Fourier transforms - Basic question 1

[megok]

I have uploaded some data into MATLAB and I am attempting to remove the funtamental frequency (1208.6Hz)
What I gather is the easiest way to remove fundamental is to apply a fourier transform to the data and then set 1208.6Hz to zero and then apply the inverse transform.
My problem is I don't know how to set the fundamental to zero.
Any help would be aprreciated

 

[VisiGoth]

You need to know the samplin rate of your data and the data length. You might not have a picke that is "on" 1208.6 Hz exactly, so maybe you could stomp on two pickets.

Here is an example

fCriticalHz = 1208.6;

fs = 48000;
nSample = 1000;
fStep = fs / nsample;

fftOfData = fft( data, nSample );
fStart = -fs / 2;
fStop = ( fs / 2 ) - fStep;

frequencyPicketHz = fStart : fStep : fStop;

indexZap = find( frequencyPicketHz >= fCriticalHz );

indexZapThese = indexZap + [ -1 0 1 ];

indexZapThese = [ ( nSample - indexZapThese ) indexZapThese ];


fftOfData( indexZapThese ) = zeros( size( fftOfData( indexZapThese ) ) );
dataZapped = real( ifft( fftOfData ) );

% Sorry I did not have time to test or review this, but someof these ideas should help in understanding.


jsolar

 

[electricpete]

You are trying to implement a digital band-stop filter.

That's a good detailed method described above.

Without providing an alternative with comparable level of detail, I am hesitant to criticize it at all. But I would point out that it implements a very square bandstop filter which will introduce artifacts in the output time waveform in some cases. More sophisticated bandstop filters have smoother transitions with a lot of mathematical thought put into them.

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[electricpete]

There are certainly a wealth of on-line resources. You might look around dspguide.com starting here:

http://www.dspguide.com/ch14/5.htm

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[VisiGoth]

electricpete, I agree with you. I gave an example of the method megok wanted to use. I never know if these are learning exercises or part of a bigger project, so I hesitate on giving adice on a different approach sometimes (sometimes I just can;t help it).