GB2057187A - Beam index colour display tubes - Google Patents

Abstract

The phosphor stripes R, G, B extend in the scanning direction, and are scanned along their length in sequence by the beam, which is kept accurately aligned along a stripe, e.g. R, by comparison of the amplitude of the two signals, of different frequency, derived from the ultra-violet emitting index segments 7A and 7B, of uniform but different pitch, located on either side of the stripe. <IMAGE>

Description

SPECIFICATION Colour cathode ray tubes and display systems using such tubes This invention relates to cathode ray tubes for producing multi-colour displays, hereinafter called colour CRTs, and to display systems using such tubes.

For many years colour display systems using shadow-mask colour CRTs have been in use and are satisfactory for general use. However, for use in aircraft, such systems have the disadvantages that the power required to drive them is too great, heat dissipation problems are too great and they become unreliable in such a high vibration environment.

A further type of display system utilising a beam indexing colour CRT is more suitable for use in environments such as aircraft.

In a typical beam index system suitable for producing raster scanned colour television pictures, a single electron beam is scanned horizontally across vertical stripes of different colour visible-light emitting phosphors. The phosphor stripes are arranged in the sequence red, green, blue (or vice versa) across the screen of the CRT, the stripes being about the same width as the CRT spot diameter and being separated by black (non light-emitting) guard bands of haif that width.

Under control of a beam indexing system the electron beam is modulated by the video signal appropriate to the phosphor colour being scanned.

Hence, as the beam is deflected along a line from stripe to stripe, red, green and blue video signals are selected in turn to modulate the beam. The timing of this selection is typically controlled by an indexing system comprising vertical ultra voilet (UV) light emissive phosphor elements or vertical conductor elements in the aluminising of the CRT screen, the indexing elements being arranged in a non-integral relationship with the visible-light emitting phosphor stripe triplets to reduce the error in index signal phase caused by changing the hue represented by the video signal.

Such a beam indexing system suffers from a number of inter-related problems as follows: (a) Indexing Phase Accuracy The phase delays in the system and particularly their stability are of crucial importance. The time taken for the electron beam to cross a phosphor stripe and guard band is about 50 to 100 ns in a practical maximum resolution system with a 25 Hz frame refresh frequency. A change of 10 to 20 ns or more in phase control therefore produces errors in hue. Such stability in a system working over a wide range environment is difficult to achieve.

(b) Screen Utilisation About a third of the screen is lost for viewing purposes in the guard bands separating phospher stripes since the beam must cross each guard band.

To achieve a bright display the energy of the beam may be concentrated in a series of larger amplitude pulses compared with a monochrome CRT and this leads to a higher peak beam current and a consequent increase in spot size and a poorer resolution. A spot dwell facility to hold the beam on each stripe for a greater proportion of the scan time can be used but this means more complexity, power consumption, radiated interference and a reduction in the time spent on the index element, making accurate phase control more difficult.

(c) Video Channel Bandwidth To achieve maximum screen utilisation within the limitations of (b) it is necessary to have a video amplifier with fast rise and fall times. For any particular circuit configuration this will mean higher power consumption than a video amplifier for a comparable monochrome display especially if higher CRT drive is needed as indicated in the (b).

(d) Resolution The resolution is limited in the direction of line scan by the widths of the phosphor stripes and guard bands and in the orthogonal direction by the raster line spacing.

Increasing the frame refresh rate to reduce peripheral vision flicker in a high brightness aircraft cockpit environment, for instance, will exacerbate the above problems proportionally.

It is an object of the present invention to provide a colour CRT and display system which will at least reduce the disadvantages of the above-mentioned prior colour CRTs and display systems.

According to one aspect of the present invention there is provided a colour CRT comprising: a screen including a plurality of interspersed sets of stripes of material, each set of stripes emitting light of a respective colour on impact thereon of electrons; an electron gun adapted to produce an electron beam to scan the screen in a raster the lines of which extend in the direction of the stripes: and sensing means associated with the stripes and responsive to the electron beam to produce an output signal for use in steering the electron beam accurately along the stripes during scanning.

In one particulartube in accordance with the invention the sensing means is located in spaces between the stripes.

In one such arrangement the sensing means comprises a row of radiation emissive segments located in the space between each adjacent pair of stripes. Preferably the segments in each row are at a constant pitch and the pitches of the rows differ so that the pitches of the rows on either side of a stripe identify the set to which that stripe belongs.

The radiation emitted by the segments is suitably ultra-violet light.

According to a second aspect of the invention there is provided a colour display system comprising: a colour CRT according to the present invention; line and field deflection means for scanning the beam produced by the CRT electron gun across the CRT screen in a raster with each raster line nominally in register with a different stripe; beam modulating means for modulating the electron beam with a video signal appropriate to the colour of the light emitted by the stripe currently nominally being scanned; and beam steering means responsive to the output of the CRT sensing means to control the deflection means in such a manner as to reduce deviations of the electron beam from its desired position on the screen during scanning.

The position of the electron beam may be controlled so as to reduce deviations in the position of the beam in the direction of the widths and/or the lengths of the stripes.

Where the CRT sensing means comprises a row of radiation emissive segments spaced along the space between each adjacent pair of stripes with the pitches of the rows differing so thatthe pitches of the rows on either side of a row identify the set to which that stripe belongs, the beam steering circuit means suitably comprises: detector means responsive to radiation emitted by the segments to provide a corresponding electrical signal: filter means for separating from said electrical signal respective components attributable to rows of segments of different pitch; selection means synchronised with the deflection means for selecting the ones of said components corresponding to the rows on either side of the stripe which is nominally currently being scanned; and means utilising said selected components to control the deflection means.

One colour CRT in accordance with the invention and a display system in accordance with the invention using the tube will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a vertical cross-sectional view of a portion of the screen of the CRT; Figure 2 is a rear elevational view of the portion of screen of Figure 1; Figure 3 is a diagram illustrating the operation of the CRT of Figures 1 and 2; and Figure 4 is a block schematic diagram of part of the display system.

Referring to Figure 1,the screen of the CRT comprises red, green and blue light emitting sets of phosphor stripes R, G and B laid down on the inner face of the glass face plate 1 of the CRT, the sets being regularly interspersed so that red, green and blue emitting stripes occur in a repeating sequence across the screen. Each adjacent pair of stripes R, G and B is separated by a non-light emitting guard band 3A, 3B or 3C, having a width about half that of the phosphor stripes. Overlying the phosphor stripes R, G and B and guard bands 3A, 3B and 3C there is a thin film of aluminium 5 which serves to improve image brightness and conduct electrons away from the screen in conventional manner.

On the side of the layer 5 remote from the face plate 1, behind each of the guard bands 3, there is a row of ultra violet (UV) light emissive phosphor segments 7A, 78 or 7C. In each row the segments 7A, 7B or 7C are at a constant pitch and the segments and the gaps 9A, 9B or 9C between them are of similar lengths. The pitch of each row of segments 7 has a first, second or third value according to whether that row lies behind a guard band 5A. 5B or 5C. Thus the colour of a phosphor stripe R, G or B is identifiable from the pitches of the rows of segments 7 on either side of it.

In use in a display system the CRT is provided with an electron beam line and field deflection arrangemenu to scan the electron beam produced by the single CRT electron gun (not shown) across the CRT screen in a rectangular raster, with the raster lines each in register with a different phosphor stripe, the electron beam current being modulated with a video signal appropriate to the stripe currently being scanned. It will be appreciated that in order to achieve a satisfactory display, the electron beam must be accurately scanned along the phosphor stripes. This is achieved by utilising UV light signals produced by the rows of segments 7 in operation.

The form of these signals will now be described with reference to Figure 3.

As the CRT electron beam is scanned along a phosphor stripe, say a green stripe G, green light is emitted from the stripe through the CRTface plate 1.

In addition, with an electron beam of appropriate diameter, UV light is emitted from the rows of segments 7B and 7C on either side of the green stripe G.

Due to the different pitches of these rows of segments 7B and 7C the UV light contains two components whose intensities vary at different frequencies F8 and Fc respectively. Moreover, the ratio of the intensisties of these components depends on the position of the centre of the electron beam relative to the rows of segments 7B and 7C, the intensities being equal when the electron beam is centred on the stripe G.

UV light containing components of frequency FA and F8 are similarly produced by the adjacent rows of segments 7A and 7B when the electron beam is scanned along a red stripe R, and UV light containing components of frequency Fc and FA are similarly produced by the adjacent rows of segments 7C and 7A when the electron beam is scanned along a blue stripe B. Figure 3 shows the intensity variation of the UV components of frequencies FA, FB and Fc and red, green and blue light as the position of the centre of the electron beam is moved in a direction transverse to the lengths of the stripes R, B and G and rows of segments 7.

Referring now to Figure 4, in the display system to be described the UV light produced by the segements is detected through a UV light transmissive window 11 in the CRT envelope by a photomultiplier 13, which thus produces an electrical signal containing components of frequency FA, FB and Fc corresponding to the UV light produced by the rows of segments 7A, 7B and 7C respectively.

The output of the photomultiplier 13 is fed to three tuned circuits 1 so, 1 SB and 15C respectively tuned to the frequencies FA, FB and Fc, the tuned circuits having the same Q factor so that variations in beam scanning speed and hence the frequencies FA, FB and Fc affect the amplitudes of the outputs of the tuned circuits equally.

The outputs of the tuned circuits 15 are rectified and smoothed in respective circuits 17A, 17B and 17C. A switching circuit 19 selects the outputs of the pair of circuits 17 associated with the rows of segments 7 on either side of the phosphor stripe R, G or B nominally being scanned by the CRT electron beam under control of a field scanning generator 21, and associated driver circuit 23. The selected signals are fed to an error amplifier system 25 which provides an output signal which controls the field deflection driver circuit 23 so as to tend to reduce the difference between the amplifier inputs to zero, and thereby laterally centre the CRT electron beam on the phosphor stripe which is being scanned.

The switching circuit 19 is driven by a timing control circuit 27 which is controlled by line and field synchronising pulses derived from a video signal source (not shown) for the display system. The timing control circuit 27 additionally drives a second switching circuit 29 whereby during each line scan the appropriate one of red, green and blue video signals supplied by the video signal source is selected for application to the CRT electron gun cathode via a video amplifier 31 to modulate the CRT electron beam current.

To reduce the time taken to correct beam position at the start of a line the correction signal generated during the leading portion of a line may be stored and used as an initial correction signal for the next line. A similar technique can be used for the start of each field.

It will be appreciated that the intensity of the UV light emitted by the segments 7 is modulated by the video signals. It is therefore preferable that the segment pitches are chosen so that the frequencies FA, FB and Fc differ sufficiently from any frequencies that may be present in the video signals to avoid beating between the video frequencies and the frequencies FA, FB and Fc. However, even if such beating occurs, the effect on beam position control will generally be very small since a small beam position error produces a large imbalance in the selected inputs to the error amplifier system 25.

Moreover, small position errors give rise to very little change in brightness or colour purity of the emitted visible light, as shown in Figure 3. For example, if the beam centre is at a position X (see Figure 3) at the edge of a green stripe G, 65% of the maximum available green light and only small amounts of blue light are produced, and the error amplifier inputs are highly imbalanced, the F8 component being virtually zero and the Fc component about 85% of its maximum possible value.

More typically the beam centre position error due to video signal interaction, or other causes e.g.

segment pitch error, is such as to give an imbalance of the order 2:1 between the error amplifier system inputs, as indicated by position Y in Figure 3. As can be seen, at this position virtually no blue light is produced so colour purity is substantially unaffected. Thus by, and large, the beam steering system is immune to video signal interaction and also to tube manufacturing and electrical circuit tolerances.

In addition to controlling the lateral position of the beam on the phosphor stripes during scanning i.e.

controlling field scan linearity, the output signal of the CRT sensing means may alternatively or addi tionaily be used to control the beam position on the stripes longitudinally so that a line scan linearity as good as the UV segment pitch linearity is obtained.

This may be achieved, for example, by using the signals of frequency FA, FB and Foe to phase lock a synthetic digital video signal source. Alternatively, the line scan generator (not shown) may be controlled so as to maintain the frequencies FA, FB and Fe constant.

It will be appreciated that in a display system in accordance with the invention the switching of the video signal is performed at line scan frequency which is at least two orders slower than the switching speed required in a system of comparable resolution using a conventional beam indexing system. Furthermore, since the electron beam accurately scans the light emitting stripes the system exhibits good screen utilisation. For example with an electron beam of diameter such that the 50% brightness points in the beam at the screen lie on a circle of diameter equal to the width of a phosphor stripe, due to the Gaussian radial energy distribution in the beam, 70% of the energy in the beam excites the stripe and only about 30% of the energy falls on the guard bands, and excites the UV segments.

Moreover, since the line scan is along the stripes there is no limitation by the CRT of resolution in this direction, and the only limitation to resolution imposed by the CRT is in the orthogonal direction due to the phosphor stripe spacing.

Thus the problems of screen utilisation, wide video channel bandwidth, and limited biaxial resolution occurring in known beam indexing display systems are overcome by a colour CRT and display system according to the invention. Moreover, the problem of indexing phase accuracy does not arise in a display system according to the present invention since no phase detection is involved in the electron beam position control system used in a display system according to the present invention.

Claims (11)

1. AcolourCRTcomprising: a screen including a plurality of interspersed sets of stripes of material, each set of stripes emitting light of a respective colour on impact thereon of electrons; an electron gun adapted to produce an electron beam to scan the screen in a raster the lines of which extend in the direction of the stripes; and sensing means associated with the stripes and responsive to the electron beam to produce an output signal for use in steering the electron beam accurately along the stripes during scanning.

2. A CRT according to Claim 1 wherein the sensing means is located in spaces between the stripes.

3. A CRT according to Claim 2 wherein the sensing means comprises a row of radiation emissive segments located in the space between each adjacent pair of stripes.

4. A CRT according to Claim 3 wherein the segments in each row are at a constant pitch and the pitches of the rows differ so that the pitches of the rows on either side of a stripe identify the set to which that stripe belongs.

5. A CRT according to Claim 3 or Claim 4 wherein the radiation emitted by the segments is ultra-violet light.

6. .A CRT substantially as hereinbefore described with reference to Figures 1 and 2.

7. A display system comprising: a colour CRT according to any one of Claims 1 to 6; line and field deflection means for scanning the beam produced by the CRT electron gun across the CRT screen in a raster with each raster line nominally in register with a different stripe; beam modulating means for modulating the electron beam with a video signal appropriate to the colour of the light emitted by the stripe currently nominally being scanned; and beam steering means responsive to the output of the CRT sensing means to control the deflection means in such a manner as to reduce deviations of the electron beam from its desired position on the screen during scanning.

8. A display system according to Claim 7 wherein the position of the electron beam is controlled so as to reduce deviations in the direction of the widths of the stripes.

9. A display system according to Claim 7 or Claim 8 wherein the position of the electron beam is controlled so as to reduce deviations in the direction of the lengths of the stripes.

10. A display system according to Claim 8 includ-.

ing a colour CRT according to Claim 4 wherein the beam steering means comprises: detector means responsive to radiation emitted by the segments to provide a corresponding electrical signal; filter means for separating from said electrical signal respective components attributable to rows of segments of different pitch; selection means synchronised with the deflection means for selecting the ones of said components corresponding to the rows on either side of the stripe which is nominally currently being scanned; and means utilising said selected components to control the deflection means.

11. A display system substantially as hereinbefore described with reference to the accompanying drawings.