Patent GB-1460911-A (GB)
Title: Colour picture and sound record and recording/playback apparatus and methods therefor
IPC: H04N5/76
Filed: 1973-03-20
Abstract: 1460911 Video disc recording and play-back RCA CORPORATION 19 March 1974 [20 March 1973] 13234/73 Heading G5R [Also in Division H4] A video disc recorder, Fig. 1, frequency modulates a carrier over a first frequency deviation range with a composite colour video signal, frequency modulates a second carrier over a second frequency range, lower than the first, with an audio signal, forms a combination signal by combining these two modulated carriers, clips the combination signal to develop a clipped signal, Fig. 3c, comprising an impulse train having a pulse repetition rate which varies in accordance with the amplitude of the composite colour video signal and having a "duty cycle" (x/(x+y) or y/(x+y) in Fig. 3c) which varies cyclically about a value of ¢, the frequency of the cyclic duty cycle variation varying in accordance with the amplitude of the audio signal, and utilizes said clipped signal or a recording signal which is recorded on a disc. The video signal source 30 combines a colour subcarrier frequency modulated with the chrominance signal into a midband portion of the luminance signal by the complementary comb-filtering/ buried subcarrier technique disclosed in Specification 1,431,378 to form a composite video signal frequency modulated in circuit 40 on to a triangular waveform video carrier to give a signal with a deviation range of 4 to 6 MHz. The audio signal carrier in modulator 20 is sinusoidal with a deviation range of 200 KHz  25 KHz. The peak-to-peak outputs of circuits 20 and 40 are in the ratio of, for example, 1 to 7. A scanning electron microscope recording apparatus 70 of the type disclosed in Specification 1,392,203 receives the output of clipper 60 to record a master disc from which replicas are made in the manner disclosed in that Specification. The electron beam is blanked for the negative pulses and unblanked for the positive pulses of the recording signal output of clipper 60 whereby successive depressions are formed in the bottom of a spiral groove, the depressions corresponding in length and position to the positive pulses of the recording signal and the unmodulated groove parts having a bottom wall forming a smooth curve with the sidewalls. The replica disc, Figs. 4-6 (not shown), has the modulated groove in a thermoplastic, e.g. vinyl, base (80) coated by an electrically conductive layer (81), e.g. of gold, covered by a layer (82) of dielectric, e.g. polystyrene, layers (81), (82) following the groove modulations. A lubricant film, e.g. a methyl alkyl siloxane compound, may be provided on layer (82). Play-back apparatus of the type disclosed in Specification 1,392,202 may be used to replay the discs, a varying capacitance 91, Fig. 7, being formed between the disc and an electrode on a groove-following stylus. A circuit 100 converts the capacitance variations to voltage variations of a signal at output D, this signal passing via a wideband pre-amp 110 to band pass filters 120, 150 passing the audio and colour video components respectively to limiters 130, 160, and thence to FM demodulators 140, 170 and audio and colour video signal utilization circuits 180, 190 respectively.