Patent GB-828794-A (GB)
Title: Improvements in colour television receiver systems
IPC: H04N9/24
Filed: 1956-02-08
Abstract: 828,794. Colour television. PHILCO CORPORATION. Feb. 8, 1956 [Feb. 15, 1955], No. 3930/56. Class 40 (3). In a colour television receiver utilizing a signal in which chrominance is conveyed by a phase and amplitude modulated sub-carrier, the accuracy of colour reproduction is improved by combining a second harmonic signal with the sub-carrier, the second harmonic signal having amplitude and phase variations proportional to those of the sub-carrier but having phase variations of the opposite sign. The invention is illustrated as applied to a receiver employing a cathode-ray tube with a coloured striped screen scanned transversely of the stripes, the sub-carrier and second harmonic after suitable frequency conversion under the control of an indexing strip signal (see below) being applied directly to the tube grid. The effect of the invention is illustrated in Figs. 2A-2C where the dotted line shows the sub-carrier alone and the solid line the result of adding the second harmonic signal. Fig. 2A represents conditions when the signal represents red, Fig. 2B, the conditions for the colour complementary to blue, and Fig. 2C the condition for the colour intermediate red and the complementary of blue. In a first embodiment, Fig. 1, the signals received at 10 are applied to filters 12 and 13 to separate a 0-3 M/c. luminance component and a 3.58 M/c. chrominance sub-carrier. A separator stage 14 extracts a sub-carrier burst and synchronizes an oscillator 15. The cathode-ray tube screen 21 is constructed with red, green and blue vertical phosphor strips and also incorporates indexing strips whereby a signal at a nominal 7 M/c. but subject to indexing variations representative of scanning and screen irregularities is obtained across a resistor 25. This signal is selected by amplifier 26, combined in modulator 27 with the 3.58 M/c. signal from oscillator 15 to derive the sum signal of 10.58 M/c., and then combined with the 3.58 M/c. sub-carrier signal from filter 13 in modulator 28 to obtain finally a 7 M/c. signal bearing the phase and amplitude chrominance modulation but subject also to variations in accordance with the indexing information. The signal then passes to an unbalanced modulator 29 where it combines with a 21 M/c. signal and produces an output via filter 32 comprising the original 7 M/c. signal and its second harmonic at 14 M/c. in the form of the difference component between the 7 and 21 M/c. signals. Since the second harmonic is produced as a difference component, its phase variations are of opposite sign to those carried by the 7 M/c. signal. The 21 M/c. carries the indexing information and is derived by a frequency tripler 30 from the 7 M/c. signal resulting from the combination in modulator 31 of the 3.58 M/c. from oscillator 15 and the 10.58 M/c. output from modulator 27. The 7 and 14 M/c. chrominance signals together with the luminance signal from filter 12 are applied to the cathode-ray tube grid. A second embodiment, Fig. 3 (not shown), operates in a generally similar manner but employs a cathode-ray tube constructed as described in Specifications 762,550 and 788,073 with a separate indexing gun receiving a pilot oscillation. Reference is made without giving details to an arrangement in which the amplitude and phase variations are extracted from the subcarrier and combined with a double-frequency signal derived from the synchronizing bursts to obtain the second harmonic signal.