Patent GB-702712-A (GB)
Title: Color-television electro-optical apparatus
IPC: H04N11/00
Filed: 1951-12-27
Abstract: 702,712. Colour television. HAZELTINE CORPORATION. Dec. 27, 1951 [Jan. 29, 1951], No. 30217/51. Class 40 (3) In a colour television system, a high-definition signal representative primarily of the brightness of an image is transmitted together with low - definition signals representative of colour characteristics of the image, and at the receiver the high-definition signals reproduce a monochrome image to which the low-definition signals add colour to reproduce the image in colour. Transmitter, Fig. 1.- Image I is projected on to three pick-up tubes 15a-c through filters 18a-c. Filter 18a is chosen, in combination with the spectral response of target 16a to develop a signal M representative of the brightness characteristic of the image in accordance with the equation 1/3R + 2/3G where R and G, respectively, represent the red and green colour characteristics of the image. The signal is applied to an adder circuit 11 through a filter 25 having a pass-band of 0-4Mc. Filter 18b is chosen in combination with the spectral response of target 16b to develop a signal representative of the red component of the image. The signal is combined in adder stage 28b with a phase-inverted brightness signal from stage 27 to produce a signal in accordance with the equation R-M, and this signal is passed through filter network 26b having a pass band 0-IMc. In a similar manner a blue signal is obtained from pick-up tube 15c and passed through adder circuit 28c to derive a signal B-M which is then passed through a filter network 28c having a pass band of 0-0.5Mc. The outputs from the filters 26b and 26c are fed to a modulator 14 which effects a time-sequential modulation of a 3.0Mc subcarrier signal supplied from generator 29 at 0 degrees and 90 degrees phase points. The output from the modulator is fed through a 2-4Mc filter 19, which passes both the upper and lower side bands of the subcarrier signal, and is combined with the brightness signal in adder circuit 11. The combined signal is then passed to power amplifier 12 which feeds a circuit 13 for subsequent transmission by line or radio. A scanning-frequency generator 23 controls the scanning of pick-up tubes and adds synchronizing signals to the combined output signals in power amplifier 12. Receiver, Fig. 2.-The signals are derived by a conventional receiver and detector 30, 32. A first filter 33 having a pass band of 1-4Mc selects signals for application to a cathode-ray tube 34, the signals developing a high definition monochrome image on the screen 35. Cathode-ray tube 34 is of the type having an image screen which functions as a light valve, the screen including a coating of a material such as alkali halide crystals which vary in transparency under control of the scanning electron beam. A second filter 48 having a pass band of 2-4 Me feeds signals to a detector circuit 44 which effects a detection at 0 degrees and 90 degrees phase points of the subcarrier signal to derive signals corresponding to the R-M and B-M siganls produced at the transmitter. These two signals are passedto filters 42b and 42d respectively and are combined with a monochrome signal (M) obtained through a 0-1Mc filter 70 in order to produce the red (R) and blue (B) component signals alone. The detector circuit also produces an inverted signal -¢(R-M) by detection at the 180 degree phase point which is passed to filter 42c and combined with a monochrome signal (M) from filter 70 to derive the green component signal G. (M-¢(R-M) = G). The red, green and blue signals produce low-definition colour images on associated cathode-ray tubes 43b, 43c and 43d. The images are combined by dichroic mirror system 46 and projected onto the screen 35 of tube 34. The resultant image is then viewed directly or projected onto an external screen 50. Line and field scanning generators 39 and 40 timed by synchronizing signals derived from separator 41 control the scanning of the four cathode-ray tubes. A modified receiver, Fig. 3 (not shown), comprises a single cathode-ray tube utilizing three electron beams in conjunction with a tri-colour screen to reproduce the colour images. Specification 689,356 and U.S.A. Specification 2,146.862 are referred to.