Control Signals, Polarity & Depth
In Part 66, you learned that a function generator produces a shape, rising then falling. But a shape by itself does nothing. It needs a destination (what to control), a direction (which way to push), and a range (how far to push). This lesson introduces the three ideas that connect a function generator to the rest of the synthesizer.
Control Signals: Numbers That Change Over Time
Every time you have dragged a slider in this series, you have sent a number to the synthesizer. A frequency slider sends a number like 440 to an oscillator. A volume slider sends a number like 0.7 to an amplifier.
A control signal is the same idea, but automatic. Instead of you dragging a slider, a function generator produces a number that changes on its own, smoothly rising and falling, over and over. The scope on the left below shows this: a number climbing from zero to its peak, then falling back down. That changing number is the control signal.
The question that matters is: what range of numbers does it produce?
Unipolar and Bipolar
A control signal that goes from 0 to 1 is called unipolar. "Uni" means one; it only goes in one direction: up from zero. The function generator in Part 66 was unipolar. It rose from 0 to 1, then fell back to 0. It never went negative.
A control signal that goes from -1 to +1 is called bipolar. "Bi" means two; it swings in both directions, above and below zero. Zero is the center. Positive numbers push one way; negative numbers push the other way.
Why does this matter? Because the range determines what happens at the destination. If you route a unipolar signal to pitch, the note only bends upward. Zero means "stay where you are," and 1 means "go higher." If you route a bipolar signal to pitch, the note swings above and below its center. That is vibrato.
Depth and Inversion: One Knob, Three Behaviors
The Depth slider in the demo below does something interesting. It has three zones:
Positive values (right of center): The control signal passes through as-is. At +100%, you get the full effect. At +50%, you get half the effect. When the function generator rises, the parameter rises.
Zero (center): Nothing passes through. The control signal is completely blocked. The parameter sits still, no matter what the function generator is doing.
Negative values (left of center): The control signal passes through flipped upside-down. At -100%, you get the full effect, but inverted: when the function generator rises, the parameter falls. Everything is backwards.
In hardware synthesizers, this kind of knob is called an attenuverter, a combination of "attenuator" (reduces a signal) and "inverter" (flips it). The name is jargon, but the idea is simple: one knob controls both how much and which direction.
Same Shape, Different Meaning
The function generator does not know what it is controlling. It just produces a number that changes over time. The destination gives that number meaning.
Route the same triangle shape to pitch, and you hear vibrato, the note wobbling up and down. Route it to the filter, and you hear a wah effect, brightness sweeping. Route it to the wavefolder, and the timbre breathes. Route it to the VCA, and the volume pulses: tremolo.
The four buttons in the demo let you switch destinations. Try each one with the same Depth and Rate settings, and notice how the same control signal creates four completely different effects.
References
- Part 10: Modulation, where you first used an LFO for vibrato, tremolo, and filter sweeps
- Make Noise: MATHS, attenuverters in action
- Learning Modular: Attenuverter
- Perfect Circuit: Learning Synthesis, Control Voltage
- Wikipedia: Control Voltage