Patent GB-1461482-A (GB)
Title: Method of and apparatus for recording and reproducing information
IPC: G11B7/12
Filed: 1974-02-04
Abstract: 1461482 Recording and playback HITACHI Ltd 4 Feb 1974 [2 Feb 1973] 05044/74 Heading G5R Information, such as a video and/or audio signal, is recorded by adding a standard signal of constant frequency to the information signal, modulating a carrier signal with the combined signal, modulating the intensity of a radiation beam 28, Fig. 2, with the resultant modulated signal, and causing the resultant beam to impinge on a rotating record 20 whilst the beam is being swung in a direction perpendicular to the direction of motion of the record at the point of impingement of the beam thereon by a signal having the same frequency and phase as the standard signal. A standard frequency signal from an oscillator 37, Fig. 2, and an information signal from a source 36 pass through frequency changers 35, 35<SP>1</SP>, respectively, so that the standard signal does not lie in the frequency band of the information signal, circuit 34 adding the two signals and frequency, pulse width or pulse position modulating a carrier with the combined signal, a light beam 28 from an He-Cd or He-Ne laser is intensity modulated in means 32 by the modulated carrier 38, the modulated light beam being deflected at 31<SP>1</SP> and condensed at 30 to form a spot 29 on disc 20, parts 30, 31<SP>1</SP> being moved to form a spiral track 26, and deflector 31<SP>1</SP> being oscillated by a driver 31<SP>11</SP> having an input from oscillator 37 so that the resultant track 26 swings about a centre line (70), Fig. 4a (not shown), at the standard frequency. Track 26 may have a width of about 1 micron with a peak-to-peak swing of 0À3 to 0À4 micron, and the disc may have a layer of photo-resist forming concave and convex parts on irradiation, or of an emulsion forming light and dark areas, or of an opto-magnetic material producing magnetic changes. By analogous means an intensity modulated electron beam may be used to record a spiral track, Fig. 3 (not shown). Playback apparatus, Fig. 5, comprises a laser 40 giving a beam 41 which is formed into a spot 29<SP>1</SP> tracked across the disc 20<SP>1</SP> by moving condenser 30<SP>1</SP> and deflector 43<SP>1</SP>, the reflected beam 44 passing to a photo-electric converter 45, the output 51 of which is demodulated at 46 to pass the information signal part via a frequency changer 47 to display 48 and to pass the standard frequency signal part to frequency changer 49. An A.M. detector 50 gives an output signal 52 corresponding to the envelope of signal 51, and a synchronous detector 53 provides to a driver 43<SP>11</SP> a tracking control signal, driver 43<SP>11</SP> actuating deflector 43<SP>1</SP> to cause the beam 41 to follow eccentricities of the disc 20<SP>1</SP>, the beam 41 following the swing centreline (70), Fig. 4a, of the track swing but not following the standard frequency swing itself. The reproducing apparatus may alternatively detect light transmitted through the disc. In modifications one or both of the frequency changers 35, 35<SP>1</SP>, and also one or both of the corresponding changers 49, 47 may be omitted, a notch filter being provided in the playback apparatus to extract the standard frequency signal if this then falls in the information signal band.