A keyboard that uses a mircoprocessor to detect which keys are pressed at a given time, and outputs this information as a stream of digital data. The scanning keyboard was essential to the design of affordable polyphonic synthesizers, starting in the late 1970s. Prior to that time, most synths used keyboards which produced a control voltage using an array of diodes and precision resistors. This worked well for monophonic synths, and clever designers managed to extend the design to produce control voltages corresponding to two pressed keys for duophonic synths, but it was unworkable for true polyphony. In 1973, designers at E-mu Systems developed a design that used a microprocessor to check each key, one at a time. This was done very fast so that the player had the impression that all keys