More progress on GR-300 project

Mark Smart smart at nn.com
Tue Jan 28 19:18:36 CET 1997


I got some more circuitry tested this weekend for my GR-300 hot-rod
project. Namely, I tested my circuit to derive square waves from the
variable-amplitude sawtooth waves from the GR-300.

As I probably mentioned before, the GR-300 generates a sawtooth wave whose
amplitude is proportional to the cycle period of the guitar string. From
this sawtooth it derives a control voltage proportional to the string's
period. This cv is used to drive a VCO which works like the
period-to-voltage converter in reverse.

Both the sawtooth wave derived directly from the guitar string and the
sawtooth from the VCO are put through interesting circuits called "chopper
gates" which clip, invert, and level shift these signals so that the output
level is the same for all different notes up and down the neck.

So deriving a square wave from either of the pre-chopped sawtooth waves is
not as easy as it usually is because the amplitude of the sawtooth wave
keeps changing. My circuit to do this just involved an Analog Devices AD633
multiplier chip and a comparator. This chip has inputs X1, X2, Y1, and Y2
and outputs a voltage = ((X1+X2)x(Y1+Y2))/10 V. Unlike most of these
multiplier chips, the AD633 only costs about $5.

For a square wave, you need a voltage equal to half the amplitude of the
sawooth wave being compared (this sawtooth wave always starts at 0V and
ramps up to between 2.5 V and 10 V, depending on the string's period).
So I just took the period control voltage from the GR-300 and ran it into
X1, with X2 grounded. Into Y1 I ran a voltage from a voltage divider pot.
5 V at this input gives you a square wave. Varying it between 0 and 10
volts will produce everything from always-high to always-low. Since all the
strings on the GR-300 are set up to generate a 10 volt period CV when
an open string is played, and a 5 volt period CV when the string is fretted
at the 12th, the same pulse-width CV can be used for all strings. I will
eventually have the pulse width adjustable with one pot, and LFO depth for
PWM with another pot. The PWM LFO signal will go into Y2 on the AD633.

I spent a lot of time playing with this, and it sounds really good. I can't
wait to get all six strings hooked up and try PWM then! The consistency of
the duty cycle is very good as you play up and down the neck. 

Normally the output waveforms of the GR-300 follow the envelope of the
guitar string in terms of volume. If you listen to the output of the
oscillators before they go through the chopper gates, you can hear what the
pitch tracker circuitry is doing when you are not playing anything on the
guitar and have the string damped...it is totally freaking out! It
sounds a lot like a digital data signal coming from a modem. When the
freaking-out sawtooth signals are put through my saw-to-pulse converter,
the output sounds like modem data run through a flanger. I think I'm going
to design the synth with this sound available, because it is so wonderfully
chaotic.

After noodling with this, I took the pulse wave output and ran it through a
double octave-divider circuit made from a 4013 CMOS chip. This also worked
fine. Having played the GR300 through an octave divider pedal, I like the
sound and have decided to try putting hexaphonic octave dividers on this
beast. I'll keep you guys posted.


************************************************
*     Mark Smart                               *
*     Network Technician                       *
*     University Communications Inc. (UCI)     *
*     smart at medusa.nn.com                      *
************************************************



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