GB1520004A - Velocity correction for video discs - Google Patents

Abstract

1520004 Automatic control systems RCA CORPORATION 21 Oct 1975 [12 Nov 1974 (2)] 43217/75 Heading G3R [Also in Division H3] In video disc apparatus, to reduce jitter in the video signal e.g. due to eccentricity of the disc, the signal from the disc is phase compared with a reference frequency, so as to control the position of the signal pick-up relative to the disc i.e. the instantaneous speed between them. As shown, a video signal from disc 12 is demodulated, and heterodyned at 18 with the output of voltage controlled oscillator 40. Only the lower sideband thus obtained (the chrominance sub-carrier) is passed by filters 20, 21 and gated by burst key signals (that follow video signal horizontal synchronizing signals) to phase comparator 22. Here it is compared with a reference frequency, the result being used to control the position of the pick-up 10 along the groove on the disc 12 (using a so called "arm-stretcher" device 34), in order to remove jitter in the video signal caused by eccentricity or warp of the disc. The output of comparator 22 is also used to control the voltage controlled oscillator 40, to regulate the chrominance sub-carrier frequency (see Division H3). Filters 30, 38 on the comparator output ensure that the pick-up position and oscillator controls respond to different, low frequency phase errors. The pick-up may capacitively sense a spiral groove on disc 12. In an alternative Fig. 1 (not shown) the oscillator 40 is controlled directly by the comparator output, and filters 30, 38 replaced by a lead/lag circuit.