Paint Filtering
This is another non-linear audio filter. The idea is that you take the original signal (which is a line of zero thickness in a time-amplitude scale) and trace over it with a thick elliptical brush. This gives an area which follows the shape of the line, but some of the little kinks will be taken out, and the peaks will be rounded. We then have to convert this area into a line, this is done by just taking a point in time and finding the average of the minimum and maximum amplitudes for that time. There are other ways of doing this step which will give different results, but I used this one because it is simple.
In my mind, I was expecting it to be a sort of low-pass filter, as the high frequency stuff would be blurred out by the "brush". What actually happened was that it has more of a comb-filter like effect. If you run it on pink noise you get a frequency response curve that looks like this:
(without filtering, this wouldn't have all the bumps in it).
Since the filter isn't linear, you can't generalise and say it will have a comb filter effect for all input signals, but for the stuff I've tried it on, it generally gives a comb filter like effect. On more aggressive settings, certain notes will sound much louder than other notes. I've chosen two examples that I'd think are usable, and while they are quite obvious, they aren't so aggressive as to add the really noticable loud notes:
Original signal, Paint Filtered 1, Paint Filtered 2.
Get the source (C, GPL3, includes makefile, and is a little bit dumb in one spot where instead of using a circular buffer I shift every value in a buffer across a spot).
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