I suppose I should clarify what the Prism is: It's a bit like a guitar with some synthesizer mashed in. It has aspects of a theremin and a laser harp thrown in to boot. In short, it's not like anything else, and you can use it to make some really neat sounds. Anything from cold Sine and harsh square waves to heavily distorted noise.
At its heart the Prism features a VCO (Voltage Controlled Oscillator), based around the very shiny XR2206 monolithic function generator. The octave range is selected by blocking one of four laser "strings," and the pitch is controlled by the position of the musician's hand on the fretboard. The musician has the option of selecting a sine, triangle or square wave. The sine and triangle waves can be skewed using a separate Skew control.
It also has two other controllable oscillators, one acting as an LFO (Low Frequency Oscillator) and the other acting as a Sync generator.
I designed the Prism to plug into any regular guitar amp, with no computer required. In fact, there is no programming involved at all in this project! It is just as portable as a regular guitar, and meant to be used at gigs or wherever else an "alternative" instrument is needed!
Here is a video showing the basic functions:
And here I demonstrate the on-board effects:
Visit the Sample page to hear me "jam!"
Step 1: How It Works
That led me to research various (all-analog) oscillator possibilities. This time simplicity was key, since it all had to fit inside the body of the guitar. There are a number of single-chip voltage controlled oscillators out there, but the only one that is actually in production is the XR2206. This neat little device takes a control current or voltage and produces either a sine, triangle or square waveform that is proportional to that control. The control can be as simple as a potentiometer, or something more complex like the infrared range finder I used. During my search I happened upon a most excellent design by Thomas Henry. It had everything I needed to get started.
The Prism is based on Thomas' design, with a few modifications. I removed some of the control inputs, such as the exponential FM and voltage-controlled Skew. I then added my own custom-designed laser-controlled capacitor bank, a hard-wired LFO generator (based on an XR2206 reference design from the datasheet) and a hard-wired "sync" circuit based on a 555 timer. Oh, and a Sharp Infrared rangefinder to control the pitch.
So how does it work?
Well, I mentioned before that the output frequency is controlled by an input voltage. The three input control voltages, Coarse (the range finder), Fine (a trimpot on the board), and LFO are all mixed together and converted to a current (taking the place of a potentiometer). This current, along with a capacitor from the capacitor bank, determines the frequency produced by the XR2206.
Normally you'd only use a single fixed capacitor, but I wanted each laser "string" to select a different frequency range. The Prism accomplishes this by having each laser trigger a phototransistor, which in turn controls a comparator. If the laser is blocked, the phototransistor turns off and the comparator goes low. This causes a solid state relay to turn on, connecting its corresponding capacitor to the XR2206's capacitor input. When no lasers are blocked, no capacitors are connected and the oscillator produces a frequency above the audible range.
To control the pitch, I used a Sharp infrared rangefinder. You've probably seen this used on autonomous robots, and perhaps some theremin-type instruments. This neat little device measures objects between 10 and 80 cm away, and generates a corresponding analog output between 2.4V and 400mV, respectively. This voltage swing is quadrupled with a simple op-amp on the board.
The desired output waveform is selected by turning a rotary switch, that selects between the sin/tri output and the square output. The frequency and amplitude is the same no matter which waveform is selected.
The skew knob causes the triangle and Sine waves to skew - that is, they get chopped up and lean to one side or the other. For instance, the triangle wave can be made into a ramp for a slightly different sound. The effect is even more pronounced with the sine wave, which goes from a nice clean sound to very harsh and metallic.
The LFO effect can be varied by turning a knob, and turned off by pushing the knob (a very clever design, if I do say so myself!) Its effect can be varied from a slow rise and fall, to a nice vibrato, to a high pitched trill sound.
The Sync only affects the Sine and Tri waves. It is also controlled by a rotary knob, and can be switched off. Each time it transitions it causes the main VCO to reset, chopping up the sound in interesting ways.
There are also a few more on-board trimpots, used for tuning the output waveform. These are only touched once when the Prism is first constructed.
The Prism can be plugged into any regular guitar amp, or it can be modified to control a separate synthesizer setup.
A separate power supply is also needed, that produces +15, -15 and +5V. The lasers are driven by a 3V regulator that "spoofs" the voltage the lasers are expecting.