Patent GB-1529954-A (GB)
Title: Optically readable records and reading apparatus therefor
IPC: G11B7/24085
Filed: 1975-09-22
Abstract: 1529954 Optical video recording/playback PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 22 Sept 1975 [25 Sept 1974] 38793/75 Heading G5R An optically readable video disc has tracks which are of constant width and are located in a plane of the disc, and the tracks exhibit an approximately sinusoidal undulation in the plane of the track structure and transverse to the track length, variation of the undulation being determined by both a first carrier wave, which is frequency modulated with the luminance information, and by one or more further carrier waves which are modulated with additional information. The additional information may be sound on one carrier, or chrominance and sound information with the sound being modulated on one, two or four carriers. The lateral track undulations may carry all the information as variations in the spatial frequencies of the undulations, e.g. with a variable short period (P 1 ), Fig. 2 (not shown), corresponding to luminance information and a longer variable period (P 2 ) corresponding to chrominance and sound information, or the undulations may have a constant period and modulated amplitude, or luminance signals may be contained in the spatial frequency of the undulations, modulated amplitude of which carries the chrominance and sound signals. The record may be readable by transmission or reflection, and the optical path difference between light falling on the tracks 2 and that on the inter-track lands 3 may be approximately (2n+1)#/4. Thus a record read by reflection may have the lands in a first plane and the tracks in a second plane 5, Fig. 3, spaced by about (2n+1)#/8 from the first plane. Recording may be effected using an acoustooptic cell (60), Fig. 9 (not shown), to wobble a write beam as disclosed in Specification 1,489,756. Playback may be effected by an objective system L 1 , Fig.,3, which transfers a read beam 12 from a laser 11 via the reflecting disc record 1 on to two radiation detectors 15, 16 which are disposed in the effective exit pupil of the objective system with the separating line between the two detectors, if projected on to the phase 5 of the track structure, being parallel to the reading direction at the region of such projection. The effective exit pupil is the actual exit pupil if accessible, or if this is not accessible is an image thereof formed by a lens L 2 . Detectors 15, 16 are connected to a differential amplifier, 17 giving an output at S l  from which the recorded information can be extracted. The record acts as a diffraction grating and the detectors 15, 16 are located in a position at which zero and first order diffracted beams overlap and interfere with one another. Track following during playback may be servo controlled so that the read spot stays centred on the average position of the track centre by extracting a low-frequency servo signal present in the replay signal due to centring errors, or such a low-frequency servo signal may be provided by modulating the highfrequency lateral track undulations, due to the television signal, with an additional signal of constant spatial period, an order of magnitude longer than the longest period of the highfrequency undulation, the phase of the additional modulation in the signal of detectors 15, 16 being dependent on the read spot centring.