Patent GB-754037-A (GB)
Title: Color television system
IPC: H04N11/08
Filed: 1954-06-15
Abstract: 754,037. Colour television. RAIBOURN, P. June 15, 1954 [June 30, 1953], No. 17550/54. Class 40 (3). Relates to a sequential colour television system and is concerned with the production of a signal having constant luminance characteristics with the object of avoiding objectionable flicker effects in the receiver display. In accordance with the invention, red, green, blue and green signals are transmitted during equal intervals (not necessarily in the sequence indicated) and the two green signals are weighted in accordance with the level of the red and blue signals so that the luminance of one of the green signals together with that of the red signal is equal to the luminance of the other green signal together with that of the blue signal. By the use of a transmission made up of four signals, simple binary switching may be employed at the transmitter and receiver. In the Figure, camera 1 produces simultaneous red, greed and blue signals on leads 2, 3 and 4 and applies them to the input terminal 7, 9, 10 and 11 of a secondary electronic switch 8. The red and blue signals go direct to terminals 7 and 9 respectively whilst the green signals for terminals 10 and 11 are applied first to weighting networks 5 and 6 which also receive inputs from the red and blue signals. The green signal GR from network 5 is weighted such that the signal equals 0.500G-0.254R+0.093B whilst that from network 6 equals 0.500G+0.254R - 0.093B. Assuming the luminance coefficient of the selected red, green and blue primaries to be 0.30, 0.59 and 0.11 respectively, the luminance of signal GR=0.200 and the luminance of signal GB=0.390. Since the luminance of the red signal on terminal 7=0.30 and the luminance of blue signal on terminal 9= 0.11, the total luminance of signals GR and R equals the total luminance of signals GB and B. In reproducing the signals, the chromaticity of the green signal is reproduced correctly from the two green signals since the red and blue weightings introduced in one signal are offset by the oppositely-phased weightings in the other signals. There are possible conditions where the signals GR and GB may become negative and since such a signal condition corresponds to blacker-than-black and may be confused with synchronizing pulses, it is arranged that no negative signal can appear at the output of networks 5 and 6. Furthermore, to maintain correct the total luminance, supplementary networks 5a, 6a comprising rectifiers or other polarity sensitive devices are arranged to deduct the negative value of either signal from the other. The output terminals 12 and 13 of the secondary electronic switch connect with the inputs of a primary electronic switch 15 whose output 17 applies signals to the transmission channel 18 communicating to a receiver. The receiver comprises a similar arrangement of primary and secondary switches 20, 24 connected to the red, green and blue inputs 31-33 of the image display 30. This may take the form of a single tube having, a line or dot colours screen, or three separate cathode-ray tubes. At the transmitter, the switches 8 and 15 may be actuated by control signals derived from the camera synchronizing or scanning circuits, whilst the corresponding switches 20 and 24 at receiver may be actuated in synchronism by means of signals communicated to the receiver either over separate channels or as part of the image signals on channel 18. Reference is made to the use of the blanking pulses and the conventional " burst " signal following each horizontal syne pulse. The switches at the transmitter and receiver may be operated in any sequence providing the operations are synchronized. The Specification refers to various dot-sequential sequences in which the secondary switching takes place at a high odd harmonic of line frequency and the primary switching takes place at line, half-line, field or frame frequency: Reference is also made to primary switching in a random manner and to secondary switching at a frequency prime to the line rate and its factors. Line sequential transmission of the four colour may be utilized and the Specification describes one sequence in detail. Reference is made to the use at the receiver of the cathode-ray tube described in U.S.A. Specification 2,692,532.