Apparatus for reading a disk-shaped optical record carrier

Claim 1

An apparatus for reading a disk-shaped record carrier containing a video signal recorded in generally parallel tracks, said apparatus comprising: an optical read unit, said read unit including means for projecting a read beam of radiation onto said record carrier so as to read the video signal recorded in said tracks, a deflection element for varying the position of the said read beam on said record carrier in a direction transverse to the direction of said tracks, said deflection element being movable in two opposite directions relative to a center position in response to a control signal, means for deriving the video signal from said read beam after cooperation with said record carrier, and means for measuring radial deviation of said read beam relative to a desired track, said measuring means producing said control signal in dependence on said radial deviation measured thereby, means for radially moving said read unit relative to said record carrier, a servo control loop for controlling the radial position of said read beam on said record carrier, said control loop including said deflection element and said measuring means and further comprising switching means for rendering said servo control inoperative during controlled time intervals and means for moving said deflection element at least towards said center position during said controlled time intervals, and means for controlling said switching means, said controlling means including means, coupled to said deriving means, for detecting field pulses or synchronizing signals in the video signal read from said record carrier, which synchronizing signals are time correlated to said pulses, and means for generating, in response to a given field pulse or synchronizing signal detected by said detecting means, a periodic signal which controls said switching means so as to alternately render said servo control loop operative and inoperative in a predetermined relationship with said given pulse or synchronizing signal in response to which said periodic signal is generated.

Claim 2

An apparatus for reading a disk-shaped record carrier containing a video signal recorded in generally parallel tracks, said apparatus comprising: an optical read unit, said read unit including means for projecting a read beam of radiation onto said record carrier so as to read the video signal recorded in said tracks, a deflection element for varying the position of the said read beam on said record carrier in a direction transverse to the direction of said tracks, said deflection element being movable in two opposite directions relative to a center position in response to a control signal, means for deriving the video signal from said read beam after cooperation with said record carrier, and means for measuring radial deviation of said read beam relative to a desired track, said measuring means producing said control signal in dependence on said radial deviation measured thereby, means for radially moving said read unit relative to said record carrier, a servo control loop for controlling the radial position of said read beam on said record carrier, said control loop including said deflection element and said measuring means and further comprising switching means for rendering said servo control inoperative during controlled time intervals and means for moving said deflection element at least towards said center position during said controlled time intervals, and means for controlling said switching means, said controlling means including means, coupled to said deriving means, for detecting field pulses or synchronizing signals in the video signal read from said record carrier, which synchronizing signals are time correlated to said pulses, and means for generating, in response to a given field pulse or synchronizing signal detected by said detecting means, a periodic signal which controls said switching means so as to alternately render said servo control loop operative and inoperative in a predetermined relationship with said given pulse or synchronizing signal in response to which said periodic signal is generated.