Focus detection system for a video disc player using a plurality of radiation sensors in the far field of the information structure

Apparatus for reading a record carrier on which information is stored in an optically readable track-shaped information structure, said apparatus comprising a radiation source;

• a radiation-sensitive information signal detector;
• objective system means for directing radiation from said radiation source to a spot on the information structure of said record carrier and for thereafter directing said radiation from said record carrier to said radiation-sensitive information signal detector;
• means for relatively moving said record carrier and said radiation spot;
• and a focus and centering error detection system comprising four radiation-sensitive focus and centering error detectors disposed in the far field of the information structure at one side of a plane defined by the optical axis of the objective system means and a line normal to the center line of the track portion on which said radiation spot is directed, said focus and centering error detectors being each disposed symmetrically in one of four quadrants of an imaginary X-Y co-ordinate system whose X-axis extends in the track direction and whose X-axis extends transversely to the X-axis, said focus and centering error detectors thereby forming a four quadrant detector, a first adder circuit having inputs connected to a first and fourth quadrant of said four quadrant detector, a second adder circuit having inputs connected to a second and third quadrant of said four quadrant detector, a third adder circuit connected to the third and fourth quadrants of said four quadrant detector, a fourth adder circuit connected to said first and second quadrants of said four quadrant detector, a first subtraction circuit having inputs connected to outputs of said first and second adder circuits, a second subtraction circuit having inputs connected to outputs of said third and fourth adder circuits, a fifth adder circuit having inputs connected to outputs of said first and second adder circuits, a first multiplier circuit providing a radial error signal, a second multiplier circuit providing a focus error signal, and coupling means for connecting outputs of said first subtraction circuit and said fifth adder circuit to inputs of said first multiplier and for connecting outputs of said second subtraction circuit and said fifth adder circuit to said second multiplier circuit and for introducing a phase shift between the output of said fifth adder and each of the outputs of said first and second subtraction circuits.