Patent GB-1097438-A (GB)
Title: Recording apparatus and medium with discrete level modulation
IPC: H04N9/82
Filed: 1965-03-18
Abstract: 1,097,438. Non-linear and pulse circuits; colour television. COLUMBIA BROADCASTING SYSTEM Inc. March 18, 1965 [March 19, 1964], No. 11593/65. Headings H3P, H3W and H4F. [Also in Divisions G4 and G5] In recording system in which a recording means makes a line-by-line transverse scan of a recording medium, a continuously variable signal to be recorded is arranged to modulate a carrier signal to different discrete values, the unmodulated frequency of the carrier being an integral multiple of the line-scan frequency. Recording.-In a colour television recording system, Fig. 1, the R, G, B signals are fed to a matrix 20 to produce chrominance signals R-Y, B-Y and luminance signal Y. The chrominance signal R-Y is frequency modulated 25 on to the carrier as a number of discrete frequency shifts, such that the modulated signal remains an integral multiple of the line scan frequency. The resulting frequency modulated signal is then limited 27, amplitude modulated, 30, with the chrominance signal B-Y and added, 32, to the luminance signal Y. The resulting composite signal, Fig. 2 (not shown), is then fed to intensity modulate a CRT 35 supplied with normal horizontal and vertical scanning signals. A film 42 is moved in front of the CRT, and the signal is recorded on to it in frames 60, Fig. 3 (not shown), separated by areas 59 corresponding to the frame blanking pulses. During this period the frame blanking pulse actuates a gate 51 to allow light source 53 to print a marker 58 on to the film. During the scanning of the CRT spot across the film, the pattern formed consists of lines 65, Fig. 4 (not shown), intensity modulated along their length by the chrominance signals the number of modulation cycles being determined by the R-Y signal. Playback.-The unmodulated CRT spot scans the film, and the resulting intensity modulated light is applied to a photo-multiplier 80. As the number of modulation cycles along each scanning line is always an integral multiple of the line scan frequency, any miss-alignment between the scanning beam and the scanned line, such that the beam scans two lines, will result in only a constant beat frequency, and will thus produce negligible interference. The output from the multiplier 80 is fed to filters 86, 87 to separate the luminance and chrominance signals, the latter being demodulated in a.m. and f.m. demodulators 90, 92. Synchronization is maintained by continuously comparing the output from the synchronizing generator 37 with signals derived from the synchronizing marks recorded on the film in comparator 110. The input and output circuits may be modified for black and white television, Figs. 12A, 12B (not shown), by using the video signal as the frequency modulation instead of the R-Y chrominance signal. Switching circuit, Fig. 8, Fig. 9 (not shown).- Input signals V and Vm-V where Vm is a constant are applied to leads 147, 148. With small values of V, valve 145 is cut-off and the output line 155 is clamped to earth via diode 151. As V rises, 145 becomes conductive and the output on line 155 follows the input until diode 153 conducts, clamping the output. Further rise in V causes the cathode potential of valve 146 to fall until diode 152 conducts and the output follows Vm-V until 146 cuts off, whereafter the output remains at zero again.