Harpo Description and Operating Instructions. Computer Printout. "The basic idea with HARPO was to allow someone with only a marginal knowledge of computer concepts to control video effects in an accurate, fast, and repeatable manner. In reality the user probably either needs much coaching or much knowledge of computer concepts to succeed. The video effects are created by sequence of control voltages applied at specified timed intervals to video hardware through digital to analog converters (DACs). Digital control signals can also be specified, to control things like switchers. HARPO also can sample the analog world and store what it finds there, for later use."