Patent GB-763352-A (GB)
Title: System of color television transmission
IPC: H04N11/18
Filed: 1953-12-01
Abstract: 763,352. Colour television. CHROMATIC TELEVISION LABORATORIES, Inc. Dec. 1, 1953 [Jan. 8, 1953], No. 33289/53. Class 40 (3). [Also in Group XL (c)] In a colour television system operating with conventional double interlaced scanning, there is transmitted during each frame a signal representative of image luminance. Coincidently with this signal there is transmitted a chrominance signal modulated on a sub-carrier the frequency of which is an odd multiple of onehalf line frequency. The subcarrier is modulated in amplitude only and the quantity modulated on it at any instant alternates in successive fields between the ratio of one colour component to the total luminance and the ratio of another colour component to the total luminance. The colour components selected are the two contributing the least to the image luminance, except where no colour contributes 50 per cent of the luminance, the colours selected are red and blue. Since the per cent contribution of two of the colours is transmitted, the per cent contribution of the third may be derived. This information, however, is communicated during the period of two fields and the per cent contribution of the third colour is presented therefore in two parts which integrate over the picture period. The manner of modulating the sub-carrier signal provides that a signal of either zero amplitude or equality with a reference amplitude represents fully saturated blue. A signal of one-third the reference amplitude represents fully saturated green, and a signal of two-thirds the reference amplitude represents fully saturated red. Signals intermediate the above levels represent various combinations of two colours. At a receiver the luminance signal is applied to the three-colour reproducing means through separate amplifying channels proportioned inversely in accordance with the relative luminance contributions such that equal input signals produce equal luminances. The sub-carrier is decoded to derive signals indicating the per cent contributions of the three colours and these signals are then applied to control the gains of the respective colour channels. Both a transmitter and a receiver are described, but only the transmitter forms the subject of the invention. Transmitter, Fig. 1. Green, red and blue colour signals proportioned in accordance with the relative contributions of the colours to the total luminance are derived from a camera or film scanning source 1 and applied to an adder 7 to derive a luminance signal on line 9 which modulates a transmitter 113 directly. The colour signals together with the luminance signal are also applied to a normalizer 11 from which a constant amplitude output is derived on line 21 corresponding to 100 per cent luminance, and further outputs on lines 23G, 23B and 23R corresponding to the per cent contributions to the luminance of the three colour signals. These outputs are positivegoing signals and corresponding negative signals appear on lines 23G<1>, 23B<1> and 23R<1>. In the normalizer, Fig. 3 (not shown), each of the signals is modulated on to an R.F. carrier, passed through a gain control amplifier and then detected to derive the original signal. The output from the detector deriving the luminance signal is applied in a negative sense to control the gain of all the amplifiers equally and is arranged to maintain the luminance signal at a constant amplitude whereby the outputs from the other detectors correspond to the per cent contributions of the colours. The positive red and blue signals are each compared with the luminance signal in comparitors 25B and 25R, Fig. 4 (not shown), and produce an output to open an associated gate 27B and 27R when they exceed 50 per cent of the luminance signal. The comparitors also control a further normallyopen gate 27G through a coincidence unit 81 which is effective to close the gate when an output appears from either comparitor. By this means, gate 27R is opened when red is the predominating colour (i.e. contributing at least 50 per cent of the luminance), gate 27B when blue is the predominating colour, and gate 27G when neither red nor blue contributes 50 per cent. The outputs from the gates are applied to control the amplitude modulation of a subcarrier generator 111 and the gate inputs are received from keyer units 29R, 29B and 29G to each of which the other two of the per cent colour signals are applied with polarities as indicated. Each of these keyer units, Fig. 5 (not shown), comprises a pair of pentode gates receiving the colour signals and which are rendered transmissive alternately each for the period of one field by multi-vibrator 82. Keyer 29G is so adjusted that with the pentode receiving the minus blue per cent signal cut-off and no plus red per cent signal to the other pentode, the current from that pentode causes a voltage drop of such amplitude as will result in a modulation of the sub-carrier to one-third of a reference amplitude. The amplitude of the red input is then adjusted so that a 50 per cent signal doubles the pentode current and produces, modulation to two-thirds of the reference amplitude. With the conduction of the gates reversed and no blue per cent input, the current from the first pentode is adjusted to produce modulation to one-third reference amplitude. The blue per cent signal is negative going and reduces the pentode current, and is adjusted so that a 50 per cent signal reduces the modulation to zero. Keyer 29B is adjusted in a similar manner, but here the positive green signal modulates from zero amplitude with zero input to onethird reference amplitude with 50 per cent input whilst the negative red input modulates from full reference amplitude with zero input to two-thirds reference amplitude with 50 per cent input. Keyer 29R is adjusted so that the positive blue input modulates from two-thirds reference amplitude with zero input to full reference amplitude with 50 per cent input whilst the negative green input modulates from two-thirds reference amplitude with zero input to one-third reference amplitude with 50 per cent input. In operation, therefore, as each picture point is scanned, the predominating colour component determines which of gates 27G, 27B and 27R is opened and thereby allows the sub-carrier to be modulated in accordance with the per cent contribution of the other two colour components, one colour being effective during one field and the other during the other field of the picture as determined by the operation of keyers 29G, 29B and 29R. The reference amplitude of the chrominance signal is made equal to 10 per cent of black level (U.S.A. Standard) and this level is communicated to the receiver one per line during the front or back porch of the line synchronizing pulse by a connection 130 to the sub-carrier modulator from synchronizing generator 3. Receiver, Fig. 2. The receiver operates in the conventional manner to derive an intermediate frequency signal and the luminance component of this signal is applied directly over lead 131 to three amplifier multiplying stages 163G, 163B and 163R forming the inputs of channels to the respective colour control electrodes of cathode-ray tube reproducer 197. The chrominance sub-carrier portion of the signal is selected by filter and amplifier 137, detected at 139 and applied through phase-splitting stage 141 to decoding stages 147G, 147B and 147R. These stages (see Group XL (c)] comprise diode circuits which are biased in dependence on the level of the reference amplitude transmitted once during each line and arranged to exhibit non-linear input/output characteristics as shown in Figs. 8A, 8B and 8C. The circuits respond to the sub-carrier modulation to produce output voltages corresponding to the per cent luminance signals applied at the transmitter. Thus a signal in one field lying between one-third and two-thirds of the reference amplitude and corresponding to, say, 30 per cent red, where green is the predominating colour, will produce an output from decoder 147R. An output will also appear from decoder 147G indicating a per cent contribution of green during that frame. During the following frame the sub-carrier will be modulated between zero and one-third in accordance with the per cent contribution of blue. An output thereupon appears from decoder 147B together with another output from decoder 147G indicating a per cent contribution of green which, together with that in the preceding field, integrates visually to produce the correct green contribution in the picture. A similar operation takes place with other colour, the operation changing continuously from point-to-point throughout the picture. The colour per cent signals from the decoders multiply the luminance signal in stages 163G, 163B and 163R by varying the amplifier gains, the resulting luminance components being detected and applied through amplifiers 193G, 193B and 193R to the cathode-ray tube. The gains of amplifiers 193G, 193B and 193R are proportioned inversely in accordance with the relative luminance contributions of the three colours taking regard of the cathode-ray tube phosphor efficiencies such that equal signal inputs produce equal luminances. Specification 722,282, [Group XL (a)], is referred to.