Patent GB-853961-A (GB)
Title: Improvements in or relating to correcting for transmission variations in colour television
Inventor: PERKINS DENIS GORDON
IPC: H04N9/09
Filed: 1956-03-01
Abstract: 853,961. Television. ELECTRIC & MUSICAL INDUSTRIES Ltd. Feb. 18, 1957 [March 1, 1956], No. 6394/56. Class 40(3). In a colour television system employing a plurality of pick-up tubes (individually responsive to different component colours) undesired signal variations in the ouput of each tube (due to variations in sensitivity over the surface of the photo-sensitive electrode of each tube) are stored in multiplex fashion in a common storage device and employed to modify the signal output of each tube to compensate for the effect of such variations. Fig. 1 shows one embodiment in which the undesired signal variations produced by red, green and blue pick-up tubes PK, PG and PB respectively are initially derived by exposing each tube to a constant light image during one complete picture scanning period and supplying the resulting output signals, in parallel to different sets of nine switches SR, SG, SB (only three being shown for each set) which are successively gated on for a duration of one <SP>1</SP>/ 9  of a line scan period by means of switching pulses T 1  -T 9  derived from a generator G. The (error) signals passed by each switch are then fed to individual storage condensers C, those in the red channel storing the signals for <SP>1</SP>/ 3  of, those in the green channel for <SP>2</SP>/ 3  of, and those in the blue channel for the whole of, one line scanning period. From the storage condensers output signals are supplied via individual amplifiers H to further individual switches RS, GS, BS, which are gated on, each for the duration of <SP>1</SP>/ 3  of a line scanning period, by means of the pulses from generator G, employed in the sequences shown, (or by means of equivalent single pulses derived by known means) to pass such signals to magnetic recording heads 1-9 associated with a magnetic record drum D which rotates at the rate of one revolution in a complete picture scanning period. In this manner the undesired variation signals (error signals) due to (closely) corresponding areas of the photo-sensitive electrode of each tube are recorded in sequence on one (circular) track on drum D error signals due to adjacent areas being similarly recorded on adjacent tracks so that additional undesired variations due to variation in the sensitivity of the recording surface are minimized. The (error) signals thus recorded are then utilized to reduce the effect of such signals in the output of each tube when the latter are employed in a tri-colour television transmission system by repeatedly supplying the "read-off" signals, which are gated via switching pulses in complementary manner to that shown in Fig. 1, to vary in a compensating manner, the gains of dynamic gain control amplifiers in the output circuit of each pick-up tube. In an alternative and preferred arrangement (Fig. 2a) for effecting the recording of the error signals, the switches RS, GS, BS, of Fig. 1 are dispersed with and only one set of nine storage condensers C and amplifiers H are required. In this arrangement the output of each pick-up tube is switched via corresponding sets of nine switches SR1-9, SG1-9 and SB1-9 (operated in the same sequence as the similar switches shown in Fig. 1) to store the signals on groups of three storage condensers C which have storage periods of <SP>1</SP>/ 3  of a line scanning period and the outputs from which are fed via individual amplifiers H to the recording heads 1-9. A similar but conversely operating arrangement (omitting the storage condensers) is then employed when the recorded signals are repeatedly employed to compensate for the undesired signal variations in the pick-up tube output signals (Fig. 2b not shown). Instead of magnetic storage devices, cathode-ray storage tubes or optical records scanned by flying-shot scanners or by camera tubes may be employed and by recording one of the error signals as a video signal and the other two as modulations of carrier waves of different frequency all three error signals may be recorded on the same area of the record surface. The invention is stated to be applicable to two colour systems and to systems for connecting sequential colour television signals to simultaneous type signals in arrangements in which three display tubes are "viewed" by respective camera tubes. Specification 847,795 is referred to.