Servo system for reading from a disc-shaped record carrier containing signals coded in optical form

Abstract

Apparatus for reading a disc-shaped record carrier in which video and/or audio information is recorded in a preferably spiral track. This information is optically read by means of a beam of radiation which via a reflecting element transmits the information present in the scanned point of the record carrier to a read detector. The radial position of the scanning point is controlled by controlling the angular position of the reflecting element which for this purpose is included in a control loop. In order to reproduce the information at a speed different from the recording speed, for example to display slow-motion pictures or still pictures, the reflecting element may be subjected by means of a control signal to an abrupt change in angular position so that the scanning point undergoes an abrupt radial displacement. This displacement preferably takes place during the frame flyback periods of the video information.

What is claimed is: 
     
       1. Apparatus for reading a disc-shaped record carrier .[.of the type.]. wherein video and/or audio signals are recorded in .[.tangential.]. .Iadd.substantially spiral .Iaddend.tracks, the apparatus being of the type wherein an optical system directs a light beam from a light source to the record carrier, and images the information from the record carrier on a radiation detector means for converting the optical image into an electrical signal, wherein position detector means for measuring the radial position of the optical system relative to a desired track provides position signals to a coarse position feedback loop for moving the optical system radially with respect to the record carrier and further provides position signals to a fine position feedback loop, wherein the fine position feedback loop controls a beam deflection means pivotally mounted in the path of the light beam of the optical system for selectively angularly redirecting said beam with respect to said radiation detector means in response to said position signals from said fine position feedback loop, the improvement wherein switching means are provided in said fine position feedback loop for selectively opening said fine position feedback loop and for providing a preselected auxiliary control signal to said beam deflection means thereby angularly deflecting said beam by a predetermined amount and for subsequently reconnecting said fine position feedback loop. 
     
     
       2. Apparatus as claimed in claim 1, wherein said beam deflection means comprises a rotatably mounted beam deflector element, and a winding which is rigidly secured to the rotatable element and is arranged in a magnetic field and to which is applied the fine position feedback signal. 
     
     
       3. Apparatus as claimed in claim 1, wherein said beam deflection means comprises a rotatably mounted beam deflector element, and a magnetic element which is rigidly secured to the rotatable element and is arranged in a magnetic field which is variable in accordance with said fine position feedback signal. 
     
     
       4. Apparatus as claimed in claim 1, wherein said beam deflection means comprises a rotatably mounted beam deflector element, and a piezoelectric torsion element which serves as a pivot for the rotatable element and to which a control signal is applied from said fine position feedback loop. 
     
     
       5. Apparatus as claimed in claim 1 further comprising means for deriving said coarse position feedback signals from said fine position feedback loop. 
     
     
       6. Apparatus as claimed in claim 1, further comprising a first transistor having a main current path included in the connection between the position detector means and the fine position feedback loop and having a control input, a second transistor, means connecting the fine position feedback loop to the signal source via a main current path of the second transistor, the control electrodes of the first and second transistors comprising means for receiving control signals for normally forward biassing the first transistor and cutting off the second transistor and during the switching cycle for cutting off the first transistor and forward biassing the second transistor. 
     
     
       7. Apparatus as claimed in claim 1, wherein the switching means comprises a first monostable multivibrator to which is applied the starting pulse for the switching cycle, a second monostable multivibrator which is controlled by the output signal of the first multivibrator, an adding circuit means for adding the output signals of the first and second multivibrators with mutually inverse polarities to one another and for applying the resulting sum signal as a control signal to the second driving device, and a third bistable multivibrator which is controlled by the output signals from the first and second multivibrators for providing an output signal for opening the control loop during the switching cycle. 
     
     
       8. Apparatus as claimed in claim 1 for reading video and audio information, further comprising means for continupously reproducing the audio information while repeating the display of the same video information. 
     
     
       9. Apparatus as claimed in claim 1, wherein the rotatable element is a reflecting element. 
     
     
       10. Apparatus as claimed in claim 1, wherein the starting instants and the sequency of the switching cycles are determined by command signals present on the record carrier and wherein said apparatus further comprises means for controlling said means for opening and closing said fine position feedback loop. 
     
     
       11. Apparatus as claimed in claim 1, wherein the switching means comprises means for detecting the frame pulses in the video signal read from the record carrier, a programming unit means connected to the frame pulse detecting means for selecting and for delivering selected frame pulses as starting pulses for the switching cycle. 
     
     
       12. Apparatus as claimed in claim 11, further comprising means in said programming unit for performing said selection in accordance with a programming signal which is read from the record carrier. .Iadd. 13. Apparatus for reading a disc-shaped record carrier wherein video and/or audio signals are recorded in substantially spiral tracks, comprising optical system means for directing a light beam to the record carrier whereby the light beam is modulated by the record carrier, and for transforming the modulated light beam into an electrical signal, means for converting said electrical signal into a radial tracking control signal corresponding to the radial position of the optical system means relative to a desired track, coarse position control means for moving the optical system means radially with respect to the record carrier in response to said radial tracking control signal, beam redirecting means in the path of the light beam in the optical system means and connected to said signal converting means for selectively radially deflecting said optical system means with respect to said desired track in response to said tracking control signal, said optical system means, converting means and beam redirecting means comprising a fine position feedback loop, and switching means in said fine position feedback loop for selectively disabling said fine position feedback loop and for providing a preselected auxiliary control signal to said beam redirecting means thereby deflecting said optical system means by a predetermined amount and for subsequently reactivating said fine position feedback loop. .Iaddend..Iadd. 14. Apparatus for reading a disc-shaped record carrier wherein video and/or audio signals are recorded in substantially spiral tracks, comprising optical system means for directing a light beam to the record carrier whereby the light beam is modulated by the record carrier, and for transforming the modulated light beam into an electrical signal, means for converting said electrical signal into a radial tracking control signal corresponding to the radial position of the optical system means relative to a desired track, coarse position control means for moving the optical system means radially with respect to the record carrier in response to said radial tracking control signal, beam redirecting means in the path of the light beam in said optical system means and connected to said signal converting means for selectively radially deflecting said optical system means relative to said desired track in response to said tracking control signal, said optical system means, converting means and beam redirecting means comprising a fine position feedback loop, and switching means in said fine position feedback loop for replacing said radial tracking control signal with a preselected auxiliary control signal thereby deflecting the optical system means by a predetermined amount and for subsequently removing said preselected auxiliary control signal and reinstating said radial tracking control signal. .Iaddend.