§ 12.11.64 - Notch Filter
Scripting command: notchf [Bandwidth=0...100] [Frequency=0...100] [Depth=0...100] [Skirts=0...100]
This is a fast Fourier transform-based horizontal feature filter. You can use it to reduce repetitive noise and artifacts with reduced effect on other image features as compared to a blur or even a RIP. It may be useful to combine the filtering techniques to really get the best of out an image suffering from noise and/or interference.
Using the Frequency 
and Bandwidth controls, you can select large or small features to be filtered out. There are many settings that will result in output that will be of no use; most useful settings will see both of these controls set past 50.
I work with a lot of images sent over radio using sideband encoded as audio with a horizontal resolution of about 256 to 320 pixels, and these tend to have some noise in them; for that, I use settings of Bandwidth = 65 and Frequency = 71; the resolution of the image you are filtering makes a difference here. The more pixels your image has horizontally, the more precise the notch tends to be.
Filtering to get some of the ripples out
of the blue sky
Bandwidth
=31 Frequency =41This is a very narrow bandwidth notch.
Depth controls how much of the effect is actually applied.
F. Skirts controls how steep the sides of the notch filter valley are. With F. Skirts set at 100, the notch is deep and the sides of the notch are steep. A setting of 50 where the sides are more gentle is generally almost as effective at removing features and at the same time results in less ringing (ringing looks like echoes of deeper vertical contrast elements in the image.)
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