Guitar as a controller

[Harry Bissell]

Hi Troy...

this sounds easy but it isn't. The guitar has a very nasty waveform that has a
habit of
having the peaks change amplitude with time... what used to be the peak gets
smaller, and the second harmonic gets "bigger" until it becomes the new peak. The
best way I can describe this is that it is much like a phase shifter... with a
notch that goes through the waveform.

Filtering helps, processing helps also. See the "Akimatu" patent for one means of
overcoming this effect. This method is used in most octave divider boxes. The
"Calman Gold" patent is a much better solution, but quite complex when you build
one. It does work very well. It gives a short pulse every fundamental cycle, and
is almost completely
free of glitches.  I'm using it now for an advanced octave divider.

You almost always have to divide the guitar wave by 2 to get a stable duty cycle.
P/V converters must have a fast response. Tach circuits like the Kong MS-20
envelope
follower circuit are no good.

The last problem is trying to decide when the string amplitude is too low to
decode anymore... and then hold the last valid data. Bob Moog has a patent on a
frequency extraction circuit that uses 2 sample/holds in series. That causes a
delay but at least you get good data. The trick is to detect a new note played,
RUSH that data to the S/H. and then start the filtering.  (Roland uses a
non-linear filter for the same effect...).

I've had good success with using a Lag processor, driven by the guitar amplitude.
This
way intial tracking is fast (well sh!t I mean relativly fast... like molasses
flowing downhill vs. COLD molasses flowing down the same hilll) and the tracking
gets slower as the string decays... eventually reaching a "hold" state.

I recommend NOT using tracking oscillators. Fuzz up that guitar string and use
waveform
processing. If you want a string pad that lasts longer than the guitar sustain...
hire a keyboard player... or multitrack... or watch Geddy Lee in an old Rush
videotape....

STD Disclaimer: This is a Guitar Synthesizer not to be confused with Guitar-like
synthesizers....  ;^)

H^) harry

Troy Sheets wrote:

> You just start with a frequency-to-control-voltage converter.
>
> Guitar sound (or any sound) gets converted to control voltage which you
> can use for you VCOs, filter cutoff, ect.
>
> Use an envelope follower to generate your VCF or VCA gates,
>
> -troy
>
> -troy
>
> On Thu, Jun 08, 2000 at 10:59:31PM +0200, Raoul Raffagli wrote:
> > Hail.
> > I would like to build a sort of synth (modular) using an electric guitar as
> > a controller. Obviously it's not a MIDI guitar. Any1 can help? Got no ideas.
>
> --
> ------------------------------------------------------------------------------
> tsheets at saturn5.com                                  <- You have found me
> http://www.saturn5.com/tsheets/audio                 <- my .mp3 tracks
> "I just don't have the discipline to be a hippie"    <- Homer J. Simpson