Patent GB-1526439-A (GB)
Title: Apparatus for detection of track locking playback apparatus for video discs
IPC: H04N5/76
Filed: 1975-10-17
Abstract: 1526439 "Locked-groove" detector for video disc player RCA CORPORATION 17 Oct 1975 [12 Nov 1974] 42670/75 Heading G5R Apparatus for playback of a video disc having a spiral track, in which corresponding sync. signal locations in successive convolutions of the spiral are non-radially aligned, has means responsive to the sync. signals of the reproduced video signal for producing an output pulse indicative of unwanted repeated playback of the same convolution of the track. The arrangement may be applied to detect a "locked groove" during the playback of a variable capacitance disc by an electrodecarrying stylus tracking a spiral groove having geometric groove bottom variations, corresponding sync. signals in successive groove convolutions being staggered in a spiral manner as disclosed in Specification 1,526,440. The separated horizontal sync. signals pass to input S, Fig. 2, of a phase locked loop circuit 60 giving an error signal at output I when oscillator 61 and input S are out of phase. Voltage comparator 70 gives an output pulse at 0 when output I is of a given polarity and exceeds a preset level, so that comparator 70 does not respond to minor phase deviations or deviations due to radially inward movement of the stylus, but responds to outward movements of the stylus obtaining in a "locked-groove" situation in which one or more convolutions are repeatedly replayed. Each output pulse of comparator 70 triggers retriggerable one-shot 80 to its metastable state to give an output to add charge to a capacitor 94 of a staircase voltage generator 90. A one-shot 100 is triggered when the potential across capacitor 94 exceeds a threshold. The period of retriggerable one-shot 80 is less than the time for one revolution of the disc but is a substantial fraction, e.g. 1/11th, of this time, so that for inputs at the once/disc revolution rate or slower one-shot 80 gives one output pulse for each input pulse, but for inputs at a faster rate, caused by transient conditions such as stylus set-down or player jarring, each input series producing only one output pulse from one-shot 80. Use of staircase generator 90 ensures that, for example, quickly selfcorrecting locked-groove situations do not trigger one-shot 100 which only responds to a sustained period of groove-lock, e.g. one-shot 100 may trigger when generator 90 is pulsed to charge capacitor 94 at least four times within about three seconds. Triggering of one-shot 100 causes a drive pulse to be applied to a transducer T from a drive pulse generator 110 via step-up transformer 118 and pulse shaper 120. Transducer T, which may be a piezoelectric bimorph element 59, Fig. 4, produces a radial inward shift of the stylus, to correct the groove lock. The pick-up stylus mounting, Fig. 4, has a holder 41 with the stylus 40 at one end, and a compliant support 42 coupling the other end of holder 41 to one end of an arm 43. The other end of arm 43 is pivotally mounted by a compliant support 44 coupled by a releasable magnetic coupler 45 to piezoelectric element 59 which flexes when actuated to pivot arm 43 and move stylus 40 radially inward. Element 59 may be mounted on an "arm stretcher" transducer 134 effecting longitudinal movements of arm 43 to correct errors in relative stylusgroove motion.