spectrograme using STFT(DSP)

I wan to build an image same left image with STFT , but I don't understand why my output image(left) is not like right image(desired).

any one can write desired code?

thanks a lot

N=512;  n=1:1:N;
y=4*cos(8*pi*n/N)+N/5;
x=cos(2*pi*y.*(n/N));
>> s = spectrogram(x);
stft(x,1000,'Window',gausswin(128),'OverlapLength',110,'FFTLength',256); colormap  jet

Answer 1

[Mathieu NOE]

hello

I was not comfortable with how you defined the basics constants in your code , so I rewrote it my way - I believe more readable (even if longer)

I used pcolor with interp shading to get the same smooth visual aspect as your target image

hope it helps

% demo
f1_mean = 100;        % mean freq (start value)
f1_amplitude = 50;    % beating amplitude of freq
f_mod = 5;     % max frequency of modulation
Fs = 512;       % sampling frequency
duration = 1;  % s

%%%%%%%%%%%
dt = 1/Fs;
samples = floor(duration*Fs)+1;

t = (0:dt:(samples-1)*dt);
mod_amplitude = linspace(0,1,samples); % linear increase of frequency modulation with time
omega = 2*pi*(f1_mean+f1_amplitude.*mod_amplitude.*sin(2*pi*f_mod.*t));
angle_increment = omega.*dt;
angle = cumtrapz(angle_increment);  % angle is the time integral of omega.

signal = sin(angle);

figure(1);
plot(t,signal)

% spectrogram demo
NFFT = 32;    % to have highest frequency resolution , NFFT should be greater than typical max length of files ( 1 second duration at Fs = 16 kHz = 1600 samples);
Noverlap = round(0.975*NFFT);
w = hanning(NFFT);

[sg,fsg,tsg] = specgram(signal,NFFT,Fs,hamming(NFFT),Noverlap);

% plots sg
figure(2);
pcolor(tsg,fsg,20*log10(abs(sg)));axis('xy');colorbar('vert');colormap  jet
shading interp
title(['Spectrogram  / Fs = ' num2str(Fs) ' Hz / Delta f = ' num2str(fsg(2)-fsg(1)) ' Hz ']);
xlabel('Time (s)');ylabel('Frequency (Hz)');

[rohollah hosseyni]

thanks a lot. it's great. I do't know how to thank you :)

[Mathieu NOE]

you're welcome