Prev: DSK 6713
Next: alternatives to correlation
From: jim on 23 Mar 2010 19:48


"Scott T." wrote:
>
> Thank you for your replies. I guess I should have made it more clear that
> I am doing the filtering in the frequency domain by multiplying FFTs. This
> corresponds to a circular convolution unless you zero pad both the input
> and impulse response to L+P-1 before taking the DFT.

OK I didn't catch that your implementation was multiplying in the
frequency domain.


The impulse response is usually short with respect to the length of the
data. Is L the length of the impulse response and P is the length of the
data? If so padding both time domain sequences to length L+P-1 before
taking the DFT sounds correct to avoid wrap around.


> I don't have an explicit version of the impulse response, so it is
> difficult to just delay the response to make it causal. Although I suppose
> I could multiply the frequency response function by a complex exponential
> to create a delay.

Yes, coming up with the correct complex exponential would amount to the
same thing as deciding where to append the zeroes. See "FFT shift
theorem".

-jim


>
> My real question is where the zeros padding needs to be in a non-causal
> filter for the output to be identical to linear convolution, not circular
> convolution. In this case, I figured out that traditional zero padding
> actually gives me a circularly shifted version of a linear convolution.
> Knowing where to take the samples from solves the problem.
First  |  Prev  | 
Pages: 1 2
Prev: DSK 6713
Next: alternatives to correlation