APPARAT FOR AVLESNING AV EN OPPTEGNINGSBAERER, I HVILKEN ER OPPTEGNET INFORMASJON, F.EKS. VIDEO- OG/ELLER LYDINFORMASJON, I MINST ETT SPOR

摘要

1428731 Automatic control of focus PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 26 March 1973 [29 March 1972] 14452/73 Heading G3R [Also in Division G1] A system for determining the focus of a radiation beam on a track on an information carrier (disc) comprises a pair of radiation detectors arranged one after the other in the direction of the track length and means for deriving a focus error signal from the relative sign and magnitude of the phase difference between the outputs of the two detectors when the carrier is in motion. In The Figure, if the plane of the disc is at e, in the focal plane of lens 18, information from a track 3 is received simultaneously by both detectors 10' 10". If there is a focus error (e.g. the disc is at g or f) an information element b cuts the extremes of beam at different times when travelling in the direction 15 so that there is a time (i.e. phase) difference in the outputs of the two detectors. This is processed by unspecified electronic means to produce a control signal for "known means" for correcting the focus. A wedge prism (14), Fig. 5 (not shown) may be placed behind condenser 19 to allow wider separation of the detectors. To avoid errors due to radial displacement of the tracks a further lens may be placed in front of the condensors Fig. 6, (not shown) and the two detectors may form a single integrated circuit element. The detectors in the above are also used to reproduce the information signal, but there may also be provided a similar system using a wide beam and specially recorded marks, e.g. in the frame flyback period of a video signal. This system Fig. 7 (not shown) has an image forming slit in front of the detectors combined with a grating structure for radial position control as described in Specification 1391335. The record may be in the form of radiation reflecting rather than absorbing form. The information may be in the form of light/dark areas, sinusoidal density changes or thickness changes (which affect the phase of the light beam).