System and apparatus for centering a light beam on an information track

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 (60), a bandpass filter (68) for deriving an inter- mediatesignal 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 dithersignal. In one embodiment of the invention, the switching circuitry includes an inverter (72), and a switch (74) 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 thecenter position of the beam moves to one side of the track, and a negative value when the center position ofthe 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.