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Plotting matrix as spectrogram

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Sania Gul
Sania Gul on 26 Aug 2022
Commented: Robert U on 26 Aug 2022
I want to plot a matrix of 256* 16, as spectrogram. But when I use surf command to do so, I got spectrogram of size 343*436. I want the image to be of same size as that of matrix i.e. 256*16. Kindly suggest any solution without cropping or resizing the image as both of them distorts the quality.
  3 Comments
Robert U
Robert U on 26 Aug 2022
testData = randn(256,16);
fh = figure;
ah = axes(fh);
surf(ah,testData);
I was not able to reproduce the described error by the given information.
Kind regards,
Robert
Sania Gul
Sania Gul on 26 Aug 2022
[Q,F,T] = spectrogram(audio,hann(512),256,512,Fs,'yaxis');
Q=Q(1:256,:);
F=F(1:256);
figure(2)
D=surf(T,F,log(abs(Q).^2));
colormap parula
shading interp
view([0 90])
axis tight
set(gca,'xtick',[])
set(gca,'ytick',[])
export_fig AA.jpg -native -c[31 52 46 72]% top right bottom left
% reading the image
I=imread('AA.jpg');
>>The value of I=343x436x3 uint8, but the size of matrix Q was only 256x16, and I want 'I' to be also 256x16x3 uint8

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Accepted Answer

Robert U
Robert U on 26 Aug 2022
Hi Sania Gul,
The values you provide are not consistent. I assume your "audio" is a vector of a certain length N, sampled with the frequency Fs.
Applying the function spectrogram with the window-length 512, overlap 256 and NFFT of 512 lead certainly to the dimension of Q of NFFT/2+1 = 257 and ((N - 512)/(512-256)).
If you want Q to be 257 x 16 you would have to provide a snippet of roughly 4600 samples.
fs = 60e3;
t = 0:1/fs:4599/fs;
x1 = vco(sawtooth(2*pi*t,0.5),[0.1 0.4]*fs,fs);
[Q,F,T] = spectrogram(x1,hann(512),256,512,fs,'yaxis');
size(Q)
ans = 1×2
257 16
Kind regards,
Robert
  4 Comments
Sania Gul
Sania Gul on 26 Aug 2022
Tnk u soooo much, but how to get rid of the frame around the spectrogram without using the export.fig?
Robert U
Robert U on 26 Aug 2022
There is still some manual work required to get the desired result.

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