sangthong
04-29-2008, 01:29 AM
Hi,
I have a problem of doing normalised cross correlation by FFT, which th
result doesn't satisfy with the result from the one from time domain metho
and I don't know where it goes wrong.
From my understanding, this is the cross covariance in frequency domain
which gives me the correct result. (If there are signals x and y)
1) Do FFT of x
2) Do FFT of y
3) Conjugates 1) or 2)
4) Multiply 1) with 3)
5) Do inverse FFT of 4)
Then when I tried to develop a programme for doing the normalised cros
correlation, I use the step below.
1) Do FFT of x-(mean of x)
2) Do FFT of y-(mean of y)
3) Conjugates 1) or 2)
4) Multiply 1) with 3)
5) Do inverse FFT of 4)
6) divide 5) by the standard deviations of x and y
In the equation in time domain method, the mean is subtracted from x an
y, and the denominator is just like in step 6) above. By this, I got th
correct result, so I applied it to the frequency domain method as show
above. However, the result is wrong, which I don't now why. Can any bod
tell me whether the steps I used above is correct or not?
Thanks, Sangthong
I have a problem of doing normalised cross correlation by FFT, which th
result doesn't satisfy with the result from the one from time domain metho
and I don't know where it goes wrong.
From my understanding, this is the cross covariance in frequency domain
which gives me the correct result. (If there are signals x and y)
1) Do FFT of x
2) Do FFT of y
3) Conjugates 1) or 2)
4) Multiply 1) with 3)
5) Do inverse FFT of 4)
Then when I tried to develop a programme for doing the normalised cros
correlation, I use the step below.
1) Do FFT of x-(mean of x)
2) Do FFT of y-(mean of y)
3) Conjugates 1) or 2)
4) Multiply 1) with 3)
5) Do inverse FFT of 4)
6) divide 5) by the standard deviations of x and y
In the equation in time domain method, the mean is subtracted from x an
y, and the denominator is just like in step 6) above. By this, I got th
correct result, so I applied it to the frequency domain method as show
above. However, the result is wrong, which I don't now why. Can any bod
tell me whether the steps I used above is correct or not?
Thanks, Sangthong