Patent GB-737212-A (GB)
Title: Apparatus for the transmission and reproduction of color television images
IPC: H04N11/12
Filed: 1953-04-21
Abstract: 737,212. Television. GRETENER, E. April 21, 1953 [April 23, 1952 ; Oct. 10, 1952], No. 10994/53. Class 40 (3). In a colour television system colour component signals representative of three component colours of the object to be televised are continuously transformed into four signals corresponding to the white content of the object colour and the three colour residues (one of which, at any instant has zero value) and at the receiver four component images are reproduced (one image having zero light value at any instant) and superimposed to form the composite coloured image. Fig. 3 shows one embodiment in which R, B and G colour component signals generated by television cameras 30, 31 and 32 respectively are supplied to a circuit 44 which selects the component signal of smallest amplitude (e.g. the green signal 23 (Fig. 1B) the amplitude 20 of which corresponds to the white content of the object at that instant-i.e. equal amplitudes of B, G and R) and by means of a switching circuit 45 connects the channel containing this signal to the " white " picture reproducing tube 58 and also to three subtraction circuits 49, 50, 51 in which signals equal to the difference between it (the smallest amplitude signal) and the R, B and G signals are produced, one of these difference signals (the output of circuit 51 in this case) at any instant having zero value. These difference signals are then fed to component colour picture reproducing tubes 55, 56 and 57 respectively and the images produced by all four tubes are superimposed to form a single coloured image 60. The selector circuit and switching device 1, 4, 45 which may be located either at the transmitter and four channels 52, 53, 54 and 48 employed for transmission, or at the receiver and the three channels 41, 42, 43 employed for transmission, may comprise a cathode-ray tube device in which the resultant of three 120 degrees spaced vectorial deflections of the beam (produced by the component signals R, B, G) causes the beam to impinge on one of three segment-shaped electrodes appropriate to the smallest deflection and so actuate a relay operated switch individual to this electrode (Figs. 4A and 4B, not shown). In an alternative arrangement (Fig. 5) the selection of the smallest component signal and the subtraction of this signal from the other signals to provide the difference signals is effected by supplying the R, B and G signals to individual rectifiers 123, 124, 125 which are so biased via the potential divider 126, 127 that the potential at point 135 assumes the value of that of the smallest of the input signals (since only the rectifier associated with this signal can conduct) so that the white content signal is produced across terminals 128. The difference signals which are now equal to the potentials across each rectifier may either be employed directly to control the appropriate picture reproducing tubes or after amplification in triodes 132, 133 and 134 so that a definite ground reference potential exists. In a second embodiment of the invention (Fig. 6) the low-frequency components of the R and B channels are derived via L.P. filters 143 and 144 respectively and combined in 156 with the high and low frequency components of at least the G and R channels obtained from a mixer 145. At the receiver the low-frequency B and R signals and the low-frequency G components of the combined high- and low-frequency signals are separated out and fed to the device 165 which produces the difference signals by either of the methods already described (in this case the low-frequency components) for application to the picture reproducing tubes 166, 167, 168 and also the white content signal which is fed to the " white " picture tube 169 together with the high-frequency components of the combined signals which are derived from the received combined (high- and low-frequency components) signals via a H.P. filter 163. To reduce distortions (e.g. change of the saturation of colours and of the ratio of the residue signals) caused by a non-linear characteristic in the picture reproducing devices or the transmission devices (e.g. a curved toe characteristic), these distortions being of importance for small amplitude signals, the effective working point of the characteristic may be shifted towards the linear portion by the addition to the colour residue signals of a constant voltage (a fraction of the white content signal) which is subsequently subtracted at the receiver or alternatively the white content signal may be reduced by this (added) amount at the transmitter. A similar method may be employed to reduce distortions caused by a curved " shoulder " of the characteristic by making the added fraction the white content signal dependent on the actual value of this signal by means of a device having a characteristic which is complementary to that of the picture reproducing tubes and this method may be combined with the arrangement for reducing distortions caused by the " toe " part of the characteristic (Fig. 8, not shown). In a further embodiment distortions caused by the occurrence of " negative brightness " values in the separated high-frequency components of the white content signal (produced by the borders between areas of great brightness contrast) which produce a slow change instead of a sharp transition in the brightness are eliminated by suppression of the colour residue signals at these points (Fig. 10, not shown)