Patent GB-778181-A (GB)
Title: Improvements in or relating to the conversion of television signals, especially colour television signals
Inventor: PERKINS DENIS GORDON
IPC: H04N11/22
Filed: 1953-10-01
Abstract: 778,181. Television. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Sept. 22, 1954 [Oct. 1, 1953], No. 26914/53. Class 40 (3). Relates to recording and reproducing apparatus for the conversion of television signals of one scanning standard to another and describes arrangements for: (1) converting field sequential colour signals at 150 fields per second to simultaneous signals at 50 fields per second; and (2) converting simultaneous colour signals to further simultaneous signals of a different line and/or frame frequency. Reference is also made to the conversion of sequential stereoscopic signals to simultaneous signals. According to the invention some of the component signals are translated to frequency bands different from the others for the purpose of discrimination in recording and subsequent reproduction. In Fig. 1, camera 1 operates with a rotating filter to produce field-sequential green, red and blue signals at a rate of 150 fields per second. The signals are passed to a gate circuit 2 which is arranged under the control of a pulse generator 3 to route the red and blue signals to separate channels 4 and 5. The signals also pass to an amplifier 6 which is controlled in gain by the generator 3 during successive fields so as to pass the green, red and blue signals in the ratios of 0.59: 0.3: 0.11 corresponding to their respective contributions to the image luminance. In channels 4 and 5 the red and blue signals are applied respectively to modulate carrier oscillations in stages 7 and 8, the resultant signals being combined with the luminance representative signal from amplifier 6 in a combining gamma correction circuit 9 from which the output is applied to a display device 10 including a conventional monochromatic cathode-ray tube scanned at 150 fields per second in synchronism with camera 1. The red and blue signals are modulated on to carriers as shown at 21 and 23 in Fig. 3a so as to occupy bands separate from the luminance signal 7. Negative modulation is employed for one or both of the carriers and the luminance signal. The modulated oscillations produce corresponding dot patterns on the screen, the dot patterns being modulated in amplitude by the red and blue signals respectively. In an alternative arrangement, Fig. 3b (not shown), the red and blue signals are transposed in position. In yet further arrangements, Figs. 4a and 4b (not shown), one of the signals modulated on to a carrier may be the luminance signal. Display device 10 is viewed by a pick-up tube 11 of the storage kind operating at 50 fields per second under the control of pulses from generator 3. The output comprises the luminance signal and also includes, due to the dot patterns, two oscillations modulated with the red and blue signals. Since the regeneration of the signals occurs at a different rate from their recording in device 10 they differ in frequency. By means of a low-pass filter 12 the luminance signal is separated into channel 16, and by means of filter and detector circuits 13 the red and blue signals are separated into channels 14 and 15. The three resultant signals are simultaneous signals and may be employed for a transmission based on the American N.T.S.C. standards. A monitor 17 is provided for the output of camera 1 and a simultaneous monitor may be provided for the outputs in channels 14, 15 and 16. The two carrier oscillations employed in modulators 7 and 8 are locked in frequency to the line scanning oscillations of display 10. Spot wobbling is preferably employed in the display to avoid line scan beating patterns. Variations in the frequency of the regenerated red and blue carriers may be employed to correct for non-linearity in the display and pick-up tube scans. Alternatively a separate carrier may be used for this purpose. Special carriers at the low and high frequency ends of the video band may also be injected into the circuit at a point prior to display 10 for the purpose of correcting variations of the sensitivity and resolution of the system. Correction for this may also be effected in a predetermined manner during each line and/or field period by controlling the gain of amplifiers by " tilt " and " bend " waveforms. Pick-up tube 11 may be operated independently without control from generator 3, in which event it is necessary to allow for possible variations in the generated signal frequencies. To this end the carrier oscillations at the output of the pick-up tube may be compared with a standard frequency and any deviations employed to adjust appropriately the pass-bands of the filters in circuit 13. Alternatively a special carrier may be injected and the regenerated frequencies stabilized by variation of the carriers applied to modulators 7 and 8. Two pick-up tubes may be utilized, one for the luminance signal and the other for the red and blue signals, and as an alternative to the use of a gain-controlled amplifier 6 in such an arrangement, a synchronously driven filter or shutter device may be arranged in the path to the tube for the luminance signal as described in Specification 778,179. As an alternative to the use of red, green and blue filters with camera 1, reference is made to the use of red, blue and luminance filters, and filters having response characteristics in accordance with the I.C.I. X, Y and Z coefficients. Fig. 2 (not shown) illustrates an arrangement for converting luminance, red and blue signals produced simultaneously at one scanning standard to similar simultaneous signals with a different field and/or line frequency. The arrangement is similar to Fig. 1, two of the signals being modulated on to carrier oscillations before application to the display device, and the pick-up tube having filters and detectors in its output to separate the signals. In the preferred embodiment the luminance signal is disposed at the high-frequency end of the video spectrum, the modulation on to its carrier being effected with a vestigial lower side-band. Corrections for variation of sensitivity, resolution, scanning linearity &c. are effected as described with reference to Fig. 1. As an alternative to the use of a cathode-ray image-reproducing tube with a storage-type pick-up tube for recording and reproduction, a reading'and writing tube of the storage type may be used. Reference is also made to the use of magnetic tape recording, the signals being fed to a recording head sequentially or simultaneously, and in the sequential case the record being so controlled that the sequential signals are recorded in superimposed relationship. Reference is made to the use of a display incorporating a light modulator as described in Specification 543,485, [Group XL], in place of cathode-ray tube 10.