Patent GB-784534-A (GB)
Title: Improvements relating to colour television apparatus
IPC: H04N11/14
Filed: 1955-02-18
Abstract: 784,534. Television. GENERAL ELECTRIC CO. Feb. 18, 1955 [Feb. 18, 1954], No. 4958/55. Class 40 (3). A colour television receiving system for converting a " simultaneous " type of signal comprising a luminance signal and a pair of phase quadrature related chrominance signals (e.g. an N.T.S.C. type of signal) into a " sequential " type of signal suitable for application to a singlegun colour picture reproducing tube includes a multiplier circuit in which at least the chrominance signals are multiplied by a signal the frequency of which is equal to twice the frequency of the sub-carrier wave on which the chrominance signals are modulated. In one embodiment (Fig. 1), in which only the chrominance signals are supplied to the multiplier circuit the latter signals Ec are separated from the composite signal Em(=Ey+Ec) (where Ey represents the luminance signal) by means of filter 4 and supplied to the multiplier 16 together with a signal of twice the chrominance sub-carrier frequency derived by means of the elements 5, 6, 7, 19 and phase shifted in 20 so as to have a phase of 5.6 degrees behind that of the sub-carrier wave which is modulated by the red colour-difference signal (ER - Ey). The output of multiplier 16 then consists, after filtering in 22, of modified high-frequency components of the chrominance signals which, when combined, in adder 27, with modified lowfrequency components of the chrominance signals, the latter being derived via filter 26 from the output of a synchronous demodulator 15, the demodulating wave of which has a phase 71.3 degrees behind the ER - EY sub-carrier and the effective gain of which is 0.581, and additionally combined (in adder 27) with the luminance signal EY (derived via filter 3), forms a signal of " sequential " type. This signal is then supplied, via filter 28, to the picture-reproducing tube 10 the cathode of which is controlled by a signal of 3 X sub-carrier frequency so that, in conjunction with the operation of the colour switching electrode 13, to which a switching signal controlled by the sub-carrier wave is applied, symmetrical sampling and sequential reproduction of the signal is effected. In another embodiment the multiplier 41 (Fig. 4) acts on the whole of the composite signal Em and its output, after amplification in a " step " amplifier 43, the characteristic of which is as shown, is combined with the low-frequency components of the detected chrominance signal in adder 45 and supplied to the picture tube 10. In a modification of the latter embodiment the input signal for the synchronous demodulator is derived from the output of the " step " amplifier, the outputs of the former and the latter then being combined for application to the picture tube (Fig. 5, not shown). Fig. 6 shows a further embodiment in which the input signal to the multiplier 87 consists of the combination of the composite signal Em and the detected low-frequency components of the chrominance signals, the multiplier output being supplied to the picture tube through the " step " amplifier 91. The Specification gives a mathematical analysis of the general problem of converting a " simultaneous " type of signal into a " sequential " type of signal and, for the particular embodiments described, expressions are derived showing the required phases of the waves, of sub-carrier and twice sub-carrier frequency, supplied, respectively, to the synchronous demodulators and to the multipliers and also showing the gain factor required for the demodulators. Details of suitable circuit arrangements for the multipliers, the synchronous demodulators (of the balanced type) and the phase shifters are described with reference to Figs. 2 and 3 (not shown).