CED (Capacitance Effect Disk)

Also referred to as "RCA Selectavision" video disks.. This is a 12 inch video disk and player system marketed by RCA, and now obsolete. It did not have consumer recording capability. No laser was used; the disks were grooved like (pre-CD) phonograph records, and the video signal was recorded as rising and falling ripples ("hill and dale" in older phonograph terminology). The needle, or stylus, does not follow the ripples exactly, it is not small enough or given enough pressure to. Under current technology it is impossible to make a stylus small and light enough at reasonable cost to do so. Instead, the rapidly varying tiny air space between the stylus and the groove bottom is sensed (using capacitance) to derive the video signal. According to specifications, the disks give about an hour's playing time on a side with 240 lines of horizontal resolution. One revolution of the disk corresponds to four video frames. The disk is never seen or touched in normal use. The viewer inserts the rigid jacket (caddy) into the player and then withdraws it leaving the disk behind. To unload the disk, the jacket is inserted again.