An Experimental Optical Videodisc Playback System

Abstract

An experimental videodisc playback system has been developed using a thin flexible transparent plastic disc as the information carrying medium. The information is read out by transmitting HeNe laser light through the disc. Information is stored on the disc in the form of pitted tracks or hill-and-dale modulated grooves. The player has a simple optical path and servo system. A magnetically driven two-axis mirror controls the light beam independently in the radial and tangential directions, providing excellent radial tracking and time-base correction. Vertical focusing of the light beam is maintained by aerodynamic stabilization of the disc. A high-frequency carrier is frequency modulated with the chroma and luminance information. Luminance bandwidth is equal to that normally used in NTSC receivers. Tentatively, the sound has been put on its own FM carrier at a low frequency. A transcoder converts the disc signal into an NTSC signal on a VHF carrier. Because the disc is transmissive, both sides can be played without crosstalk merely by adjusting the focal plane of the laser light spot; the disc itself need not be repositioned. Track spacings as close as 9,000 tracks/cm (0.39 in), corresponding to a playing time of 4 min 43 s/cm, have been achieved experimentally with acceptable crosstalk between tracks.