AA filter question - Joe?

Started Feb 9, 2012 | Discussions thread
hjulenissen Senior Member • Posts: 2,070
Re: AA filter question - Joe?

Peter 13 wrote:

DSPographer wrote:

Actually, as long as the frequency response never goes to zero you can apply a sharpening filter with the inverse frequency response. The down side is that you also sharpen noise, so the end result will be noisier than if no sharpening was needed. Still, this is better than the situation with aliasing where it is impossible to determine exactly which of several possible scenes created the image.

This can only be "successful" if the softening is not so strong. Otherwise, you run into a small divisor problem. Instability of such a deconvolution is a textbook example of a unstable transformation.

You can always use a linear (FIR) filter that is guaranteed to be stable for any input (or set of coefficients), but when the signal is small relative to noise, all you are doing is amplifying noise.

The moral is that high-frequency rolloff in the OLPF can be compensated by sharpening/deconvolution to a point, but you will probably never be able to get the exact same image as you could have had if there was no OLPF filter.

Using an OLPF will also significantly reduce the magnitude of certain errors that are hard to mask afterwards. I guess that is why most of the manufacturers choose to include that cost...

-h

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