1451799 Automatic control of position THOMSON-CST 22 Aug 1973 [25 Aug 1972] 39870/73 Heading G3R [Also in Division G5] In an arrangement for optically playing back frequency or phase modulated signals recorded as topographic features 14, which can be projections or depressions, in a track formed in a e.g. transparent disc 1, light from source 5 is focused by object lens 7 on the track to illuminate an area 10 of two photo-cells 12, 13 when a space between features is illuminated and an increased area 11 when the beam is diffracted by a feature 14. The output of the cells is added and amplified 18 to provide the output signal s(t). In addition the output of the cells is connected to a differential amplifier 17 to provide a difference error signal E which via a transducer 8 controls the position of lens 7 to shift the beam to compensate any deviation in track position. The transducer 8 which is supplied with the error signal via a low pass filter 21 is a ceramic electrostrictive bimorph device. In a signal reflection arrangement, Fig. 6 (not shown) the record is non-transparent and can be provided with a reflective coating and the beam is transmitted via a semi-reflector plate which then directs the signals reflected from the record to the pair of photo-cells.
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