"dspchick" <sarah.ali1@gmail.com> wrote in message 
news:1100t92ljn6vdc0@news.supernews.com...
> Just say I am using the FFT (in matlab) to compute the crosscorrelation of
> 2 discrete time signals, each of length N. I've tried 2 cases
>
> 1) zero-padding to a length of 2N, in order to avoid circular convolution
> (aliasing effects)
>
> 2) zero-padding to a length of 8N.
>
> I expected that case 2) would give me a better estimate on the lag, but it
> seems like the estimate is worse!  Is there such thing as too much zero
> padding?

Hi Sarah,

The result of the convolution should be exactly the same, except the 8N 
output buffer will have an extra 6N zeros in the middle.  I can only guess 
that you're estimating the lag incorrectly.   Remember that in the 8N 
convolution output, the 8N-xth sample corresponds to lag -x, not lag 8N-x, 
for x<=4N.  Similarly, for the 2N output, the 2N-xth sample is for lag -x.

Hope that helps,

Matt

Hi,

Just say I am using the FFT (in matlab) to compute the crosscorrelation of
2 discrete time signals, each of length N. I've tried 2 cases

1) zero-padding to a length of 2N, in order to avoid circular convolution
(aliasing effects)

2) zero-padding to a length of 8N.  

I expected that case 2) would give me a better estimate on the lag, but it
seems like the estimate is worse!  Is there such thing as too much zero
padding? 

Thanks!!
		
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