FIELDS, TRACKS AND MODULATION
There are two ways of arranging the television fields on the surface of the disc, both illustrated in Fig. 20.1. At (a) is depicted the ‘active’ disc in which each TV field occupies one half of the disc’s rotation, with all the field sync pulses lined up across a diameter of the disc. These play at a constant speed of 1500 r.p.m. and have the advantage of excellent trick-speed replay: the pick-up is ‘skipped’ sideways during the field blanking interval for still-frame or special-speed playback. This disc is a CAV (Constant Angular Velocity) type, and since the track-length for 1 field of 312^ lines varies greatly from the middle to the outside of the disc the pit-density is great at the beginning of play, but sparse at the end. CAV discs have 36 minutes’ playing time per side.
The alternative arrangement is the CLV (Constant Linear Velocity) disc shown at Fig. 20.1(b). Here the pit-density along the tracks is constant so that each TV field has the same track length. At replay start (disc centre) 2 fields are read out per rev at 1500 r.p.m. As playback continues the disc slows down to maintain constant track- scanning speed until at the outer edge the pick-up is reading six fields
per rev at about 550 r.p.m. This disc is capable of 54 minutes playing time per side, but no trick-replay is possible.
The pit-track pattern is shown in Fig. 20.1(c). The track pitch is 1.6 micron, the pit width is 0.4 micron and pit depth 0.1 micron. The surface of the disc is scanned by a very fine (0.9 micron) spot of light from a laser, and the information is read out by a photodiode which discriminates between the reflected light level during the presence or absence of a pit. In the absence of a pit most of the light is reflected onto the pick-up diode which passes high current; when a pit is present most of the light is scattered and little returns to the photo- diode, whose current is now low as a result. The diode output contains all the video, colour and sound information.