Patent GB-1108463-A (GB)
Title: Improvements in projection systems
IPC: H04N5/74
Filed: 1965-04-23
Abstract: 1,108,463. Light modulation. GENERAL ELECTRIC CO. 23 April, 1965 [7 May, 1964], No. 17298/65. Heading H4F. In a colour television image projection system, Fig. 1, not shown, in which two vertical phase diffraction gratings corresponding to the red and blue video signals of the image and a horizontal phase diffraction grating corresponding to the green video signal of the image are formed by an'electron beam in a light modulating medium, e.g. a layer of oil, substantially as described in Specification 1,096,741, the carrier wave on to which the green video signal is modulated instead of being a standard sine wave is arranged to have a substantially linear rising and falling portion in order that a good dark field may be obtained in the green channel for the projected image. In the green channel since the amplitude of the carrier wave, varies inversely with the amplitude of the green colour signal it is necessary in order to obtain good green dark field to provide highly uniform deposition of charge over the light modulating medium since green dark field is produced by maximum carrier modulation of the electron beam scan. With a sinusoidal high frequency waveform being used to form the green diffraction grating bunching of charge is produced at the peaks of the sinusoidal wave which produces deformations whereas for good green dark field there should be no deformations in the light modulating medium. To avoid these undesired effects and yet keep the ratio of the electron beam spot size to raster line spacing down in order to obtain a good contrast ratio such waveforms as shown in idealized form in Figs. 4B and 4C, which have more uniform rates of travel in the vertical direction than a high frequency sine wave, are applied to the vertical deflection plates of the electron beam device in Fig. 1, not shown, to produce modulation or wobbulation of the beam. Since the waveforms shown in Figs. 4B and 4C, are difficult to obtain at high frequencies, waveforms as shown in Figs. 4D and 4E, are used since these are relatively simply obtained. The waveform shown in Fig. 4D, may be obtained by combining with the waveform of Fig. 4A, a waveform as shown in Fig. 4F, which is the second harmonic of the wave of Fig. 4A, of one-third the amplitude of the waveform of Fig. 4A. The waveform shown in Fig. 4E, may be obtained by combining with the waveform of Fig. 4A, a waveform as shown in Fig. 4G, which is the third harmonic of the wave of Fig. 4A, of 10% of the ampiltude of the waveform of Fig. 4A. Further, a wave such as shown in Fig. 4H having a smaller amplitude and a frequency at least twice the frequency of wave 4A when applied to the vertical deflection plates simultaneously with the modulated sine wave 4A produces the desired charge distribution for good dark field while leaving unaffected the operation of the system under light field conditions. A wave such as shown in Fig. 4J, may be produced by applying pulses as shown in Fig. 41, occurring at the same frequency rate as the sine wave 4A to a transducer such as a tuned circuit, Fig. 8, not shown. Figs. 5, 6 and 7, not shown, illustrate in block form various ways of carrying out the invention in which the additional green wobbulating frequency sources of higher frequency than the fundamental green wobbulating source are applied to the vertical deflection plates simultaneously with the modulated fundamental green wobbulating source or to the bias source which is connected between the cathode and the control electrode of the electron beam device and controls the electron current flow in the electron beam.