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In the DIY synthesizer world, folks tinker with lowly CMOS logic
chips to create everything from oscillators to sequencers. CMOS
(Complimentary Metal Oxide Semiconductor) chips were an advancement
in logic chips that eliminated most of the drawbacks of older
TTL (Transistor Transistor Logic) parts.
CMOS chips are inexpensive, easy to find, and easy to use.
This article presents a butt-load of CMOS sound generation circuits
for your amusement.
What do CMOS Synthesizers Sound Like?
In their simplest form, these circuits generate digital squarewave
sounds. They are not hi-fi and generally lack amenities like
envelopes. However, by combining various CMOS oscillators, with
a few other logic tricks, you can create complex waveforms that
are decidely lofi, grungy, and drone like.
Chip Basics
For our experiments, we'll use a couple basic chips:
40106 Hex Inverting Schmitt Trigger: This chip contains
6 individual Schmitt triggers. A Schmitt trigger is a comparator
circuit that incorporates positive feedback. What this means
is that when a high signal (positive voltage defined as '1')
is applied to the input pin, the output pin generates a low
signal (ground voltage defined as '0). This simple circuit can
be connected to a resistor/capacitor pair to rapidly turn on
an off. When the frequency of this on/off conversion is in the
audible range, we hear a square-wave tone.
4040 Binary Counter/Divider: This simple logic chip
takes an input square wave and generates oscillations at 8 outputs
with a frequency related to the input frequency. Q1 generates
a frequency one quarter of the frequency of the input square
wave, Q2 genertes an eigth, and so on through Q12. This allows
us to harness octaves of frequencies.
4051 Eight Channel Analog Multiplexor/Demultiplexor:
The voltage status of the three address pins A, B and C
determines which of the eight channels presents its voltage
to the common connection pin.
Note: All schematics on this site use the following notation
for potentiometer tapers: A = Logarithmic, B = Linear
Experiment 1: An Astable Multivibrator
What the hell is an astable multivibrator? It is
simply an oscillator that we will tune to put out a
tone in a given range. Apply 5-9 volts DC to pin 14,
connect pin 7 to ground. Then connect a capacitor and
potentiometer as shown in the schematic. The values
of the capacitor and potentiometer determine the frequency
range--I chose values that give a good range within
audible frequencies. The B10K pot adjusts the frequency.
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Experiment 2: Line Out Signal and Volume Control
This version adds a decoupling capacitor C2, a voltage
divider (R1+R2) and a volume control. This tames down
the output to a level more appropriate for hooking up
to line-level inputs, such as an audio amplifier.
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Experiment 3: Simple Low Frequency Oscillator (LFO)
Here's a simple LFO that can be used to drive other
circuits such as modulation effects.
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Experiment 4: Simple LFO with LED Rate Indicator
Here we add an NPN switching transistor, C2 as a
decoupling cap, Diode D1, an LED and a current limiting
resistor (R1) to provide a visual indication of the
LFO operation.
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Experiment 5: Oscillator with Adjustable Duty Cycle
This modification of the Simple Oscillator adds an
additional pot and diode to control the Duty Cycle,
or the amount of time spent outputing one side of the
square wave. The result is more interesting variation
in the tone of the output.
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Experiment 6: Dual Oscillator
Here we take the output of one oscillator and feed
it into the input of the second oscillator.
Diode D1 keeps oscillator two from feeding back into
oscillator one.
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Experiment 7: Mixing with Diodes
Here we take the output of two independent
oscillators and mix them together.
If you simply connect both outputs together, you
will cancel the sound. This is because the output of
each oscillator will feed back into the input,
thereby creating chaos and other non-functional
behavior. To solve this, you need to isolate the
output signal from the input signal.
This can be done with a resistive mixer (simply
put two resistors, 33kohms should work, at the
outputs), or use diode mixing as shown here.
Resistive mixing will simply attenuate the sound
but will not change its character. Diode mixing acts
as a half-wave rectifier and creates a very
interesting variation on the standard square wave
sound.
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Let's Build Something Fun
In our first experiment with a flexible tone generator, let's
look at the Heterodyne Space Explorer. This design is comprised
of four 40106 square wave oscillators. The supply voltage can
be sagged using VR1 to create starved sounds as each oscillator
competes for voltage. Each output stage goes to a switch that
flips between a resistor mixer and a diode mixer. Each sounds
different. Each stage also has its own volume knob--this makes
it easy to create beating heterodyning noise walls.
More Fun: Heterodyne Peyote Space Explorer
This piece is a conglomeration of various things: three independent
oscillators, a complex waveform generator, a white noise circuit
and a mixer. The complex waveform generator circuit from the
excellent "Fun
with Sea Moss" article. Sea Moss. Get it?
Using Your Effects!
You can take the output of your CMOS sound generator circuits
and feed them into your effects and pedals. This opens up an
entire new set of sounds. Fuzz, distortion, delay, phasing,
chorus, all will allow you to create some very strange, good,
or atmospheric sounds.
If you have a multiple oscillator circuit like the Heterodyne
Space Explorer shown above, the application of a delay pedal
(analog or digital) to the output circuit will remove the
two-dimensional nature of the sound and open it up
dramatically. Reverb can also have a big impact on the
spatial spread of the sound.
References and Further Reading
Flux Monkey's Excellent Looney Board:
http://www.fluxmonkey.com/electronoize/looney1.htm
Art Harrison's Beautifully Chaotic Cacophonator:
http://theremin.us/Circuit_Library/cacophonator.html
Fun with Sea Moss -
http://www.milkcrate.com.au/_other/sea-moss/
Nicholas Collins, Handmade Electronic Music -
http://www.nicolascollins.com/read.htm
Lunettas, circuits inspired by Stanley Lunetta -
http://electro-music.com/forum/forum-160.html
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