Compute PSD from fft compared with spectrogram
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Hi there,
Recently I am working on translating some Matlab code to Java, as I want to do some pattern recognition in mobile phones. In the Matlab code from my teammate, he used this function "spectrogram" to get PSD, which I couldn't find a complete open-source implementation online. Therefore I have to do it by my own.(who doesn't!!)
I got some algorithms from the forum such as:
y = data;
Fs = 90;
L = length(y);
y = y(:) - mean(y);
NFFT = 2^nextpow2(L);
Y = fft(y,NFFT);
f = Fs/2*linspace(0,1,NFFT/2+1);
psd = abs(Y(1:NFFT/2+1)).^2/L;
psd = psd/Fs;
which has the same result with
periodogram
But NONE of them has the same result as the spectrogram. Would any one help me out here? Thank you very much.
采纳的回答
Wayne King
2012-12-3
编辑：Wayne King
2012-12-3
spectrogram optionally returns the short-time periodogram estimates. I'll show you how to establish the correspondence for the default Hamming window used in spectrogram. I'll establish the correspondence only for the 1st segment in the following example, but it is straightforward to extend it over the entire data record with the proper overlap.I'll just use a noise vector and assume a sampling rate of 1 kHz.
x = randn(1000,1);
[y,f,t,p] = spectrogram(x,256,250,256,1000);
% now to create an equivalence between the column vectors
% in p
win = hamming(256);
xdft = fft(x(1:256).*win);
psdx = abs(xdft(1:length(xdft)/2+1)).^2;
psdx = 1/(1000*norm(win,2)^2)*psdx;
psdx(2:end-1) = 2*psdx(2:end-1);
% now compare
subplot(211)
plot(psdx)
subplot(212)
plot(p(:,1))
3 个评论
Yifei
2012-12-6
Brandon Madsen
2017-6-8
编辑：Brandon Madsen
2017-6-8
This is a very helpful explanation, but I am unsure about the purpose of a certain line of code, namely: psdx = 1/(1000*norm(win,2)^2)*psdx; . Could someone explain what this part is doing and why it is needed?
EDIT: Oh, nm, it's for normalization to Hz I assume.
HYZ
2021-10-15
If I want to plot for different frequencies, let's say 0 to 300Hz in the y-axis [y,f,t,p] = spectrogram(x,256,250,1:300,1000); how can I change the below?
win = hamming(256);
xdft = fft(x(1:256).*win);
psdx = abs(xdft(1:length(xdft)/2+1)).^2;
psdx = 1/(1000*norm(win,2)^2)*psdx;
psdx(2:end-1) = 2*psdx(2:end-1);
更多回答（1 个）
Vieniava
2012-12-2
0 个投票
Spectrogram is time-frequency (3D=time vs freq. vs amplitude) representation of a signal and periodogram/fft is frequency only (2D= freq vs amplitude) representation. Spectrogram shows how the frequency spectrum is changing over the time. Spectrogram is a set of consecutive fft's. Spectrogram is a matrix and fft/peridogram is a vector.
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