Electrical systems come in two flavors, digital and analog. Digital systems use incremented inputs and outputs, 1s and 0s to communicate information. Analog systems accept inputs and then output information as they sense it from the outside world.
To understand the difference between digital and analog systems, compare digital and analog clocks. Digital clocks are updated electronically as opposed to analog clocks which use motors and gears to move hands that point to numbers on the clock face.
VPI designs analog to digital, and digital to analog systems, which convert inputs into a format that the system can use.1