Patent GB-1068592-A (GB)
Title: Improvements in or relating to colour television display devices
IPC: H04N9/22
Filed: 1964-05-11
Abstract: 1,068,592. Colour television. SCOPE Inc. May 11, 1964 [May 23, 1963], No. 19454/64. Heading H4F. In a system for converting a conventional monochrome television display into a colour display a colour mask 9, Fig. 1, parts of which vibrate, is placed in front of the screen of the monochrome cathode-ray tube (C.R.T.) 4 to which colour information is supplied field-sequentially, the colour field sequencer 2 (Fig. 2, not shown), having converted the composite viedo signal arriving via lead 3 into a sequence of fields with selected colour information. The colour mask 9 comprises a fixed rigid striped colour plate 24, Figs. 3, 4 and 5, having clear, green, red and blue stripes arranged in consistent succession and perpendicular to raster lines 30 of the C.R.T. screen 25, a flexible aperture plate 23 having alternate opaque and transparent stripes and a rigid transparent plate 57, shown cut-away in Fig. 5, which is spaced from frame 45 which supports the colour plate 24 by washers thus leaving enough space for free sliding motion of aperture plate 23. The distance between transparent stripes 31, 32 &c. is equal to the distance between the similarly coloured stripes of colour plate 24, e.g. stripes 27 and 34. Aperture plate 23 is tilted slightly with respect to the colour plate 24 so that the top of a transparent stripe such as 31 is horizontally displaced by slightly less than one colour strip from the bottom of that transparent stripe and thus a horizontal movement to the right in Fig. 4 of apertured plate 23 causes the colours corresponding to various stripes to move from top to bottom of the viewing field. The waveform of Fig. 7 shows how the aperture plate position varies with time measured in field intervals, i.e. three zones, corresponding to the three colours green, red and blue, the retrace of the plate being shown at 20. By using a motion waveform such as that of Fig. 7, it ensures that the colour fields created by the mask 9 progress in synchronization with the vertical sweep from top to bottom of the raster of the C.R.T., thus reducing any variation in hue due to parts of the raster having different dwells on the primary colours. The aperture plate 23 is moved by means of a solenoid 40, Fig. 5, having a movable armature 41 attached by way of a number of pivots to the plate 23. When the solenoid is de-energized the armature 41 is held against a felt limiting member 42 by means of a spring 43. The position of the aperture plate is adjusted, with solenoid 40 de-energized, so that the tilt angle of the transparent stripes 31, 32 is correct with respect to the colour stripes of plate 24 and so that the transparent stripes 31, 32 coincide with the colour following the motion retrace, ji.e. colour green with reference to Fig. 7. The solenoid 40 pulls the armature 41 against the spring 43 during energization, i.e. during the slope 37, 38, 39, Fig. 7, and the spring 43 provides the retrace 20 the solenoid being de-activated. The Specification includes a detailed description (Figs. 6, 8, 9 and 10, not shown), of how a driving saw-tooth waveform is produced from a switching waveform derived from the colour field sequencer 2, Fig. 1, so that it is of such a shape as to cause the motion of the aperture plate to correspond to the motion waveform of Fig. 7. Improvement in wave shape and stability of mask motion is obtained through the use of a feedback circuit, Fig. 1. The feedback circuit may comprise a light source 82 (Fig. 8, not shown), which casts a shadow of the edge of the aperture plate 23 on to a light sensitive cell which provides an output voltage that follows the action of the aperture plate. This output voltage is then used to cause differences between the mask motion and the driving waveform to cancel so as to cause the mask to follow the driving waveforms. Alternatively, a direct current feedback or automatic gain control may be used (Fig. 10, not shown). A photo-electric technique for sensing the mask motion is preferred because it does not mechanically load the mask but electromagnetic or static pick-ups may be used. Aperture plate 23 may be made photographically using a plastic sheet and although it is preferable to move the aperture plate because it is light, colour plate 24 may be moved instead. The colour plate 24 may be made by silk-screening transparent coloured ink on to a glass surface or by colour photographic methods. When black and white presentation is desired the aperture plate 23 is so positioned that the transparent stripes 31, 32 lie over clear stripes 26, 33 and the motion of the mask is terminated the solenoid 40 being energized so that the solenoid armature 41 is pulled down against adjusting screw 97, Fig. 5. The screw 97 is adjusted until the transparent stripes of the aperture plate 23 lie over the clear stripes in the colour plate, the tilt angle of the aperture plate being so slight as not to impair the clarity of the black and white display because of the higher light transmission of the clear stripes 26, 33 compared with the adjacent colour stripes.