A videodisc system utilizes a beam of light which tracks the center of an interrupted spiral information channel. The light beam is "dithered" from side to side of the channel in order to determine the relative "direction" of a beam deviating from the center of the channel. The beam oscillates by approximately one-third of the width of the channel. By multiplying the returned signal by the drive signal, a signal is derived whose mean value is positive on one side of the track, negative on the other side and zero when in the center of the track.

Synchronous detection tracking of video disc

A control system for maintaining proper focus of a radiant recording or writing beam, such as a laser beam, with respect to a recording surface, by measurement of the duty cycle of an oscillatory information-carrying signal recorded from the recording surface immediately after recording. The system includes a photodetector for detecting a reading beam used to recover the recorded information, a focusing lens movable with respect to the recording surface to focus the writing and reading beams, a dither oscillator to generate a dither signal employed to move the focusing lens in an oscillatory manner, a duty cycle detector to provide a signal indicative of the duty cycle of the information-carrying signal recovered by the reading beam, and a multiplier for comparing the phases of the duty cycle signal and the dither oscillator signal, and thereby providing a correction signal indicative of the magnitude and polarity focus error. This control system provides fine focus control, and is utilized in conjunction with a coarse focus control comprising a focus detector, differential amplifier and feedback path to the movable focusing lens. In addition, the signal from the duty cycle detector can be coupled to maintain the power of the writing beam at an optimum level and to maintain the duty cycle at a constant level, such as fifty percent.

Apparatus and method for controlling focus in a recording system

A video disc player is described for use with a video disc having frequency modulated video information recorded thereon in the form of a plurality of concentric circles or a single spiral. The information track comprises successively positioned light reflective and light non-reflective regions. A focused light beam is caused to be positioned over the center of an information track and the light reflected from the information track is gathered by an objective lens for application to electronic circuitry for recovering the recorded frequency modulated video signals. Radial tracking means are described for maintaining the focused light spot to impinge upon the center of an information track. Lens focusing means are described for positioning the objective lens at the optimum focused position above the information track for gathering the maximum amount of reflected light from the information track. FM processing means are described for reconstructing the recovered frequency modulated video information such that the ratio between the amplitude of the signals as recorded is essentially the same in the signals as recovered from the video disc member. Further servo means are described for handling the selective change of the intersection of the reading beam with the video disc member in a predetermined preferred mode of operation.

Focusing system for video disc player

An optical information processor comprising a semiconductor laser device which has a rectangular light emitting region, a disk, and an optical system which includes a prism for converting a non-circular beam emergent from the laser device into a circular beam, the circular beam being projected onto the disk by the optical system thereby to record or play back predetermined information.

Optical information processor with prismatic correction of laser beam shape

An automatic optical focusing system suitable for an apparatus for reading an information track on a record carrier with the aid of a read spot which is focused in an imaginary focusing plane and which is formed by a radiation beam which is produced by a radiation source, more in particular suitable for a video disc player which employs optical reading, and serving for automatically keeping the read spot focused at the plane in which the information is disposed (information plane) during playing of a record carrier, comprising an optical focusing device, an oscillating optical element which cooperates with the radiation beam and which can effect high-frequency oscillations so that a high frequency oscillation of small amplitude about the coarse position of the focusing plane is obtained, an electromagnetic drive system for the high-frequency drive of the oscillating optical element, a radiation-sensitive detector which responds to the radiation beam which has been modulated by the information track, and a control circuit for producing a control signal which depends on the output signal of the detector, which control signal is applied to the control coil of the optical focusing device. The oscillating optical element is constituted by an oscillating mirror which reflects the radiation beam which mirror effects high-frequency bending oscillations of which at least a component has the direction of the reflected part of the radiation beam. Preferably, the oscillating mirror is round and effects high frequency bending oscillations between a convex and a concave shape.

Automatic dithering optical focusing system using an oscillating mirror in the path of the radiation beam

A controlled system for focussing a read-out light beam onto a moving track carrying information, comprising: a focussing objective lens for forming a periodic motion along its optical axis, under the action of a pilot device; a comparator comparing the pilot signal with the envelope of an electrical read-out signal, the comparator thus producing a focussing error signal, and means for applying said signal to the focussing objective lens for correction purposes.

Controlled system for focussing a read-out light beam

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Controlled system for focussing a read-out light beam