US 4,232,201 A · Dithered center tracking system

Abstract

A servo control system for maintaining an information detection device, such as a light beam, centered on a moving information track, such as a spiral recording track on a video disc. The system of the invention includes a dither signal generator, a bandpass filter for deriving an intermediate signal indicative of the effect of dither on the detected signal derived from the information track, and switching circuitry for deriving an error signal from the intermediate signal and the polarity of the dither signal. In one embodiment of the invention, the switching circuitry includes an inverter, and a switch for selecting either the intermediate signal or the inverted intermediate signal, in accordance with the polarity of the dither signal. The error signal thus derived is low-pass filtered and has a positive value when the center position of the beam moves to one side of the track, and a negative value when the center position of the beam moves to the other side. In an alternate embodiment, a pulse generator provides two trains of pulses occurring at the positive and negative peaks of the dither signal, respectively, and the pulse trains are employed to control two sample-to-hold circuits, respectively supplied with the intermediate signal and its inverse.

Drawings

Drawings are temporarily unavailable. Open the PDF at USPTO

Drawings

Claims14

Dependent claims

For use in an information storage and retrieval system having a detection means for deriving an information signal from an information track, apparatus for controlling the position of the detection means to follow the information track, said apparatus comprising:

first transducer means for moving the detection means transversely with respect to the information track;
second transducer means for deriving an information signal from the detection means;
means for generating an oscillatory dither signal for application to said first transducer means;
means for bandpass filtering said information signal to obtain an intermediate signal indicative of the effect of dither on said information signal;
means for correcting the phase of said dither signal to obtain a phase-corrected dither signal for correlation with said intermediate signal;
means for deriving an error signal from said intermediate signal and the polarity of said phase-corrected dither signal, wherein said error signal at any instant has a magnitude proportional to that of said intermediate signal and a polarity derived from the product of the intermediate signal polarity and the phase-corrected dither signal polarity;
and
means for combining said error signal and said dither signal for application to said first transducer means.

5 more elements

For use in a video disc record player having a reading beam for deriving a modulated light signal from an information track on a video disc record, a system for controlling beam position to follow the information track, said system comprising:

first transducer means for moving the beam radially;
second transducer means for deriving a modulated light signal from energy reflected from the disc;
means for generating an oscillatory dither signal for application to said first transducer means;
means for bandpass filtering said modulated light signal to obtain an intermediate signal indicative of the effect of dither on said modulated light signal;
means for correcting the phase of said dither signal to obtain a phase-corrected dither signal for correlation with said intermediate signal;
means for deriving an error signal from said intermediate signal and the polarity of said phase-corrected dither signal, wherein said error signal at any instant has a magnitude derived from said intermediate signal and a polarity derived from the product of the intermediate signal polarity and the phase-corrected dither signal polarity, and wherein said means for deriving an error signal includes
means for inverting said intermediate signal,
first and second sample-and-hold means connected to sample said intermediate signal and its inverse, respectively, and
pulse generation means responsive to said phase-corrected dither signal to generate a first train of pulses corresponding to positive peaks of said phase-corrected dither signal and a second train of pulses corresponding to negative peaks of said phase-corrected dither signal, said first and second trains of pulses being coupled to said first and second sample-and-hold means, respectively, whereby said sample-and-hold means supply said error signal;
and
means for combining said error signal and said dither signal for application to said first transducer means.

8 more elements

Dependencies are inferred from the claim text.

Description3,317 words

Summary1,073 words

#This invention relates generally to systems for the reproduction of recorded information, such as video information recorded on a disc, and, more particularly, to systems for centering a light beam on an information track on such a disc.

#Video and other types of information can be stored on a disc as a succession of light reflective and non-reflective regions along a spiral information track on the surface of a disc-shaped record carrier. In reproducing the video information, a video disc player employs an optical system for directing a radiant reading beam, such as a laser beam, onto the information track, and for detecting a reflected beam indicative of the reflectivity of the disc surface scanned by the laser beam as the disc is rotated. In a disc player of the type with which the invention may be employed, the intensity of the reflected beam takes the form of a high frequency carrier signal which is frequency modulated by the video information.

#In order to store a reasonably large amount of information on the disc, successive turns of the information track must be so closely spaced that even a slight degree of disc eccentricity would cause the beam to traverse a number of adjacent tracks on each revolution. Consequently, some means must be provided for applying transverse or radial corrections to the beam position, so that it substantially follows along the center of the information track.

Brief description of the drawings10 figures

#FIG. 1 is a simplified diagrammatical view of an optical playback system for video disc recordings;

#FIG. 2 is a block diagram of a beam position servo control system utilizing the principles of the present invention;

#FIG. 3 is a block diagram showing an alternate form of a beam position servo control system utilizing the present invention;

Detailed description1,997 words

#As shown in the drawings for purposes of illustration, the present invention is concerned with a servo control system for maintaining a beam as nearly as possible in alignment with the center of a recording track on a video recording disc. In the optical system of video reproduction with which the present invention is principally concerned, video information is stored on a disc in the form of raised bumps or lands on the disc surface, and is read by means of a light beam scanned across the surface. The bumps exhibit a lower total light reflectivity than the flat areas between the bumps. Moreover, since the scanning beam has a width comparable to the bump width, the reflectivity signal obtained when the beam is scanned over a series of the bumps is substantially sinusoidal in nature. Information is encoded on the disc by forming a large number of bumps in a spiral track on the disc surface, such that the resultant reflectivity signal has the form of a sinusoidal carrier, which may be frequency modulated with the video information.

#Since most discs, especially those produced for consumer use, will have relatively large eccentricities, some means must be provided for maintaining the reading beam centered on the information track as closely as possible. When only a single reading beam is utilized both for obtaining information recorded on the track and for obtaining an error signal for centering the beam, a radial oscillatory motion, usually referred to as a dither, must be applied to the beam in order to determine in which direction a correction signal should be applied to keep the beam centered on the track. As set forth in the aforementioned copending patent application, an error signal can be obtained by a process of analog multiplication of the dither signal with an intermediate signal derived by filtering the reflectivity signal.

#As shown in FIG. 1, in a typical optical playback assembly a laser light source, indicated by reference numeral 10, directs light onto a disc surface 12. The reading beam from the laser is first reflected by a plane mirror 14, and then focused by a lens 16 onto a beam splitting prism 18, the transmitted portion of the beam, indicated at 20, then being directed through a quarter-wave plate 22, and thence reflected from two successive fixed mirrors 24 and 26, and then from a movable mirror 28, which is pivotally mounted as shown at 30, and is adjustable in angle by means of a transducer 32. Finally the beam is passed through an objective lens system 34 and focused on the disc 12. A beam reflected from the disc follows essentially the same path back to the beam splitting prism 18, at which point the reflected beam is further reflected by the prism to a light detector 36.

Dithered center tracking system

Life of this patent

Dithered center tracking system

Life of this patent

Life of this patent

Abstract

Drawings

Drawings

Claims14

Description3,317 words

Drawings

Classification3 codes

Life of this patent

Description3,317 words