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How to Build a 2 Note Ribbon Controller

by John Simonton

Ribbon controllerEven if you've never played with a ribbon controller you have at least seen one and know what they are. They are far and away the most organic and intuitive controller available for synthesizers. Sliding a finger up and down the ribbon can control the pitch of an oscillator or some other parameter.

And here's some secret insider info: many a gliss'ing lick attributed to theremin was in fact played on a ribbon controller such as the Tannerin or PAiA Gnome, including the most famous riff of all from Brian Wilson's "Good Vibrations". Not that there are no differences between a theremin and a synth played with a ribbon controller, there are. For instance, the synth can produce a greater variety of more interesting timbers than a theremin and even beginners can actually play a melody on a ribbon controller (1).

On many instruments the ribbon controller is small and intended as a replacement for pitch or modulation wheels, even though this is not their highest calling. But it's unusual to find full size ones and when you do they're considered "classics", which you can take to mean expensive.

We can deal with that problem here and now by exploring the design and construction of a very contemporary ribbon controller with features, like the ability to play 2 notes at once, not found on any previous commercially available units. Best of all the circuit uses readily available parts and materials that will not be obsolete 2 years from now.

Ready? We begin.

Ribbon SimpleIn its simplest form there's nothing more to a ribbon controller than a voltage divider like that shown. This basic configuration is not as useful as it could be primarily because it has no provisions for holding a note once the probe is removed from resistive strip. Also there is no circuitry to provide a gate signal when the probe is touched to the divider. Though a gate could be provided with a push button, like in the Gnome, this is not a very elegant solution.

It shouldn't take much explanation for this circuit. The voltage supplied by the battery distributes evenly on the length of the conductive ribbon and touching the probe to this surface picks off the voltage corresponding to the point touched, e.g. the "bottom" is 0V., the "top" is 10V, the middle is 5V and so on.

There are a couple of design issues to be addressed even in this simplest possible controller; what is the conductive ribbon made of and what does the probe look like.

Conductive vinyl sheet intended for antistatic work bench surfaces is available from companies like Emerson& Cumming but their Eccosorb VF-20 material is thicker than needed and rather pricey as well. An alternative is the lowly anti-static bag that many supplier use to ship sensitive components. Now, sorry to say, most of these bags are too high resistivity to be useful. Most bags have resistivity of many megOhms per square(2). Ribbon OrthoSome bags have resistivity of only a few kOhms and you will need to find one of these. Cut a square of convenient size and measure the resistance from one edge to the opposite edge. Either your meter won't deflect at all (high resistivity, no good) or you will read some value less than a few kOhms (Hurray, a good one).

You need a strip of material 1 in wide by 13 in. long. Attach the ribbon strip to the case bottom with a strip of double sticky cellophane tape. In the prototype case a pair of bare wires under the end pieces press into the ends of the ribbon. Brass thumbtacks with wires soldered to them could also be used for the end contacts.

For the "probe", which is really more of a wiper, we use a guitar "G" string. If you prefer a big old gnarly "E"or even a bass string, go for it. Wirewound strings gives a little better contact as each ridge of the winding presses down into the vinyl but flat wound or unwound strings are OK too. On one end the string is secured by passing it through a hole in the end piece of the case until stopped by the collar on the end. At the other end it passes through a similar hole into the electronic housing part of the case and around a securing screw and solder lug. It should be suspended about 1/8" above the ribbon and only has to be tight enough to keep from sagging into the ribbon, you probably won't be strumming it.

2 CVs from one ribbon

One of the secrets to generating two CVs from a single ribbon controller is to drive the conductive ribbon, R1 in the schematic at the right, with a current rather than a voltage. Pressing the ribbon in two places effectively shorts circuits a section causing the total resistance of the strip to decrease. which decreases the voltage at the collector of Q1. This will be used to calculate a CV Hi value.

The voltage at the low touch point is the same regardless of a higher point because the resistance from the low point to ground is the same regardless of how much the resistance above it changes. Constant current flow produces constant voltage at the probe regardless of changes in a higher touch point. The probe connects to a voltage follower circuit. IC2B, with such a high input impedance that negligible current is drawn from the ribbon. Capacitor C2 adds a slight lag to how quickly the voltage from the probe can change. This lag is handy when it comes time to switch the Sample and Hold circuits to their Hold state.

The voltage at the top of the ribbon is sensed and inverted by IC2A, the first step in the sequence of calculating CV Hi. C3 adds an integration time that serves the same function as the lag in the probe circuitry.

The basic CV Hi calculation consists of subtracting the voltage at the top of the ribbon from the voltage at the probe. This is done as a summation in IC2D (remember that the ribbon voltage was inverted in the previous stage). Since it will always be useful to set a single touch point so VCOs are in unison or set to some interval a transposing voltage is also summed into CV Hi from the wiper of R34.

The Unison control is also used for setting the diff (difference) output, which is a CV that is proportional to the distance between the two touch points. A possible application would be where CV Lo controls the pitch of an oscillator while the diff CV controls the oscillator's pulse width. Sliding two fingers upscale would produce a pitch change in the oscillator at an unchanging pulse width. Sliding the the higher finger upscale while holding the lower one fixed produces change timbres as the pitch stays fixed as the pulse width changes. A full spectrum of tone colors available at your fingertips - VERY organic.

Triggering and Sample and Holds

Any CV higher than ground turns the S/H switches on to track the CVs.When the voltage on the probe, CS0, rises above ground the output of threshold detector IC3C goes from low to high and D1 lights to indicate the Gate is active. The high output also charges C1 relatively slowly through R25 allowing time for the CVs to settle before turning on bilateral switches IC1A and IC1C. When contact is released and IC3C's output again goes low C1 quickly discharges through R26 and the steering diode D4. D3 clamps the Gate voltage to a lower value a fraction of a volt below ground

Sample and Hold is something of a misnomer since these circuits actually track dynamically changing touch points until the probe is released. IC3A and IC2B along with capacitors C4 and C5 hold the last CVs present when the low gate turns off switches IC1A and IC1C. The voltage followers IC3A and IC3B read the voltage on the capacitors with a high impedance that keeps them from discharging too quickly.

Touch Switches and Power Supply

Plug foot switches into J5 and J6 for external control of HOLD Hi and LoTwo touch pads, T1 and T2 allow CV Lo and/or CV hi to be frozen at whatever their value when the pads are touched. A typical application might have CV Lo and CV Hi driving the pitch of a pair of oscillators tuned to unison or some interval with the Unison control. A single touch point will transpose the pair of VCOs chromatically when run up and down the ribbon until the HOLD Lo pad is touched then the CV Hi oscillator can be gliss'd to new intervals while CV Lo stays fixed as a drone.

The touch switches work simply by the conduction of your skin. Taking Hi touch as typical, R11 pulls the gate of the bidirectional switch IC1B high to turn it on for a low voltage at R9/R10 holding Q3 off. No collector current. Touching the pad pulls the switch gate low and the now high voltage at R9/R10 turns on Q3. When on, Q3 acts like a switch to pull the gate of S/H switch IC1C low causing it to disconnect from the driving source and hold the last CV.Half wave bridges convert single phase AC to biplar voltages.

The power supply is a pair of opposite polarity half wave bridges. Sometimes you hear criticism of the higher power supply ripple possible with this topology but these problems are easily avoided with large filter caps. Years ago a cartoon showed a 1 Farad capacitor being delivered on a semi flatbed with the caption "I think you dropped a decimal point". Very funny then but today super capacitors of several Farad are small and relatively inexpensive - though not needed here, C6 and C7 are more than enough. A pair of +/- 12V regulators, VR1 and VR2, complete the power supply.

Dual Ribbon (tm) Complete Schematic

Dual Ribbon Complete Schematic

The case I designed is pretty cool in an old school sort of way and the striped ply is striking when stained and varnished but don't be afraid to use your imagination on the housing for the ribbon and electronics. Adapting an old guitar or baritone uke' comes to mind and be sure to check out the Hall of Fame for ideas on alternative enclosures.

So there you have it. A two output ribbon controller of a size large enough to be useful and if you like you can scale it larger or smaller. The Dual Ribbon can be used with linear response VCOs like older PAiA 2700 and 4799 series but is not recommended because all the notes will be bunched at the low end of the ribbon and oscillators will not chromatically track a constant interval.It is an ideal compliment to the current 9700 series gear.

As with any new device there will be some learning curve on the more complex applications of using the ribbon as a pitch and expression controller simultaneously but with a little practice you'll get it. Experimentation is an important part of the process, there is no "wrong" way to use the PAiA Dual Ribbon Controller.

(1) The theremin zeitgeist of making music by simply waving your arms in front of a box is so compelling that most people underestimate the difficulty of actually playing them. The most appealing idea of not touching anything is precisely what makes it so hard; there's no tactile feedback to assist in the process of locating pitches. The importance of touching a fret board or keyboard or even a ribbon can't be overstressed. With only proprioceptors, which are the sensors on joints and muscles that give you a subconscious feel for where your limbs are, exactly locating a precise point in space and holding that position with no wandering is very tricky. Using only audio feedback takes excellent pitch perception and considerable practice to learn.

(2) Resistivity is measured in so many Ohms per square because a square of a material, no matter how large or small, will measure the same resistance between opposite edges. Why? Because a square gets wider as it gets longer ... imagine it as strips, as each strip gets longer, and consequently increases in resistance, there need to be more strips in parallel, which lowers the resistance back to the constant per square value.

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