Patent GB-1073375-A (GB)
Title: Improvements in colour television systems
Inventor: GARGINI ERIC JOHN
IPC: H04N9/43
Filed: 1963-02-20
Abstract: 1,073,375. Colour television. COMMUNICATIONS PATENTS Ltd. Feb. 18, 1964 [Feb. 20, 1963], No. 6781/63. Heading H4F. A colour television system is characterized by the use of a whiteness signal, i.e. a signal which is representative of the white content of the light from the elements of the scene. The signal is generated by forming an equi-energy signal from red, green and blue component colour sources 61-53, Fig. 2, with the aid of matrix network 57-59 and then subtracting in a circuit 63 a correcting signal 64 which has an amplitude equal to the equi-energy signal for saturated colours. In this manner the whiteness signal, which appears at 65, falls to zero for saturated colours. The correcting signal is derived by forming a phase and amplitude modulated colour carrier signal with the aid of three modulators 72-74 which are supplied from a carrier source 75 through 120 degrees phase shifters 76, 77 and detecting the amplitude of the carrier in a demodulator 80. The signals applied to modulators 72-74 represent the complementary colours cyan, yellow and magenta and are derived with the aid of pairs of gain-controlled amplifiers 66a-66b, 67a-67b and 68a-68b which are connected to receive the original red, blue and green signals as shown, one connection to each amplifier being the controlled signal and the other connection being the gain-control signal. A colour signal to accompany the whiteness signal is derived as a conventional phase and amplitude modulated sub-carrier signal with the aid of three modulators 87-89 which respond to the red, green and blue signals directly and also receive 120 degrees phase-displaced carrier signals from source 75 via phase shifters 90, 91. This sub-carrier signal, which falls to zero for white light, is said to represent a single primary colour as a mixture of two primary colours when coloured light is present. The whiteness signal is allotted a band 0-5 Mc/s. by filter 82, the carrier source 75 is made 6 Mc/s. and the colour signal is allotted a band 5-7 Mc/s. by filter 94. If desired the frequency of source 75 may be selected to cause the whiteness and colour signal bands to overlap. The composite signal is modulated on a carrier and prepared for transmission in circuits 95-97. Circuit 101 controls scanning synchronization; circuit 104 controls the production of a colour synchronizing "burst"; circuits 81, 84, 85, 86 are gamma control circuits. At a receiver, Fig. 3, the incoming signal after amplification 1 and demodulation in circuits 110, 114 is separated into the whiteness and colour components in filters 116 and 117. The whiteness signal is applied to modulate a first cathode-ray tube 120 which produces white light, whilst the colour signal is synchronously demodulated at 120 degrees phase intervals in demodulators 131, 132, 133 and applied to control three further tubes 147, 148, 149 which produce red, green and blue light respectively. The final image is obtained by combining the light outputs from the four tubes. Scanning synchronizing signals are separated at 121 and the colour synchronizing " burst " is separated at 124 and applied to synchronize an oscillator 128 by way of a retroactive loop including a phase comparator 127. The signals derived from demodulators 131-133 do not represent solely the amplitude of the related colour (red, green or blue) but contain negative component contributions due to the other two colours. This undesirable result is nullified by adding to each demodulated signal, in adders 139-141, a signal equal to saturation which is derived by detecting the amplitude of the colour subcarrier signal in a demodulator 143. According to modifications: (1) the colour component signals are converted into colour residue signals by subtracting from each the whiteness signal, Fig. 1 (not shown); (2) the coloured light at the receiver is provided by a stripe-screen, beam-indexing cathode-ray tube, Figs. 4 and 5 (not shown); (3) the coloured light at the receiver is provided by flat cathoderay tube, e.g. a Gabor tube, the white cathoderay tube being viewed through the phospors of the colour tube, Fig. 6 (not shown); (4) the coloured light at the receiver is provided by a single dot-screen, cathode-ray tube, the colour signals being employed to steer the electron beam to dots of the correct colour and a saturation signal derived by demodulating the amplitude of the colour sub-carrier signal being applied to modulate the beam intensity, Fig. 7 (not shown); (5) the colour component signals are compressed in duration to one third of a line by swept delay devices and then transmitted in sequence, so that the three components occupy the time of one line, and are displayed on a single cathode-ray tube which is scanned at the normal rate and produces the three component colour images side-by-side, Figs. 8-9 (not shown). In order to transmit fine detail in fully saturated areas (in Fig. 2 such fine detail is not transmitted since the wide-band whiteness signal disappears on saturated colours), the amplitudes of the two signals applied to subtractor circuit 63 are adjusted so that there is incomplete cancellation and a residual whiteness signal remains with saturated colours. At the receiver the white cathode-ray tube is given an appropriate bias such that light is only produced for signal amplitudes above the residual level, whilst signal levels below the residual level are separated by a biased diode and applied to intensity modulate the colour cathode-ray tube(s). The system is said to be compatible in that a monochrome receiver may respond to the composite whiteness and colour signals, the colour signal providing a grey dot pattern when the whiteness signal disappears on saturated colours.