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 known as a resistor ladder. 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 responded simultaneously. Yamaha independently developed the concept at about the same time, in Japan, although their version saw little use (other than in the massive and expensive GX-1) until later in the decade.
Later, the design had the additional advantage of making it easy to add velocity sensing. And, once the cost of microprocessors dropped, it was cheaper to manufacture than the resistor-ladder keyboards.
E-mu licensed their design to several other manufacturers; it was notably used by Oberheim in its Four Voice series, and a few years later by Sequential Circuits in the Prophet-5. Royalties from the scanning keyboard patents were a lucrative source of income to E-mu for a number of years. Today, most synthesizers on the market use some form of scanning keyboard.