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Spectrogram to get the frequency contents

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dil639

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frequency content of a signal

I want to read the signal frequencies that can be seen in the spectrogram using matlab function(spectrogram).

But when i give different sampling frequencies i am getting the display of my signal at different frequency points.This is explained below.

I am using the below code to create a sine wave with freq = 100hz. and display its frequency.

t =0:0.001:2.047;
f = 100;% 100hz frequency
y = sin(2 * pi * f * t);
spectrogram(y,512,256,512,1000,'yaxis'); % Display the spectrogram with FS = 1K hz
figure;
spectrogram(y,512,256,512,10000,'yaxis'); % Display the spectrogram with Fs = 10Khz.

Here i observed that when i gave sampling frequency as 1000 hz. I am able to see my signal content at 100hz, which is correct and expected. but when i gave sampling frequency as 10000 hz i am seeing my signal at 1000hz instead of 100hz. What does this mean. I also observed that time axis also scales proportionally.

I am not understanding this phenomenon.

Actually by getting the spectrogram i want to extract the (mean)frequency contents in it. but if the display of frequency content depends on Fs then plese suggest me how to get the mean frequency content.

Thanks for your reply.

Regards,
Dileep.D
 

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