The capacitance (that is, the amount of electrical charge necessary for current to pass from the disc to the electrode) is measured, and because this varies based on the depth of the stylus in the groove (the higher it is the more charge is required) a signal can be reconstructed.
A diamond stylus sits in the groove, a metal electrode attached to it. The video signal is stored on the disc by varying the depth of the grooves to correspond to the waveform of a frequency-modulated (FM) video signal – similar to a broadcast television signal. The system worked by making the disc conductive – that is, electricity could pass through it. But by 1972 the team had produced a prototype disc system capable of holding 10 minutes of colour video. RCA first began researching a method of recording video on to a disc in 1964, however they didn’t devote a lot of resources to it and with only four people working on it development was slow. While Asian manufacturers were feverishly working on perfecting videotape, American RCA was working on a videodisc system – not a laserdisc system, which did not contact the surface of the disc, but instead a system similar to a phonograph record, where a needle sat in a groove carved into the disc.
