Patent GB-822465-A (GB)
Title: Improvements relating to television receiving apparatus
IPC: H04N11/14
Filed: 1955-10-26
Abstract: 822,465. Television. GENERAL ELECTRIC CO. Oct. 26, 1955 [Oct. 27, 1954], No. 30659/55. Class 40 (3). A simultaneous colour television signal of the N.T.S.C. type is modified for application to a single-gun picture tube the (three) component colours reproduced by which are scanned in a sequence in which one alternates with the other two taken in turn. Because of this sequence it is necessary to sample the signal four times per cycle of the chrominance subcarrier wave and the Specification shows that to allow this, the original signal, which has the form Em = Ey + [M cos wt. + N Sin wt] must (when the colour sequence is R, G, B, G &c), be transformed into a signal En = L + K cos (wt + #1) + H cos 2 (wt + #2); where En is the desired modified signal; L = ¢ Eg +  (En + Eb); K = ¢ (Er - Eb) ; H =  (Er = Eb) - ¢ Eg, and the remaining symbols have the usual significance. In one embodiment, Fig. 1, the signal En is derived from the received original signal Em (supplied by conventional receiving circuitry 2), by combining in adders 44 the outputs of a number of parallel paths to the inputs of which the signal Em is supplied and which, respectively produce the terms involving L, K and H. To produce the L term, the luminescence signal Ey, selected by L.P. filter 3, is combined in an adder 8 with the detected chrominance sub-carrier wave Ec, the latter being selected by B.P. filter 4 and detected in synchronous demodulator 10 by means of a reference wave of sub-carrier frequency (derived in conventional manner via units 5, 11, 12) the phase of which is adjusted by shifter 15 to derive particular chrominance components which when combined with the Ey signal in adder 8 (the relative gains of the two channels being adjusted having regard to the composition of the Em signal) produce the signal L as defined above. For the production of the term involving K and H the chrominance signal Ec is supplied to two parallel paths via respective " multiplier " circuits 20, 50 which operate as described in Specification 809,506 to produce desired chrominance components when a signal of 2X sub-carrier frequency is supplied thereto. By correct adjustment of the phase shifters 41 and 51 the signals of the form K cos (wt + #1) and H cos (wt + #2) are respectively derived at the outputs of B.P. filters 42, 52. The former signal which is in the desired form, is fed to adder 44, but the latter signal, which is required to be in the form H cos 2 (wt + #2), is converted to the latter form by combining it with a suitably phase signal of 3X sub-carrier frequency in a mixer 53 the output from which is fed to the adder 44 in B.P. filter 55. Delay networks 7, 18, 43 and 56 are provided in the different paths to equalize the inherent delays in such paths. The output of adder 44, which is the desired signal En is then supplied to the control grid of the (single-gun) picture tube 60 the colour switching grid 64 of which is controlled in conventional manner and the cathode of which is supplied with a signal of 4X subcarrier frequency to effect the required 90 degree sampling. In a modification of the arrangement described, the elements 3, 6, 7 and 8 are omitted and the Ey signal derived together with the signal term involving K, in the channel which includes multiplier 20 by supplying the composite signal Em as input thereto (Fig. 4, not shown), Fig. 5 shows a modification of this latter arrangement in which the Ey signal together with the signal term involving K are derived via elements 220, 242, 243 and combined with the detected chrominance signal, derived via elements 204, 210, 217, 218, in adder 282 the output from which supplies the control grid of the picture tube. To produce the signal involving the term H the chrominance signal Ec is first combined with suitably phased wave of 3X sub-carrier frequency in mixer 253 the output from which is supplied via B.P. filter 255 to a multiplier 250 and there multiplied by a signal of 4X subcarrier frequency. From multiplier 250 the signal is supplied to the cathode of the picture tube over two paths one of which including B.P. filter 287 supplies the signal involving the term H and the other of which passes the 4X sub-carrier frequency to effect the necessary sampling. The Specification refers to the particular phase angles and absolute and relative magnitudes of the signals which are necessary for correct operation and considers the form which the signal En should have when the colour which repeats twice is the blue or the red. Specification 784,534 also is referred to. Reference has been directed by the Comptroller to Specification 784,055, [Group XXXVIII].