Patent GB-702182-A (GB)
Title: Color-television system
IPC: H04N11/16
Filed: 1951-12-14
Abstract: 702,182. Colour television. HAZELTINE CORPORATION. Dec. 14, 1951 [Jan. 22, 1951], No. 29354/51. Class 40 (3) In a colour television sys tem of the type wherein the colour-component signals are sequentially modulated upon a sub-carrier wave signal (i.e. a dot-sequential type of systen wherein the colour-componen signals are effectively sampled in sequence), the sequence o the colour "sampling" periodically reversed, prefer ably in alternate frames. By this means errors in colour reproduction due to sligh errors in the synchronisn between the " sampling " at the transmitter and receiver and due to certain interference effects may be substantially eliminated. In the transmitter shown in Fig. 5, the green, red and blue outputs from the colour-camera unit 80 are ap plied respectively to low-pass filters 83a 83b, 83c, the out puts from which are applied to modulators 85a, 85b, 85c wherein the colour-componen upon a sub-carrier-wave sig nal generated at 81. To ob tain the desired phasing of the " sampling " operation the sub-carrier-wave signal inputs to the modulators 85b, 85c are delayed with respect to that to modulator 85a by means of delay lines 76b, 76c which effect respective phasedelays of 120 degrees and 240 degrees. The outputs from the delay lines 76b, 76c are passed through a switching circuit 82 which is synchronized from the camera unit 80 and which periodically reverses the connections between the delay lines and the modulators 85b and 85c, thus resulting in a periodic reversal of the modulation sequence. In addition the three colourcomponent signals from the camera unit 80 are combined in an adder circuit 84 to develop a monochrome signal which is combined in adder circuit 87 with the lower side-band of the modulated sub-carrier wave signal delivered from adder- and-filter circuit 86. The resultant video signal is applied to amplifier 88 and transmitting aparatus 89. At the receiver, Fig. 1 (not shown), which is similar to that described in Specification 689,821, the lower side-bands of the sub-carrier wave signal are applied in parallel to these synchronous detectors to which are also applied suitably phased signals of the sub-carrier-wave frequency, the sub-carrier-wave signals applied to the redsignal and blue-signal detectors being periodically interchanged in synchronism with the switching at the transmitter to allow the demodulation to take place in the correct order. The demodulation signals are individually combined with the composite video signal to restore the three colour-component signals as described in Specification 689,821. In a modification, the relative-amplitudes of the colour-component signals are proportioned in a manner similar to that described in Specification 689,356 so that crosstalk between the various colour channels due to errors in phasing of the demodulators at the receiver does not present in brightness variations of the reproduced picture. At the transmitter, Fig. 6, the " monochrome signal is supplied by adder circuit 184 and comprises red and green components only as represented by the equation M=2/3G + 1/3R, whence it may be seen that (R-M)=-2 (G-M), so that a single signal (R-M) may be made to convey both the red and the green colour information. The red and blue component signals from the colour camera are combined in adders 84b, 84c with the monochrome signal to give respectively signals (R-M) and (B-M). These are passed by low-frequency filters 83b, 83c to the modulators 185a, 185b where they are modulated in quadrature upon the sub-carrier wave signal from generator 81 the signal from generator 81 being passed through a delay line before application to the modulator 185b. This signal is switched between a 90 degree delay line 176b and a 270 degree delay line 176c in alternate frames by a switching circuit 182 so that the modulation sequence is periodically reversed. The modulator outputs are combined in circuit 86 where the lower sidebands are selected and the combined signal is then passed to adder circuit 87 where it is combined with the monochrome signal from adder circuit 184. At the receiver, Fig. 3, the synchronous detector 127c extracting the signal (BM) is supplied with sub-carrier-wave signal from generator 31 through alternately switched delay lines 132a, 132b in synchronism with the corresponding operation at the transmitter. The outputs from the synchronous detectors are filtered and applied to amplifiers 130a, 130b, 130c, the input to amplifier 130a being derived by inversion at 40 of the output of detector 27a Amplifier 130b which has twice the gain of amplifier 130a delivers signal (R-M), whence it can be seen from the relation given above that amplifier 130a delivers signal (G-M). Amplifier 130c has a gain three times that of amplifier 130a and delivers a signal (B-M). The amplifier outputs are then combined individually with the monochrome signal, passed by filter 23 and isolation amplifier 24, to restore the original colour-component signals. In a modified receiver, Fig. 4 (not shown), the modulated sub-carrierwave signals applied to detector 127c are changed in phase alternately by 90 degrees and 270 degrees, the output of generator 31 being applied in phase to the two detectors. In this arrangement also, isolation amplifier 24 is dispensed with, the monochrome signal being applied to the control grid of a three-cathode three-colour cathoderay tube to the individual cathodes of which the signals (G-M), (R-M) and (B-M) are applied In afurther receiver arrangement, Fig. 7 (not shown), the sub-carrier wave signal, modulated at 120 degrees spaced phases by the colourcomponent signals is heterodyned in alternate frames with a signal of twice the sub-carrier frequency. The resultant signal is again of the sub-carrier frequency but the modulationsequence is reversed in alternate frames to correspond with the periodic reversal at the transmitter which may be effected in a similar manner. Specification 701,830 also is referred to.