Patent GB-1492389-A (GB)
Title: Reading and displaying video disc recordings
IPC: G11B7/004
Filed: 1974-11-11
Abstract: 1492389 Video disc playback apparatus PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 11 Nov 1974 [14 Nov 1973] 48725/74 Heading G5R [Also in Division H4] In a method of reading video information from a disc record 1, Fig. 1, and displaying such information on a TV display device, a playback transducer, 7 to 18, is repetitively caused to jump between adjacent substantially circular tracks, each containing one video frame, and frame sync. signals are generated in coincidence with such jumps in such a manner that the picture on the display device is formed by consecutive parts of the two adjacent frames recorded on the disc with the junction on the displayed picture between the two frames moving vertically to give an upwardly or downwardly rolling picture effect. A scanner for an optically readable video disc 1, having a spiral or concentric circular tracks, focuses a light beam b 1  on the disc 1 and the beam b 2  modulated by the record passes via a mirror 12 to a detector 14 giving a playback signal at output 16. Track following is effected by a coarse control motor M 2  operative to shift optical system 4 radially of disc 1, and by a fine control 19, e.g. a piezoelectric element, operative to tilt mirror 12 so that light from the wanted track is maintained on detector 14. Control 19 receives an error signal from a servo amplifier V 1  having an input from an error detector 15 which is illuminated by light from tracks either side of the wanted track, the beam b 1  illuminating a plurality, e.g. 50, of adjacent tracks. Motor M 2  receives a signal derived from the output 17 of detector 15 or derived from the capacitatively or inductively measured mean deviation of mirror 12, motor M 2  operating continuously or intermittently. The scanner and the coarse and fine servo control loops may be as described in Specification 1,429,882 (see Division G3). A scanner control and signal processor A, has a stage a receiving the signal from output 16. The signal on disc 1 is recorded with line sync. pulses but vertical sync. pulses may be omitted, the disc 1 then having means for identifying the start of each recorded video frame, such as marks in the signal tracks detectable by the scanner 7-18 or marks at the outer or inner disc peripheral region detectable by a fixed additional photoelectric device. The stage a detects the frame start signals and also supplies the line sync. pulses to a jump command pulse source c and a frame sync. pulse source b. Source c may comprise a variable frequency oscillator which when set to a frequency equal to the rotational frequency of disc 1 allows still picture display, and when set to a lower frequency produces jump command pulses which act to close a switch b 5 , thereby supplying frame sync. pulses from source b to adder 20 and to simultaneously move a switch S to position S 2  whereby control 19 is caused to shift mirror 12 for a track changing jump. Thus for the track configuration of Fig. 3b wherein each track is one frame of two interlaced fields, source c causes tracking changing at the arrow positions, whereby if the first played back frame starts at T 1 , the playback pictures are (1) T 1 , T 2 , T 10 , T 14 , T 3 , Tk, T 12 , T 5 ; (2) T 2 , T 10 , T 14 , T 3 , T 7 , T 8 , T 4 , T 12 , T 5 , T 6 ; (3) T 10 , T 14 , T 3 , T 7 , T 8 , T 11 , T 12 , T 5 , T 6 , T 9 ; &c., whereby one frame gradually moves up on the display with the top part being gradually lost and the top of the frame next on the disc gradually advances up the display from the bottom. A downward picture roll is produced if the oscillator of source c is set to higher than the disc rotation rate. Effectively continuous movement of the picture by increments of at least one line may be produced by the above means, or such movement may be intermittent with the same picture being repeated in between. An alternative control circuit, Fig. 6 (not shown), gives picture movements each corresponding to a fraction of a line, e.g. “ line intermittently every “ second requiring about 4 or 5 minutes for a complete picture change for 525 or 625 line standards respectively, or 1/10 line per field period requiring about 1À3 or 2 minutes for a complete picture change. This circuit has a counter (54) which on reaching a preset count controls a generator (53) which gives frame and field sync. signals to adder (20) and track change signals to the scanner. A generator (56) responds to the replayed line sync. pulses to give pulses at a multiple, e.g. 4 or 10, of the line sync. pulse rate, the pulses of generator (56) pass via a gate (57) to be counted in counter (54), and the gate (57) is controlled by pulses from an adjustable frequency pulse generator (58) to block certain pulses from generator (56) to cause an upward picture roll, or to add pulses to those from generator (56) to cause a downward picture roll.