Autofocus circuit for a videodisc playback device

An apparatus for reading a flat reflecting record carrier on which information is stored in an optically readable structure, comprising a radiation source means for supplying a read beam;

• an objective system for focussing the read beam on the plane of the information structure;
• a radiataion-sensitive signal detection system for converting the read beam after having been modulated by the information structure into an electrical signal;
• and an objective position control system, the objective position control system comprising focus detection means for providing an electrical focussing signal corresponding to the direction and magnitude of the deviation of the objective system from a predetermined distance from the information structure of the record carrier at which predetermined distance the read beam is focussed on the information structure, a signal controlled objective drive means for advancing the objective system toward the record carrier information plane from a rest position at a distance exceeding said predetermined distance to a position closer than said predetermined distance and for moving said objective system away from said record carrier, first signal controlled electrical means for selectively connecting said focussing signal to said objective drive means in response to a first timing signal, second signal controlled electrical means connecting said focus detection means to said drive means through said frist signal controlled electrical means for selectively amplifying the focussing signals that advance and move back the objective system in response to a second timing signal and for selectively amplifying and amplitude limiting only those focussing signals that advance the objective system in response to an absence of said second timing, signal, third signal controlled electrical means responsive to an absence of said second timing signal for selectively adding to said focussing signals an electrical bias signal that tends to advance the objective system at a substantially uniform rate toward the record carrier and for providing velocity feedback to said objective drive means through said first signal controlled electrical means, thereby advancing said objective system at a substantially fixed velocity in response to an absence of said second timing signal, and clock means for providing said first timing signal for said first signal controlled electrical means and for concurrently providing an absence of said second timing signal for said second and third signal controlled electrical means to advance the objective system from said rest position toward said record carrier at a substantially fixed velocity and for providing said second timing signal at a time after the initiation of the first timing signal calculated to be such that the objective system is closer to the record carrier than said predetermined distance