A New Ribbon Based Gestural Controller The primal instinct to build and operate a ribbon controller has been a consistent component of the human psyche since the dawn of man. Cave drawings and the fossilized remnants of early ribbon devices indicate that Neanderthal Man indeed had mastered the ribbon controller several thousand years ago. Harnessing the natural power of lightning, these devices likely used an array of ferret-like creatures as the sound generating device. The resistive element of the first ribbon controller has long since disintegrated, leaving the actual material subject to speculation. The prevailing theory holds that it was almost certainly charcoal, either from a campfire or from the remains of the previous operator.
It wasnt until the twentieth century that ribbon controllers became practical as a non-lethal musical instrument. One of the most ubiquitous devices of the last half of the twentieth century was developed by Dr. Robert Moog. This controller was made famous (perhaps infamous) through its use by the progressive rock musician Keith Emerson. Emerson was known to drive home a particular musical point by shooting flames from the end of his controller, perhaps in homage to the unsung Neanderthal musicians of yore.
A ribbon controller can be used for a number of purposes. The dominant conception is that a ribbon controller is used for controlling pitch. As a pitch controller, without quantization, the ribbon controller ranks at rather difficult, a notch or two below the extremely difficult rating of the theremin. However, when mastered as a pitch controller, it is a sublimely expressive instrument.
The ribbon controller can handle other tasks as well for example, rather than being the sole source of pitch control, a ribbon controller can be used to modulate the pitch signal generated by another device, such as a keyboard or a sequencer. In this application, the ribbon controller excels at applying very expressive vibrato and slides from note to note.
The signals produced by a ribbon controller can be used to control other less pitch-related events. Filtration, amplitude, note timbre and modulation can all be controlled quite dynamically with a ribbon controller. If the ribbon controller is so equipped, it may also be used to generate gate and trigger signals within a larger musical system.
So, now that we know what a ribbon controller is good for, what, pray tell, is a ribbon controller?
Quite simply, a ribbon controller is a long resistive element that, when pressed at any point, comes into contact with a conductive element, thus creating a path of variable resistance from either end of the resistive element to the conductive element, allowing applied voltage or current to generate signals that will, in turn, influence voltage controlled musical devices.
Huh?
OK, a ribbon controller is just a long, drawn out potentiometer with some extra junk added. If you think of a slider potentiometer, youre pretty close to the idea. But, instead of moving the slider to change the resistive value, you press on the ribbon at various points with some object, such as (but not limited to) a finger.
The Mechanics of a Voltage Divider Based Ribbon Controller
A normal potentiometer consists primarily of two elements: a strip of resistance that possesses a constant resistance along its length and a tap that is used to provide contact at any point along the resistive strip. From one end of the strip of resistance to the other, the value is always fixed. For example, from one end of a 1000 Ohm pot to the other, it will always read 1000 Ohms. As mentioned, there is a third connection to the pot besides these two outside points - this connection is the tap. The tap is always in contact with the resistive strip, and is the element that moves when you turn a knob on a rotary pot or slide the slider on a slide pot.
Figure 1: The Potentiometer or Pot Lets say the tap is in the dead center position. Therefore the tap is contacting the resistive strip right in the middle. If the resistive strip is linear (IE, its a linear pot), then from one end of the resistive strip to the tap, it is 500 Ohms, and from the other end of the resistive strip to the tap, it is 500 Ohms. If you move the knob or slider so that it is only a quarter of the way up, then from one end of the resistive strip to the tap it is 750 Ohms, and from the other end of the resistive strip to the tap it is 250 Ohms. Thus, the resistance from either end of the strip to the tap is completely variable with a potentiometer, which is why, I guess, its called a potentiometer. Figure 2: Moving the Tap Changes Its Contact Point Along the Resistive Strip If voltage is placed on one end of the resistive strip, and the other end of the resistive strip is left unconnected, then the current at the tap varies with the position of the tap this is because the resistance between the end of the strip to which the voltage is connected and the tap varies with the position of the tap. The pot is simply following Ohms Law, which states that voltage is equal to current times resistance. Actually, Ohms law is just like a bad preacher it repeats itself three times in slightly different ways, yet is always saying the same damn thing: I = V/R
or...
V = I*R
or.... R = V/I
If you know this, you know electricity, but dont start re-wiring your house yet..theres something called the Watt that may get in your way.
For those of you new to electronics, I means Inductance (which is a fancy term used instead of current). Inductance is expressed in Amperes, or Amps for short, and because it sounds cool. V is, of course, for Voltage, which is expressed, coincidentally, in Volts. Voltage is very useful for reanimating dead corpses, or assemblages of dead body parts sewn into one conglomerate being, among other things. R stands for Resistance, which is expressed in Ohms, because thats the name of the feller that invented resistance. You will often see Ohms represented by the Omega symbol () as its secret, special code.
As you can see, these three properties are locked in a death ménage-a-trois jacking with one always affects at least one of the others, because thats the way Mother Nature likes it. And thats the way ribbon controllers and pots like it, too.
 Figure 3: The Rheostat a Pot Configured to Control Current Figure 3 shows what happens when one end of a pot is wired to a voltage source and the tap is wired to ground. The three horizontal lines at the end of the tap represent ground (another secret, exclusive electro-symbol). Current flows from the voltage source to ground. Well, an engineer will tell you the opposite is true the engineer will tell you that the current actually flows from ground to the voltage source, but most people with a life look at it the way all electronic symbols seem to indicate, so dont worry about that. Anyway, when a pot is configured to control current, its often called a rheostat.
On the left side of Figure 3, the tap is situated in the middle so the current flows through half of the strip (which, in this example, is one half of 1,000 Ohms, which is 500 Ohms). Ohms law tells us that 20 mA (.02 Amps) will flow through the 500 Ohms on its way to ground.
On the right side, the tap has been moved so that only a quarter of the resistive strip is passing the current. That means 10V is flowing through 250 Ohms, which gives you a current of 40 mA. Now, guess what happens if you move the tap so that the current is flowing through 10 Ohms that will give you 1 Amp, which is one hell of a lot of juice; enough juice to easily cause burns, sparks, and general mayhem. This may be a clue that using this document as your sole research in re-wiring a house may not be such a good idea.
Now, actually, the Appendage is not really configured as a rheostat, but rather is configured as a voltage divider, which is another job a pot handles with extraordinary ease. A voltage divider uses Ohms law to present a signal better expressed as voltage rather than a current at the tap output.
Remember that V=I*R, right? Well, that means if you shove I through R, that makes V. This is the coolest part of Ohms law there is, in my humble opinion. Lets say you connect 10V on one end of the resistive strip, and connect ground on the other end, ignoring the tap for the time being. 10V/1000 Ohms = 10 mA. So, you get 10 mA flowing through your 1000 Ohm strip. Ground is 0V, so that means your resistive strip has dropped 10 V across it (this is a very important principle, dont forget it. Ever.). In other words, one end of the strip is 10V and the other end is 0V.
Figure 4: The Variable Voltage Divider a Pot Configured to Control Voltage If you place the tap right at the middle of the strip, thats like chopping your strip into two separate resistors. The top resistor is 500 Ohms and the bottom resistor is 500 Ohms. Each of those two resistors is passing 10 mA to ground. Remember, we drop 10V across the 1000 Ohm strip. Well because the two resistors are equal in value, each one is dropping 5V so that 10V is the total drop. The top resistor drops 5V, but lets pretend we dont know that. Well just act like lunkheads and solve for the unknown value which is voltage (wink, wink): I*R = V
.01 Amps * 500 Ohms = 5 Volts
Ah! So we know the top resistor is dropping 5V. Well, it started out at 10V, so we subtract the 5V from 10V, so we know that the tap will be right at the 5V mark. An easier way to do it is to determine the voltage drop of the bottom resistor. This will tell you immediately what the voltage is at the top of this resistor, without having to subtract it from the starting voltage. But, we just figured the voltage here, so lets move on.
Lets move the tap of the pot to one quarter of the way down from the top of the resistive strip. Now the two resistors formed by tapping the strip at this point are unequal. One resistor will have to drop more voltage than the other in order to get to 0V at ground. The largest resistor will drop the most voltage. The top resistor is now 250 Ohms, and the bottom resistor is now 750 Ohms. Lets do it the easy way and figure the voltage drop of the bottom resistor:
I*R = V
.01 Amps * 750 Ohms = 7.5 Volts
So, we have 7.5 Volts at the tap. Congratulations, youre well on your way to having the privilege of getting in touch with your Appendage. The ribbon of the Appendage is just a pot, and the tap is where you press your finger. Voltage is applied to one end, and ground is applied to the other, just like in Figure 4. Dont worry you wont get zapped by pressing the ribbon because (A) its insulated the electrons cant jump out and kill you and (B) the voltage is so low, you wouldnt know it if you did touch it. Youd be dead instantly. No, just kidding, its such a low potential and is supplied by so little current, it couldnt be felt.
When the ribbon is pressed, the voltage at that point is present on the tap, and, if that tap is connected to a voltage controlled object, such as a VCO, the pitch of the VCO will change to whatever value the voltage at the tap tells it to change to. If you slide your finger up the strip, the VCO goes up in pitch. If you slide your finger down the strip, the VCO goes down in pitch. Beautiful, innit?
But, theres a catch: A ribbon controller differs slightly from a normal potentiometer in that its tap is not in constant contact with the resistive strip. The tap only comes into contact with the resistive strip when the ribbon is pressed. That means, as soon as you release pressure, the voltage seen by your VCO will go to some other value. Which means press, I get note, not press, I get crap, which is basically true. The VCO will only hold the pitch for as long as you press the ribbon. You better figure out some way to mute that VCO when you release, or find some way to make the voltage either stay put or go to a known value until you do something else with the ribbon.
The easiest thing to do is to put a large value resistor from the tap to ground. This way, at least the VCO will always go to some known pitch the pitch that corresponds to 0V. So, now you release your ribbon and your VCO always drops back to the same pitch. Its an improvement, but not much of one. It does provide a certain playing style that depends on that known 0V pitch of the VCO, but it ultimately is still a limiting factor.
The tap voltage could be used to generate a gate signal that could be used to gate a Voltage Controlled Amplifier (VCA) that the VCO signal is passing through. Then, the VCO will only sound on the tap voltage notes. Some designs use an additional pressure sensor or a capacitive sensor to detect when the ribbon is pressed to generate this gate signal. But, ultimately, the gate must shut off immediately after the ribbon is released, or the base pitch of the VCO will be heard on each note.
The best solution in the analog world is some sort of device that could sample the tap voltage when the ribbon was pressed, and hold that voltage when the ribbon was released; some device like, oh, I dont know, a Sample and Hold.
Thats exactly what Dr. Moog did with his ribbon controller. When the ribbon was pressed, its output fluidly changed voltage with position of the pressure (IE, sliding the finger up and down the controller) and held the last value the tap voltage was at when the finger was lifted up. And, thats exactly what the Appendage does as well, though through a different method, with two sample and holds instead of one. It is from these two sample and hold devices that the Appendage derives most of its voltage outputs.
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