[sdiy] Filtering for chromatic scale frequencies?

rsdio at audiobanshee.com rsdio at audiobanshee.com
Thu May 31 08:52:14 CEST 2018

On May 30, 2018, at 10:49 PM, Mike Beauchamp <list at mikebeauchamp.com> wrote:
> This might be bit of a strange one.. I'm wondering if there's any way to filter an audio signal to favour frequencies that are within the chromatic scale?
> So, sort of like "autotune" - but instead of correctively altering the pitch of 'wrong' notes, the filter is just lowering their amplitude significantly.

It’s not gonna be easy.

Note that the typical filter slope only changes gradually over the span of an octave, dropping only by 6 dB to 24 dB, and a single filter slope continues on for several octaves, so it’s not terribly surgical. You can make filters with very narrow bandwidths, but those filters are a lot heavier weight than a Moog 4-pole.

You could make 7 narrow band pass filters per octave, but that would be a lot of filters. In addition, the high Q of these narrow band pass filters would tend to enhance frequencies above their natural amplitude.

Probably the easiest way to do this would be with a digital filter.

The problem is that such a filter would change the timbre of notes that are not ‘wrong’ for the chosen scale, but their harmonics would fall sometimes in the right bins and sometimes in the wrong bins.


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