Abstract: The clock signal output of an oscillator is synchronized to the frequency of a data signal by detecting the phase difference between the two signals. The control signal signifying the difference is quantized into a number of standard units and a remainder. The remainder is accumulated and, when it is equal to a standard unit, added to the number signal signifying the number of standard units. The frequency of a fixed frequency oscillator is then adjusted in a direction tending to nullify the number signal.

Abstract: An apparatus for optically reading a disc-shaped record carrier, which apparatus comprises a radial tracking control system which employs a radial error signal obtained by comparing two signals supplied by two radially adjacent detectors. In order to stabilize the loop gain of the control loop at a constant value, a measuring signal of predetermined frequency is injected into the control loop. The response of the control loop to said injected signal is compared with the injected signal, which comparison results in a control signal for a controllable amplifier circuit. Suitably, said comparison is a phase comparison. The injected measuring signal causes the scanning beam to oscillate relative to the track to be followed.

Abstract: An apparatus for optically scanning a disc-shaped record carrier, which apparatus comprises a radial tracking control system which employs a radial error signal obtained by comparing two signals supplied by two radially adjacent detectors. In order to correct tracking errors caused by deviations between the signals supplied by the two detectors the correlation between the radial error signal and the amplitude of the information signal being read is measured. The control signal obtained by means of this correlation measurement controls a balancing amplifier-circuit for balancing the two signals supplied by the detectors. Suitably a signal of predetermined frequency is injected into the control loop to cause the scanning beam to oscillate relative to the track so as to improve the correlation measurement. This injected signal may also be used for stabilizing the loop gain in the radial tracking control loop.

Abstract: Disclosed in an apparatus for optically scanning a record carrier in which the scanning beam is focussed on the record carrier by a focussing system. For automatically correcting an incorrect control point of the focussing system, the apparatus is provided with a phase detector to which are applied the focussing error signal generated by the focussing system and a signal corresponding to the amplitude of the information signal being read. The output signal supplied by the phase detector is applied to the focussing system a correction signal so as to automatically correct for an incorrect control point of the focussing system.

Abstract: A focusing system for focusing a radiation beam onto an object, in particular an optical disc. The focusing system comprises focusing-error detection means and two control elements for correcting the focusing depending on the detected focusing error. The detection means are constructed so that via one of the two control elements a feedback servo system is obtained while via the other control element a servo system without feedback, i.e. a feed-forward system, is obtained.

Abstract: An apparatus for reading a disc-shaped record carrier on which per track circumference one picture of a color television signal is recorded. The signal being read is converted to a standard PAL color television signal. The apparatus comprising a command device which by applying command signals to control means can produce a jumpwise radial change in the scanning position on the record carrier, so as to enable the scanning sequence of the recorded pictures to be changed. In order to maintain a standard PAL color television signal in the case of a changed scanning sequence, the apparatus comprises a correction device for the chrominance signal. This correction device is controlled by the command device and comprises a mixing circuit for mixing the chrominance signal with a reference signal of twice the chrominance subcarrier frequency during time intervals prescribed by the command device, and phase correction means for introducing a phase shift of 0.degree., 90.degree., 180.degree., or 270.degree.

Abstract: An optical read unit for reading radiation-reflecting video discs comprises a semiconductor laser which with the aid of a scan wobbler and a focussing wobbler is moved at high frequency in two directions relative to the video disc, a radial direction for tracking purposes and a direction normal to the disc for the purpose of automatic focussing. The semiconductor diode laser is mounted on a metal radiation source carrier which is connected to a heat sink for carrying off the heat with a low temperature gradient, and is driven by piezo-electric plates.

Abstract: An optical scanning device which scans a rotary video disc with the aid of a light beam comprises an objective which performs focussing movements, for focussing a light spot on a recording surface of the video disc, as well as tilting tracking movements and time-error correction movements. Near its lower end the objective is suspended in an impregnated corrugated loudspeaker diaphragm of fabric fibre which enables the various movements of the objective to be made without giving rise to annoying resonant effects. The tracking movements and the time-error correction movements are obtained with the aid of coils which are connected to the objective near the upper end and which are disposed in air gaps of associated permanent magnetic stators.