[sdiy] Filters

[harrybissell]

john mahoney wrote:
<snip>

>
> > I.E. What if I want to make a guitar fx pedal?
> Most filter fx for guitar use an envelope follower to track the level
> ("volume") of the guitar signal, so higher output from the guitar increases
> the output of the envelope follower, which in turn increases the VCF's
> cutoff frequency. There is often a lag control to slow down & smooth out the
> envelope follower's response.
>
> When you play a chord on the guitar (for example), it quickly goes from soft
> to loud, then slowly gets softer again -- the envelope follower will thus
> cause the VCF to open up and then slowly close down, so you hear the filter
> sweep up and back down. Again, the lag allows you to reduce the speed at
> which the filter opens up.

anyone even dreaming about a guitar env. folower needs to check out "Envelope
Follower
combines fast response, lowest ripple" by Harry Bissell Jr (me) in the DEC 25
2002 issue
of EDN.  (this is just not tooting of the horn... I published it so that people
could get the
benefit of a better design).  This circuit is ideal for guitar of bass, where
the lowest frequency
can be accurately predicted i.e. guitar = 80Hz

I just spent some quality time with a design that uses the traditional "peak
detector"... gack
what a piece of sh!t. You can either make it slow enough to get rid of ripple...
giving you about
a 100-200ms decay time (thats like next week to a guitar player) or make it fast
and get
horrible IM diustortion because the ripple in the CV acts as an unwanted
modulation source.

Full wave rectifier and a good filter (maybe 12db) can do a little better.

The crown of all env. followers... the Mutron III... uses LED/Photocell modules
which
are slow enough that the ripple is not a problem, with tasty response times.
I'd even consider
using an LED/Photocell module just to condition the CV before driving a VCF...

> Of course, someone will LIKE the IM distortion from ripple in the CV...

H^) harry