[sdiy] FB design

[Ian Fritz]

Hi Scott --

At 05:35 AM 12/15/2002, Scott Gravenhorst wrote:
><ignorance>
>Could someone here please provide a brief technical
>description of what a "filter bank" is and what it is
>generally (or can be) used for?
>
>Also, why is the 2.1^(1/5) spacing significant?  Are there
>other useful spacings?
>
>Thanks much.
></ignorance>


The term "filter bank" usually refers to a large set of bandpass filters 
operating in parallel.  They typically are set up to produce a zig-zag 
frequency response with a peak-to-valley ratio of 3-5 or so.

A filter bank can provide a tonal enrichment through two effects:

1.  The harmonic series of different musical notes will align differently 
with the filter frequencies, giving each note a slightly different timbre, 
ie an uneven musical scale is produced.

2.  With frequency vibrato, the harmonics of a tone interact with the 
frequencies of the filters in a complicated manner giving rich dynamic 
timbral modulation.  It is believed that this mechanism is important in 
producing the rich tones of string instruments.

You may think of other interesting uses.  For example, I can reduce the Q 
of my bank and use it as a super EQ.  Or I can use part of it for formant 
filtering and part for timbre enhancement.

The 2.1^(1/5) ratio starting at 50 Hz gives a series of frequencies that do 
not overlap with the musical scale over 10 octaves.  So you don't usually 
hit a filter right on resonance.  It would be interesting to experiment 
with different ratios, but you'd want to do this with C-sound or something 
rather than in hardware.  :-)

I've been thinking for some time that this would be a DSP application.

   Ian