that you do not get a plot similar to the one you'd get, say, with surf.
Furthermore, I am also not quite sure why you would want to use plot3, maybe to get the labels on the time and frequency right ? you could do that all the same with surf:
surf(T, F, sp_frame_array,'EdgeColor','none');
As a matter of fact, the reason why your plot3 does not give the same figure is because the arguments of plot3 must be three matrices of the same size (check it on help plot3). Your code should actually be broken on Matlab, which it's not, according to my test. Well, once again Matlab allowing people to mess around without warnings (go Python! :D)... Anyway, try to set the matrices more like the following:
F=linspace(1/Fs,Fs/2000, rows); % note: has to be rows, not cols here!
Fmat = F(:) * ones(1,cols); % or use repmat
T=1/Fs:(frame_shift*Fs/1000):(cols*(frame_shift*Fs/1000));
Tmat = ones(rows,1) * T(:)';
plot3(Tmat,Fmat,sp_frame_array);
While this will normally produce something more in line with what I would expect in drawing a spectrogram, I'd still make some remarks:
your F vector should go up to Fs, because of the way you filled sp_frame_dft in. More specifically, it should go from 0Hz to Fs - Fs/rows:
F = linspace(0,Fs*(1-1/rows)/1000,rows); % in kHz
you would probably like to draw the amplitudes in dBs:
plot3(Tmat,Fmat,db(sp_frame_array));
plot3 draws one line per column of the provided matrices. That means potentially lots of lines to draw! As #atul-ingle asked, are you sure this is what you want? Maybe waterfall would provide a better rendering at a lower cost?
waterfall(T,F,db(sp_frame_array));
Well, you'll get the lines for the rows, instead of the columns, so you might need to transpose if the latter is what you want.
You might also prefer to visualise only the first half of the matrix (because the frequencies higher than Fs/2 are only mirrors of the other half of the matrix).
Hope that helps!