Patent GB-973646-A (GB)
Title: Improvements relating to colour television
Inventor: JAMES IVANHOE JOHN PENFOUND
IPC: H04N9/69
Filed: 1959-11-24
Abstract: 973,646. Television. ELECTRIC & MUSICAL INDUSTRIES Ltd. Dec. 8, 1960 [Nov. 24, 1959], No. 39894/59. Heading H4F. A colour television system utilizes a gamma corrected luminosity signal and two colour difference signals having the form Ey<SP>1</SP>/γ, (E R <SP>1</SP>/γ- Ey<SP>1</SP>/γ) and (E B <SP>1</SP>/γ-Ey<SP>1</SP>/γ) respectively. The luminosity signal may have one of several forms, e.g. Ey<SP>1</SP>/γ=(0.3E R +0.59E G  + 0.11E B )<SP>1</SP>/γ or Ey<SP>1</SP>/γ= (0.334E R  + 0.585E G  + 0.081E B )<SP>1</SP>/γ. The signals may be generated by any of the camera arrangements shown in Figs. 1 to 4. In Fig. 1 the Y signal is produced by combining the outputs of linear pick-up tubes for the R (red), G (green) and B (blue) components in suitable proportions by means of the resistor matrix 4, 5, 6 and then effecting gamma correction in amplifier 8. The gamma corrected red (R<SP>1</SP>/γ) and blue (B<SP>1</SP>/γ) signals from amplifiers 7 and 9 respectively are then supplied together with the gamma corrected luminosity signal Y to respective subtraction circuits 7 and 9, from which the required colour difference signals are derived. To avoid registration errors in the Y signal due to producing it by combining the outputs of three separate pick-up tubes viewing the same scene the pick-up tube for the green component may be replaced by a linear monochrome pick-up tube 2a as shown in Fig. 2. Modifications of the circuits of Figs. 1 and 2 for use with self-gamma type tubes (e.g. Vidicons) are shown in Figs. 3 and 4 respectively. In all the arrangements the pick-up tubes may be replaced by flying-spot scanners and photo-electric cells. The colour signal specified above may be transmitted by conventional means, as e.g. for an N.T.S.C. type signal or, alternatively, the colour-difference signals may be transmitted as A.M. or F.M. of the sub-carrier during alternate lines. The overall modulation of the transmitter may be either positive or negative. The Specification gives an analysis of the reception of a colour signal according to the present invention by a standard N.T.S.C. receiver and shows that distortion in the reproduced green signal results. To avoid this the receiver shown in Fig. 5 in which the signals E R <SP>1</SP>/γ and E B <SP>1</SP>/γ (derived by known means from the received signal) are supplied to the modulator grids of two guns 24, 25 of a tri-gun colour picture tube and the cathode outputs, consisting of E R , E B , combined in a matrix with the signal - Ey to produce a signal E G <SP>1</SP>/γ for application to the modulator grid of the green gun 26 may be employed. The green signal from the matrix is supplied to gun 26 via a high gain amplifier 35 which operates with heavy negative feed-back via the grid to cathode path of gun 26 and since the latter (as with guns 24, 25) has a gamma characteristic the required gamma corrected signal E G  appears on the modulator grid. Alternatively, the cathode current of gun 26 may be controlled by one section of a cathodecoupled double-triode tube the grid of the other section being supplied with a signal from the matrix (Fig. 6, not shown). A circuit arrangement for reducing errors in saturated colours when a normal unmodified N.T.S.C. type receiver is employed and which includes means for preventing reproduction of the green signal unless it exceeds the larger of the signals R or B is described with reference to Fig. 7 (not shown). The receiver of Fig. 5 may be provided with switch means to enable it to receive either the colour signal according to the present invention or a standard N.T.S.C. type signal.