Abstract: A focus servo control system for automatically adjusting a relative distance between an objective lens unit and a recording surface of an optical recording medium in which a laser light beam is converged through an object lens on a recording surface of a recording medium and the laser light is modulated by an optical pattern on the recording surface and reflected therefrom and converted into an electrical signal wherein the focus control system includes a component adapted to be energized by an electrical current to shift the object lens perpendicularly to the recording surface, a component for detecting the laser light reflected from the recording surface for producing an error signal which represents the deviation of the focus of the lens from a desired position with respect to the recording surface, and a servo amplifier adapted to amplify the error signal thereby to produce the electric current for energizing the lens shifting components, and in order to avoid wasteful electrical power consumption and the

Abstract: A tracking servo system of a video disc player, in which the switch provided in the servo loop of the system is closed and accordingly the servo system is locked in when the level of the reproducing signal read out from the video disc is at its peak or close to the peak and concurrently the amount of error between the scanning spot and the track to be scanned is of a minimum value.

Abstract: A focus lens protecting and in-focus detecting device has light receiving circuit and a comparator for comparing the output of said light receiving elements with a reference potential. A switch network is controlled by the output of the comparator to supply signals to a focus control amplifier from the light receiving circuit.

Abstract: A two-way CATV system having a substantial number of terminals incorporates a central processor and transmitter system which continually interrogates the terminals at high speed, monitoring changes of usage of premium programs and other data while also permitting central control of access to restricted premium programs. For inexpensive but reliable intercommunication, short, fixed word length digital messages are frequency multiplexed onto the cable in both upstream and downstream directions, and subscriber groups are controllably selected in turn, with each subscriber being scanned in what may be an arbitrary order by transmission of unique private messages which are recognized at the terminal and in response to which previously prepared fixed length messages are immediately transmitted in return.