Patent GB-704791-A (GB)
Title: Improvements in and relating to frequency-interlace colour television systems
IPC: H04N11/12
Filed: 1951-07-16
Abstract: 704,791. Colour and stereoscopic television. BRITISH THOMSON-HOUSTON CO., Ltd. July 16, 1951 [July 28, 1950], No. 16822/51. Class 40 (3). [Also in Group XL(c)] In a colour television system of the simultaneous type, the low-frequency portions of the red and blue component video signals are transmitted as modulations of respective sub-carriers which do not coincide in frequency with integral multiples of the line-scanning frequency, the subcarriers being subsequently modulated on to a main carrier upon which are also modulated directly the green-component video signal and also the high-frequency portions of the red and blue component video signals. The highfrequency portions of the component video signals are consequently transmitted by the " mixedhighs " technique, while owing to the discrete nature of a television video-signal frequency spectrum the low-frequency components of the red and blue signal frequency spectra are interlaced between those due to low-frequency-green and mixed-high signals. The system may be modified to provide stereoscopic reproduction, the two component images being reproduced in complementary colours. Fig. 2 illustrates one form the resultant frequency spectrum may take in a system operating in accordance with present day United States transmission standards for black-and-white transmissions. The " green " signal and component of the "red" and "blue" signals above 1 Mc/s and 0.2 Mc/s respectively are modulated directly upon a main or " green " picture carrier. The remaining red and blue components are modulated upon respective subcarriers which are odd multiples (the 441st and 63rd respectively in the example given) of one half the line frequency, these sub-carriers being then modulated as indicated in the figure upon the green carrier. The red and blue carriers and side-bands thereof resulting from this modulation are consequently all spaced by one half the line frequency from the nearest frequency components of the combined green and mixed high side bands and may then by means of suitable circuits be separated therefrom at the receiver. Figs. 6 and 8 (not shown) illustrate alternative arrangements. In Fig. 6 the blue sub-carrier is modulated upon the red sub-carrier and the resultant sub-carrier and side bands (which includes the blue sub-carrier) are modulated on to the green carrier. In Fig. 8, vestigial sideband techniques are used for transmitting the red and blue low frequency signals. A further sub-carrier at an odd multiple of the one-half the line frequency is also modulated on to the green carrier and used at the receiver for automatic gain control. Details of transmitter.-Fig. 3 illustrates a transmitter for generating a signal having the spectrum indicated by Fig. 2. Camera 16 supplies green, blue and red signals to conductors 18, 20 and 40 respectively. The high-frequency portions of the blue and red signals are selected by high-pass filters 22 and 42 respectively and then applied to inner circuit 19 to which the green signal is also applied directly. The lowfrequency components of the red signal are separated out by low-pass filter 41 and are subsequently modulated in 45 on to the output of oscillator 31 generating the red sub-carrier. The modulated sub-carrier is also applied to mixer 19 the output of which has synchronizing and blanking pulses from generator 49 added at 47 and is then modulated at 12 on to the green carrier which is generated by oscillator 10 and frequency-multiplier 11. The low-frequency components of the blue signal are selected by lowpass filter 21 and are applied via circuit 25 to modulate the blue carrier applied to circuit 26. The blue sub-carrier is generated by frequency division at 32 of the output of oscillator 31 and is applied to balanced modulator 27 to which the green carrier is also applied through buffer amplifier 29. The resultant difference-frequency signal forming the blue carrier is modulated at 26 and then added at the output stage of the transmitter to the output signal from circuit 12. Frequency divider 50 and frequency multiplier 51 derive a signal of double the line frequency for synchronizing the generator 49. In the transmitter described in connection with the spectrum illustrated in Fig. 8 (not shown) wherein both the blue and the red carriers lie within the upper sideband of the main carrier, the blue sub-carrier modulated by the low-frequency blue signal is applied to mixer 19 instead of to the output stage of the transmitter. A further subcarrier which is an odd multiple of one half the line frequency is also applied to circuit 19 to produce the carrier for automatic gain control. In the transmitter described in connection with Fig. 6 (not shown) the modulated blue subcarrier is added to the red low frequency components before modulation to the red sub-carrier. Details of receiver.-Fig. 4 shows a receiver for use with the transmitter of Fig. 3. The output of the I.F. amplifier 63 is passed through a band-pass filter 64 which attenuates the lower side-bands (and consequently the blue carrier and side-bands) and the output thereof is then detected at 65. The amplified video signal from 66 is applied over line 67 to the input of a threecolour cathode-ray tube 68 to produce the green components of the image on the screen. The output of second detector 65 is also applied to high-pass filter 81 which selects that portion of video signal including the mixed-high components and the filter output is applied through video amplifier 84 and 85 to the red and blue inputs of the cathode-ray tube respectively. Band-pass filter 88 selects the red sub-carrier and its side-bands which are detected at 89 and then applied through a low-pass filter 90 and video amplifier 92 to the red input of the tube. The output of I.F. amplifier 63 is also applied to band-pass filter 101 which selects the blue carrier and side-bands and the green carrier (at their respective intermediate frequencies) and applies them to second detector 102, the output of which consists of the modulated blue sub-carrier. This is filtered at 103 detected at 104, filtered at 105 and applied through video amplifier 107 to the blue input of the cathode ray tube. Synchronizing signals are separated at 69 and used to control the line and frame scanning generators 70 and 71. It will be seen that each input to the cathode-ray tube has its respective low frequency colour components applied to it as well as the mixed-high signal. The relative-amplitudes of the latter signal are adjusted in the three inputs to cause it alone to produce a dark-grey picture on the screen. It will furthermore be apparent that unwanted components will be present in each channel; thus the red input will contain those components of the mixed-highs which lie within the red side-bands in Fig. 2. However, it may be shown that, since these components are not integrally related to the line frequency, they ultimately produce no visible effect owing to an integrating action in the eye. Nevertheless demodulation in the second detector 65 results in some degree of demodulation of the red sidebands owing to the presence of the red carrier, and this produces colour cross-talk in the green channel. This is eliminated by feeding a portion of the output of low-pass filter 90 in the appropriate polarity, in the green channel over line 99. Figs. 11 and 12 (not shown) illustrate the frequency relationships occurring in a modification of the system wherein the interlaced frequency components are spaced by one-third the line frequency. Specification 675,887 is referred to.