Patent GB-1501141-A (GB)
Title: Optically readable record carrier and apparatus for reading same
IPC: G11B7/24085
Filed: 1975-02-07
Abstract: 1501141 Optically readable recordings PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 7 Feb 1975 [12 Feb 1974] 5302/75 Heading G5R An optically readable record carrier stores information in an optically readable structure of tracks 2, 2<SP>1</SP>, Fig. 2, each track comprising first areas g, g<SP>1</SP> which alternate along the track with intermediate second areas t which have a different effect on an optical read beam to that of the first areas, the information being contained in the variations of the spatial frequency of the first areas g, g<SP>1</SP>, and the first areas as a whole comprise at least two different types of diffraction grating which types differ in the sense that the direction of their grating lines relative to the local direction of the track length are different to one another, the term "grating" being defined to embrace grating-like diffracting structures having as few as two or only one "grating" line. Information may also be contained in variations in length of the first areas g, g<SP>1</SP>. The record carrier may be a tape, or a cylinder, e.g. a flexible flat sheet curved into cylindrical form, or a disc 1 with circular or spiral tracks. The first areas g may be all of the same grating type within each track having grating lines locally at 90 degrees to those of the first areas g<SP>1</SP> of the immediately adjacent tracks, all the grating lines in all the tracks being at 45 degrees to the local direction of the track length. Such an arrangement may be formed by areas g and g<SP>1</SP> being on a first and a second spiral respectively. Alternatively, Fig. 3, only the end parts of each first area may have grating areas, with each first area having a grating area S of one said type at one end and a grating area s<SP>1</SP> of another said type at the other end so that in each track areas s alternate with areas s<SP>1</SP>. In playback apparatus, Fig. 7, for such a record carrier, light focused thereon from a source 5 by a lens 6 is received after reflection, or transmission, and diffraction on to detectors, or detector pairs, which are each orientated to correspond to the grating line orientation, at the position of incidence of the read beam on the record, of a respective type of grating. For a record having only two grating types, lenses 11, 12 direct light from one type of grating on to detector 15, and lenses 13, 14 direct light from the other type of grating on to detector 16, a circuit 17 processing the detector signals to give a video and/or audio replay signal at Si. When reading a record, Fig. 2, in which adjacent tracks have different grating line directions to one another, the circuit 17 may process the outputs of detectors 15, 16 to give an error signal Sc for use in correcting, e.g. by movement of a mirror in the path from source 5 to lens 6, track centring errors of the read spot. Track centring may also be effected in the manner disclosed in Specification 1,489,756 in conjunction with a carrier having tracks which exhibit periodic lateral excursions. If the read beam is not properly focused it will produce a spot illuminating the wanted track and adjacent tracks, so that if the latter have different grating line directions to the wanted track, circuit 17 may process the outputs of detectors 15, 16 to give a focusing error signal S<SP>1</SP>c, e.g. for use in effecting axial movement of lens 6.