Patent GB-709496-A (GB)
Title: Improvements in and relating to colour television systems
IPC: H04N3/34
Filed: 1952-03-18
Abstract: 709,496. Colour television. BRITISH THOMSON-HOUSTON CO., Ltd. March 18, 1952 [March 19, 1951], No. 6969/52. Class 40 (3) In a colour television system employing signals corresponding to three addition component colours, a first group of signals comprising the low-frequency components of the green signal together with the mixed high-frequency components of all the three signals is transmitted continuously whilst the low-frequency components of the red and blue signals are transmitted alternately in a sequence at line frequency to form a second signal group, said first and second signal groups being transmitted by means of two separable forms of direct modulation of a main carrier wave. The first group may be transmitted as amplitude modulation whilst the second group may also be transmitted as amplitude modulation but in phase quadrature to the first group. Alternatively the second group may be transmitted by frequency and/or phase modulation. To avoid the effects of cross-modulation between the two signal groups, means may beprovided for reversing the phase of the second group modulation components after each picture field, frame line or group of lines. The distortion due to such cross-modulation is thereby rendered selfcancelling due to integration in the eye. In a modification of the system, fractions of the red and blue low frequency components may be added to the first signal group before modulation at the transmitter and corresponding fractions subtracted at the receiver to restore the brightness. By this means the main signal group is made more nearly panchromatic to aid reproduction of the signal on a normal black-and-white receiver. In one embodiment, Fig. 8, employing amplitude modulation for both signal groups, the red, blue and green signals from camera 26 are separated into high or low frequency components by means of filters 31-36 having pass-bands as indicated, and the high frequency components are combined in an adding circuit 37 together with the low frequency green components. This signal is then combined in mixer 38 with synchronizing and blanking signals derived from a master pulse generator 50a and supplied to an amplitude modulator 49 whose output modulates the carrier wave from source 20 in modulation amplifier 21. The low frequency components of the red and blue signals pass to gated amplifiers 54, 55 which are operated alternately in a sequence at line frequency by keyer circuit 56. The output appears on conductor 60 and is supplied to a balanced modulator 61 which also receives the carrier signal over conductor 175 and operates to produce modulation side-bands with the carrier suppressed. The output is suppressed during blanking intervals in amplifier 64. The two modulated carrier signals are then combined in an adder circuit 23, that from amplifier 21 being shifted first by 90 degrees in unit 22 whereby the two signals are in precise phase quadrature. The complete signal is further amplified in a stage 24, which include a filter unit to provide vestigial side-band transmission, and radiated from antenna 25. Fig. 3 illustrates the radiated signal, the alternating red and blue signals being in phase quadrature with the remaining signals. At the receiver, Fig. 14, the intermediate frequency signal is applied to two band-pass filters, the first, 93, passing the entire range of video frequency whilst the second, 94, selects only the range including the alternating red and blue low frequency signals. The filter outputs are applied respectively to detectors 95, 96 where they are heterodyned against a locally-generated intermediate-frequency carrier signal of high level which has the correct phase to demodulate the desired video component. The carrier signal is generated in an oscillator 97 and applied through amplifier 98 and individual phase adjustors 99 and 100. The wave supplied to detector 96 is exactly in phase with the wave from filters 93 whilst that to detector 96 is in precise phase quadrature. The oscillator is synchronized by the incoming intermediate frequency wave which is passed through a gated amplifier 101 and compared in phase with an output from amplifier 98 to produce a control voltage operating on a reactance stage 103. The resultant video outputs from the detectors pass through amplifiers 104, 105, the green signal plus mixed highs appearing on conductor 106 and the alternating red and blue low-frequency signals on conductors 107. Conductor 106 connects with the green gun of the cathode-ray tube 254 whilst conductor 107 connects with a pair of gated amplifiers 129, 130 which are operated in alternation by Keyer 126 in the same sequence as the transmitter switching so as apply the red and blue low-frequency signals to the appropriate gun in the cathoderay tube. As illustrated, the mixed high-frequency signals are applied to the green gun only. If desired a band-pass filter may be employed to add the mixed high-frequency signals to the red and blue signal channels also. The synchronizing signals are separated in stage 120 and control the sweep generators 121, 122. The line scanning voltage is also applied over conductor 123 for gating amplifier 101 during blanking intervals. In an alternative receiver, Fig. 4 (not shown), the signals are reproduced on three separate cathoderay tubes. The switching between the red and blue lowfrequency signals is effected during line blanking intervals in a sequence at line frequency. This may involve alternate red and blue lines although preferably the sequence should be such that an integral number of complete groups occur in one picture frame. With 525 lines double-interlaced scanning a suitable group comprises two red lines and one blue line, (RRB). Alternative groups comprise: RRRBB and RRRRBBB. These sequences eliminate crawl from the reproduced fmage. To prevent the resultant bar structures of the red and blue images from appearing in the combined image, the widths of the red and blue cathode-ray beams are increased. At the transmitter suitable waves for switching between the red and blue signals are generated by the Keyer 56 the waves also being fed back to the master pulse generator 50a over conductors 67 so as to modify the horizontal synchronizing pulses whereby the colour switching at the receiver may be synchronized. Fig. 5a shows one suitable arrangement operating in the sequence red, red, blue where the trailing edges of the synchronizing pulses 140, 141 preceding the blue lines are given a different shape from those, 142, preceding the red lines. Fig. 5b (not shown), shows a slightly modified arrangement. At the receiver, the synchronizing pulses from separator 120 pass to a decoder 127 where in a series of differentiating, integrating and clipping circuits, Fig. 6 (not shown), pulses are produced indicating each change over from red to blue. These pulses are applied to an automatic frequency control circuit to synchronize a sine wave oscillator from which a saw-tooth wave is generated and combined with a saw-tooth wave at line frequency. The resulting stepped wave is then clipped to provide a pair of rectangular output waves having the required two-to-one switching ratio for gating amplifiers 129, 130. To avoid the effects of cross-modulation between the green signal and the red and blue signals due to incidental nhase modulation of the carrier or lack of stability in the local carrier wave at the receiver, the phase of the quadrature modulation components may be reversed at both the transmitter and receiver after each picture field or frame. In the transmitter shown in Fig. 8 this is effected by a circuit 171 which reverses the phase of the carrier signal applied to modulator 61. The circuit is keyed by 15 CPS square waves from a Keyer 170 so as to reverse the phase during alternate picture frames. Fig. 10 (not shown) shows details of a suitable circuit. In order to synchronize corresponding switching actions at the receiver, the 15 CPS square waves are fed back to the master pulse generator 50a over conductors 174 and control the injection of short bursts of high-frequency sine-wave signals near the ends of the vertical blanking intervals. For example. as illustrated in Fig. 9, 200 kc bursts are added at the end of fields 1, 5 &c. and 500 kc bursts at the ends of fields 3, 7 &c. In an alternative arrangement, Figs. 12 and 13 (not shown) the phase of the carrier signal remains fixed but the phase of the alternating red and blue low frequency signal is reversed. The receiver of Fig. 14 functions from signals sent by transmitters operating in accordance with either of the above arrangements and includes a phase reversing circuit 250 similar to that employed at the transmitter connected in the channel receiving the alternating red and blue lowfrequency signals. The circuit is operated by a Keyer 251 synchronized by a burst selector 252 including circuits tuned to the 200 kc and 500 kc sine wave signals. As an alternative to the above arrangement the phase reversal may be effected at the end of each field, scanning line or group of scanning lines. In another embodiment of the invention, Figs. 15 and 16 (not shown), the green low-frequency signals and mixed high-frequency signals are carried by amplitude modulation of the carrier whilst the alternating red and blue low-frequency signals are carried by phase or frequency modulation. In a modification phase modulation is gradually changed to frequency modulation within the region 10-200 kc of the red and blue lowfrequency signal band. The phase of the red and blue low-frequency signals may be reversed as described above in connection with Figs 8 and 14 to eliminate cross-modulation effects. The Specification includes a mathematical analysis of the system operating with double amplitude modulation. It is shown that a conventional black-and-white receiver may operate from the signal. Specifications