[sdiy] Low Cost Comb Filter Methods

[Ben Bradley]

I have these articles (I have a 13-CD set of JAES archives for cheap,
an amazingly lucky hamfest find). I think I can describe them here
adequately without running into legal or moral problems.

In short, this "comb filter" is a bunch of spaced-out-in-frequency
analog bandpass filters, similar to those in graphic equalizers, but
with substantially higher Q. The thread from last year (see the last
paragraph below) is, IMHO, actually more informative than both of
these articles.

The first article describes an op-amp-based bandpass filter with
trimpots to change frequency and bandwidth. There's a PC board shown
with a quad opamp and R's and C's making up four such filters (and
thus 8 trimpots). The author made nine of these boards (I doubt this
was "low cost" back then), with each filter having different R and C
values. There's a table giving the resonant frequency and  component
values for each frequency. The frequencies for the 36 filters range
from 50Hz to 9000Hz, in an approximate geometric progression so that
there are about four filters per octave.

The second article describes a clavichord with its output going
through this filter.

Both articles have references to previous JAES articles, but the
second one also has this to an article by the author in Electronotes:
[5] R. W. Burhans, "Non-Uniform Comb Filter Spacings with Respect to
Equally Tempered Scales," Electronotes, vol. 5, pp. 15-16 (June 20,
1974).

There was a discussion on SDIY just last year of this type of filter
bank (used to simulate the many resonances in an acoustic instrument),
and many posters had something to say about it. The subject is:
Resonator type filters, started 6/29/17.