Think of a square wave. If it is symmetrical it is 50% positive and 50% negative.
To modulate it, you can vary the duty cycle from 1%-99% through to 99%-1%.
This is commonly used in switch mode power supplies to regulate the voltage. The advantage being that output devices are more efficient when working in on/off states than they would be somewhere in between as they would waste energy as heat.
Pulse width modulation can also be used in amplification. If you run an oscilator at about 50kHz and pulse width modulate it with an audio signal, the digital output can be used to drive a speaker.
Commercially, these units have a filter on the output to prevent the 50kHz getting to the speaker but in a version I built, I didn't bother as the speakers impedance at this frequency is far too high for it to be a problem.
Steve.
