Patent GB-1472933-A (GB)
Title: Optical disc read-out system
IPC: G11B7/12
Filed: 1974-07-25
Abstract: 1472933 Automatic control systems MANSEI KOGYO KK 25 July 1974 [31 July 1974] 32804/73 Heading G3R [Also in Division G5] An optical system for reading video and/or audio signals recorded on a disc comprises means for detecting the recorded information and converting it into electrical signals, means for detecting tracking error and controlling radial movement of the reading head in response thereto, and means for detecting errors in the position of focusing elements and thereby controlling focusing means. The Specification has thirty one figures, none of which are reproduced here. The record comprises a disc of glass or synthetic resin coated on both sides with aluminium or copper on which is etched a spiral track of alternate reflective and non-reflective areas to carry the information as pulse-width modulation. A transparent protective film is added, (Figs. 1, 2, 20). Recording is made by use of a pulsed electron beam and an electro resist coating (Figs. 6, 8). The signal comprises a frequency modulated carrier which provides control signals during silent passages. The reading head is traversed beneath the disc and receives reflected light from a fixed laser (Fig. 9). The head itself contains the means for correcting tracking and focus errors.  Tracking. In one embodiment tracking error is corrected by a mirror in the incident light path, the mirror being attached to a rod clamped at one end and having a waisted portion to provide a torsion spring, twisting movement being imparted by an armature attached to the rod and arranged between the poles of a permanent magnet which also carries modulating windings (Fig. 11). These windings are energized by signals obtained from a low pass filter to which the output of a pair of information reading photo-cells is applied. The filter blocks information frequencies but passes variations in amplitude which result from tracking errors (Figs. 12 to 16). In an alternative embodiment (Figs. 29, 30) a half-mirror is rotated to correct the error. Focusing. In one embodiment (Fig. 18) focusing lenses are mounted in a spring-suspended vertical cylinder which also carries a coil arranged in the gap of an annular permanent magnet. The top of the cylinder carries a metallic plate surrounding the lens which forms a capacitor with the disc coating. Changes in the position of the lens cylinder in relation to the disc alters the capacitance, which is connected in a high frequency circuit, the output of which energizes the coil so as to keep the spacing (and hence the focus) constant. In one alternative arrangement (Fig. 24) focusing error is determined by the difference in output between the two photo-cells (which comprise a single unit) which are mounted on a mirror in the  light path. The error signal adjusts the position of a focusing lens element (86). In one modification of this (Fig. 26) the mirror is adjusted instead of the lens, and in another (Fig. 28) the photo-cells are separate from the mirror, and stationary, while the mirror moves. In each of these arrangements tracking errors cause the reflected light spot to  travel along or parallel to the junction between the photo-cells, thereby producing noise on this signal (Figs. 25, 27 and 31); in a final modification (Figs. 29, 30) the spot moves across the junction only, due to focusing error, so eliminating the noise.  This arrangement has fixed photo-cells, adjustable lens for focusing and tilting half mirror for tracking. Video signals are recorded with one "picture" per revolution; to facilitate slow motion reproduction the reading light spot may be made elongated radially by a semi-cylindrical lens (Fig. 23) so as to read two tracks at once (Fig. 22). It is shown that the long axis of the spot must be truly radial.