Keyboard Controller

We'll start the "lecture" with a keyboard, as that is the typical human-machine interface.  A synth keyboard is fairly simple but I won't spend much time on "how" it works, but I hope to explain "what" is does.  A "synth keyboard" is a piano-like keyboard that may span between two and seven (or more) octaves worth of notes.  There might be various outputs unrelated topics on this page.  There might even be a separate "box" that connects to the synth keyboard to provide the outputs required for an analog synthesizer.  There are simply too many possibilities to discuss a specific keyboard, so my comments are confined to the most elemental keyboard possible.  No, this most elemental keyboard does not really exist!  Real keyboards do far more than is needed for control of an analog synthesizer so I refuse to clutter this page addressing all of the possibilities.  Today, most analog synthesizers have two requirements that must be fulfilled in order to control a big box of analog sound modules.

The first requirement is that we get a DC voltage whose amplitude is linearly proportional to the last note that was pressed.  In addition, this voltage shall increase by exactly 1.000V for each jump by one octave.  If say note C0 produces 1.000V, then note C1 will produce 2.000V, note C2 will produce 3.000V, note C3 will produce 4.000V, and so on.  This output of a synth keyboard is called the control voltage, or CV.  When a key is pressed, this analog voltage output assumes a voltage potential that is proportional to the key that was pressed.  The industry standard is 1 volt per octave.  That means each increase in an octave will increase the output by exactly one volt.  That also means that each note is 0.08333333 volts different from the next key.  (1 volt per octave/12 notes = 0.0833333V per note.)  The lowest key might produce 0.000 volts.  In that case, the very next note will produce +0.083333V and so on.  Five octave keyboards might produce a 0 to 5.0000V range of output.

The second requirement is that we get an indication that a note has been pressed.  This  output is a logic level signal that is either on or off (1 or 0).  This is the "Gate" output.  When *any* note on the keyboard is pressed, the "Gate" output is on (1).  When no note keys are pressed, this output is off (0).

Keyboard Controller Recap

A synth keyboard produces at a minimum, two signals:

CV - A control voltage linearly proportional to the last note pressed

Gate - A logic signal that is (1) when any note is pressed and (0) if no note is pressed

Note: The control voltage (CV) stays at the corresponding voltage of the last note pressed.  Just because you are not playing a key, the CV output will stay at the voltage of the last key you did play, even if that key was played an hour ago!  It is the Gate signal that communicates whether or not a note is currently pressed.  What is the value of CV when the unit is first turned on (there was no "last" note!)?  Very probably it is the lowest note of the keyboard ... but check it out for yourself.

Note: Nobody said that the CV output could only be a positive voltage with respect to ground!  The CV output may be negative with respect to ground, as well.  But the convention is that as you move up the the keyboard, the voltage will increase in the positive direction.  Lower notes produce lower (or more negative) CV values, while higher notes produce positive (or less negative) voltages with respect to ground.

Note: A typical analog synthesizer keyboard will likely have a CV output within the range of -10V to +10V.  Some keyboards might be -3V to +5V, or some other asymmetrical range.  Most of the time, the keyboard can be coaxed into a -5V to +5V range.  That is a ten volt change which corresponds to 10 octaves.  No other instrument in an orchestra has a pitch range that exceeds the typical analog synthesizer.