When i plot spectogram with the following code, it shows frequencies at 0.4 and 0.8. Why it is so? Why not 1.256 and 2.51?

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Anees khan 2016-3-30

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https://ww2.mathworks.cn/matlabcentral/answers/276285-when-i-plot-spectogram-with-the-following-code-it-shows-frequencies-at-0-4-and-0-8-why-it-is-so-w

回答： Star Strider 2016-3-31

N = 1024;
n = 0:N-1;
w0 = 2*pi/5;
x = sin(w0*n)+10*sin(2*w0*n);
s = spectrogram(x);
spectrogram(x,'yaxis')

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Answer 1

Star Strider 2016-3-31

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https://ww2.mathworks.cn/matlabcentral/answers/276285-when-i-plot-spectogram-with-the-following-code-it-shows-frequencies-at-0-4-and-0-8-why-it-is-so-w#answer_215740

The spectrogram function presents those data because it normalises the frequency axis from 0 to pi. You defined ‘w0=2*pi/5’. Since the arguments to the sin functions are ‘w0’ and ‘2*w0’, the frequencies are 2/5*pi and 4/5*pi, or 0.4 and 0.8 of the full frequency axis range respectively.