Patent GB-1108464-A (GB)
Title: Improvements in colour projection systems
IPC: H04N5/74
Filed: 1965-04-23
Abstract: 1,108,464. Light modulation. GENERAL ELECTRIC CO. 23 April, 1965 [8 May, 1964], No. 17299/65. 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, substantially as described in Specification 1,108,463, the fixed high frequency carrier waves which are amplitude modulated by the red, blue and green video signals are maintained during the occurrence of each line of scan in a predetermined fixed phase relationship in respect to one another so that spurious image effects such as streaks, herring bone patterns, fine bar structures and fine cross hatch patterns are avoided. A finely sectioned or checkered pattern may occur on the background of the projected picture due to the beat frequency between the red and blue carrier frequencies and this may be avoided by reversing the phase of the red frequency source every other line, Fig. 7, not shown. This line phase reversal effectively doubles the frequency of the difference pattern and so eliminates it from view. To avoid streaks extending in the direction of horizontal scan and having heights embracing several lines of scan, which are caused by differences in the phase of the carrier waves of either the red or blue frequency grating in one line with respect to an adjacent line, the horizontal fly-back pulses are applied to two keyed oscillator circuits to initiate in phase the outputs from the red and blue grating frequency sources, Fig. 4, not shown. To avoid clusters of streaks appearing at angles with respect to the horizontal and vertical dimensions of the picture in the form of herring bone patterns, which are caused by a lack of phase coherence between the wave from the green wobbulating frequency source and the horizontal sweep wave, the horizontal fly-back pulses are applied to a keyer to initiate in phase with the trailing edge of the fly-back pulses the green wobbulating frequency source, Fig. 5, not shown. Striations which are lines of alternating intensity and/or hue which reoccur at intervals along the horizontal or vertical axis of the projected picture, and are caused by a shifting of one of the primary colour component carrier frequencies with respect to either or both of the other two carrier frequencies to produce a difference frequency greater than the horizontal scan frequency, are avoided by using a fundamental frequency source and a series of frequency multipliers multiplying the frequency of the fundamental frequency source to obtain the red, blue and green carrier frequencies, Fig. 6, not shown. Further, to avoid a harmonic beat pattern or striations the relationship between the blue, red and green carrier frequencies is set at 3 to 4 to 12, however the relationships of 3 to 4 to 9, 3 to 4 to 8, 3 to 4 to 6, or 2 to 3 to 6 may be employed but the likelihood of spurious patterns due to beat of the blue or red carrier harmonics with the green carrier would be greater.