[sdiy] Fixed filter bank questions

[David G Dixon]

Concerning changing the centre frequency from 1000 Hz to 640 Hz...

I've done a bit of analysis, and my assessment is that it really doesn't
matter.  

If you use 1000 Hz as the centre, then you have 6 BP filters below that
going down to 125 Hz, with the LP at 88 Hz, and 5 BP filter above that going
up to 5657 Hz, with the HP at 8000 Hz.

If you use 640 Hz as the centre, then you have 5 BP filters below that going
down to 113 Hz, with the LP at 80 Hz, and 6 BP filters above that going to
5120 Hz, with the HP at 7241 Hz.

The result is that all of the BP filters have shifted a little less than two
semitones downward.  Nobody is going to notice this difference.  I'm going
to stick with the more Moogy frequencies out of a sense of tradition.  Plus,
I like putting the rounder numbers on the front panel.