U.S. Philips Corporation

Cornelis Henricus Vos

Antonius A Van Alem

Nicolaas Alphonsus Verhoeckx

Johannes W Coenders

Herman Van Der Hoff

An apparatus for reading a disk-shaped record carrier, the radial scanning position being controlled with the aid of a first servo control loop, which controls a positioning member for the read element depending on a control signal which is read from the record carrier. If this control is disturbed a second servo control loop is rendered operative. This second servo control loop comprises a position indicator for supplying a position signal which is representative of the radial position of the positioning member, a memory circuit for storing the value of this position signal at the instant of failure of the first servo control loop, and a differential amplifier for applying a control signal to the positioning member, which signal corresponds to the difference between the instantaneous and the stored value of said position signal. Furthermore, there is provided a control circuit for changing-over between the first and the second servo control loop and vice versa depending on a built-in criterion.

Dual radial position servo to compensate for radial tracking failure in

In a videophone system sound, video, synchronizing and signalling signals are to be transmitted through a single pair of telephone cables of existing telephone communications. In that case sound, synchronizing and signalling signals are transmitted in a digital form and the video signals are transmitted in an analog form. It is then possible to achieve optimum picture quality at the given bandwidth of the existing telephone communications. If for the given bandwidth the video signal were also digitalized, this would be at the expense of the picture quality. However, it is then necessary to transmit information regarding clock pulses for decoding at the receiver end. To be able to synchronize at the receiver end only on two frequencies instead of on clock pulse frequency, line frequency and field frequency, the information regarding the clock pulses is cotransmitted in such a manner that a clock pulse generator at the receiver end can be synchronized therewith which generator supplies clock pulses. A field code word is also transmitted during each field flyback period. In the receiver the regenerated clock pulses are divided in frequency in a divider stage and the divided signal is compared with the detected code word in a synchronizing verification circuit. As long as there is no phase equality between component signals and detected code word, an output signal from the verification circuit resets the divider stage.

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Emmasingel, Netherlands

Cornelis Henricus Vos