ANORDNING FOR AVLESNING AV EN PLAN OPPTEGNINGSBAERER

1434834 Automatic control of position PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 8 May 1973 [11 May 1972] 21901/73 Heading G3R [Also in Divisions G1 and G5] An optical playback apparatus includes a device for centring the read beam 5 1a on the track 3 by forming three light spots A, B1, B2 spaced across the track, a detector 55-57 being provided for each spot. A comparison of the output from spots B 1, B2 at the track edges is used to centre the read spot A. The spots are formed from the zero and Œ1st order images of a source 50 in a diffraction grating 52, higher orders being suppressed, the grating lines being projected on to the disc record 1 at an angle to the track direction. The difference between the outputs of detectors 56, 57 (shown superimposed on the record tracks) controls the position of a pivotable mirror 59 to centre the spot A. Further control signals may be derived as shown by directing the outer beams 51b, c at an acute angle to the record 1, rather then normally as in Fig. 4, (not shown), and dividing their detectors 56, 57 into component detectors 56a, b, 57a, b to monitor the beam positions. The difference in output from detectors 56a, b controls the position of a half-silvered mirror 54 to compensate errors in the position of the source 50, while the difference in output from detectors 57a, b controls a lens 53 focusing the beams on the record 1. The track Fig. 2, (not shown) is a series of coplanar marks and spaces of differing reflectivity, or of the same reflectivity but not coplanar. The lengths of the marks and spaces represents a video signal. The system is applicable to transmission arrangements.