Patent GB-761433-A (GB)
Title: Colour television apparatus
Inventor: LUCK DAVID GEORGE CROFT
IPC: H04N11/14
Filed: 1952-11-25
Abstract: 761,433. Television. RADIO CORPORATION OF AMERICA. Nov. 25, 1952, No. 29813/52. Class 40 (3). In a compatible colour television system in which the transmitted signal comprises a luminosity signal in combination with a sub-carrier wave modulated at different phases by separate tinting signals and in which both double and single side bands exist for the transmission of the different modulated phases of the sub-carrier, the frequencies of all but a first tinting signal are restricted such that the products of modulation due to these tinting signals fall completely within the double sideband region. This arrangement reduces the distortion which would be caused by " crosstalk " (resulting from the loss of sideband information) if the sidebands of the modulated sub-carrier exceeded the cut-off frequency of the transmission system and, by suitably selecting the colour (tinting) signals, the effect of the loss of the colour information contained in the restricted signals in the colour image reproduced by the receiving system may be minimized. Fig. 3 shows one transmitter arrangement according to the invention in which three cameras 8, 10, 12 supply different component colour signals to a network 14 in which they are combined in required proportions to produce a luminosity signal L and first and second tinting signals T1 and T2 respectively. The luminosity signal is fed via a delay network 71 which compensates for the delay introduced into the colour channels by filters 58, 64, 65 to an adding circuit 63 where it is combined with the colour sub-carrier which is modulated in phase quadrature by the tinting signals T1, T2, the former being fed to a doubly balanced modulator 62 (in the output of which only the sidebands appear) via a filter 58 and the latter to an unbalanced modulator 66 via a filter 64. Filter 58, which has a characteristic as shown by graph 60, passes the frequencies of T1 lying below a frequency equal to the difference between the cut-off frequency of the transmission channel (Fco) and the frequency of the subcarrier wave (Fs) at unity amplitude and the frequencies lying between this frequency and a frequency not higher than 2Fs-Fco at a relative amplitude of two units, the increased amplitude 6 in the region 2Fs-Fco being necessary because these frequencies of T1 are transmitted with a single side-band whereas the frequencies below this region are transmitted with two side bands. Filter 64 has a characteristic as shown by graph 68 and passes the frequencies of T1 lying below Fco-Fs at unity amplitude to modulator 66 the output of which is fed to the adder 63 via a H.P. filter 65.which has a cut-off frequency at 2Fs-Fco to prevent signals other than the 90 degrees sub-carrier and the side-bands resulting from the modulation process from reaching the adder. This subcarrier is neutralized by a sub-carrier wave of 270 degrees phase which is supplied directly to the adder from the phase splitter 72. Alternatively, the sub-carrier may be balanced-cut by employing a doubly balanced modulator for modulator 66 or both modulators 62 and 66 may be unbalanced and the resultant 45 degrees subcarrier neutralized by supplying a wave of 225 degrees phase to the adder. The composite signal produced by adder 63 is then transmitted in any desired manner and at a monochrome receiver the luminosity signal may be employed for black and white reproduction. However, in order that a correct colour image may be reproduced at a colour receiver the coupling network 14 is so adjusted that the two-colour information lying in the band between (Fco-Fs) and (2Fs-Fco), the direct transmission of which is prevented by the restrictions imposed by the filters, may be produced by a suitable combination of the luminosity signal L and the tinting signal T1 whilst at the same time allowing the production of the three different component colours when L is combined with T1 and T2 in the low-frequency double sideband region. At a colour receiver suitable for use with the transmitter of Fig. 3, the tinting signals TI and T2 are recovered by homodyning the received composite signal in modulators 82 and 84 (Fig. 4) respectively with a wave of sub-carrier frequency at phases 0 and 90 degrees respectively, filters 51 and 53 preventing the transmission of the composite video signals and signals of sub-carrier frequency through modulator 84 and filter 91 eliminating the sub-carrier from the output of modulator 82 which is singly balanced against video signals and also eliminating much of the " dot " patterns produced by the low frequency of signal L beating with the sub-carrier in the modulator. If desired, modulator 82 may be unbalanced and a filter 85, which however limits the maximum frequency of T1 in the twocolour regions to ¢Fs, inserted between the composite video signal input and the modulator. Alternatively a doubly balanced modulator may be employed. The recovered tinting signals T1 and T2 together with the luminosity signal L are then combined (delay devices 87 and 89 compensating for the delay introduced by filter 51, 53) in a network 14<1> (similar to 14, Fig. 3) the three output channels of which supply the component colour (Green, Red, Blue) reproducers 90, 92, 94. The network 14 (Fig. 3A, not shown) is arranged to supply output signals at various amplitudes in either positive or negative polarity and by suitable adjustment (at the transmitter) any desired type of luminosity signal and colour information in the intermediate frequency region may be transmitted. Other embodiments for carrying out the invention and employing signal generating apparatus including cameras producing signals individually representative of the Red, Blue and the black/white or luminance aspects, respectively, of the image and further including matrixing arrangements for deriving from the signals produced by the cameras the signals actually transmitted and the reverse operation at co-operating receivers are described with reference to Figs. 5-13 (not shown).