Finding max value and its index

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Edo Christianto
Edo Christianto 2014-6-12
回答： Malcolm Hawksford 2017-12-19
So, here is the problem.
I have a wav file (attached), one tuts piano sound, and I analyzed it. Using this.
[wave,fs] = audioread('file01.wav');
n=length(wave)-1;
figure
t=0:1/fs:n/fs;
subplot(3,1,1), plot(t,wave)
subplot(3,1,1), xlabel('Time (Second)')
subplot(3,1,1), ylabel('Amplitude')
f=0:fs/n:fs;
wavefft=abs(fft(wave));
subplot(3,1,2), plot(f,wavefft)
subplot(3,1,2), xlabel('Frekuency (Hz)')
subplot(3,1,2), ylabel('Magnitude')
[max_value, index] = max(wavefft(:));
subplot(3,1,3), plot(f,wavefft)
subplot(3,1,3), axis([0 index 0 max_value])
The output is like this.
When I search max value using
[max_value, index] = max(wavefft(:));
I got this. Surely the peak would be on the right side, right?
When I inspect it, the max_value is in index 1111 Hz.
But when I zoom in manually, it's on index 740 Hz.
Did I do something wrong?
  1 个评论
Star Strider
Star Strider 2014-6-12
Seems you did everything correctly. The data in ‘subplot(3,1,3)’ is correct.
Explore the documentation on fft to understand why. (See the documentation on fftshift to understand the reason subplot(3,1,2) is misleading you.)

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采纳的回答

Image Analyst
Image Analyst 2014-6-12
Index 1111 is not at 1111 Hz necessarily. From the plot above, it looks like it's at about 750 Hz. But you msight want to use fftshift to shift the ends to the middle.
  4 个评论
显示 2更早的评论
Image Analyst
Image Analyst 2014-6-12
If you shift the spectrum to put the 0 frequency at the center, you need to subtract ~2.25 * 10^4 (actually x(floor(length(x)/2)) I believe) from your x axis so that 0 shows up at the middle, with the middle index of the array you're plotting.
Edo Christianto
Edo Christianto 2014-6-12
编辑：Edo Christianto 2014-6-12
Ahh... Actually I'm so confused right now... If you don't mind, I would ask again about this in the near future.
Anyway, thanks for all the help, kind Sirs/Mesdames.
(Note for future reference.)

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更多回答（1 个）

Malcolm Hawksford
Malcolm Hawksford 2017-12-19
Lets assume you have a stereo .wav file and simply want to find the peak amplitude and sample index (time domain) in each channel. Assume the file is called Cohen.wav then this works for me: [p q]=max(abs(audioread('Cohen.wav'))); p gives the peak values and q the corresponding indices.

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