US3522368A - Color display system - Google Patents

Description

July 28, 1970 R. E. SMITH 3,522,368

COLOR DISPLAY SYSTEM Filed April 10, 1967 FIG. 1

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OUECE 75 pl/OSPHOE Seed-EN United States Patent 3,522,368 COLOR DISPLAY SYSTEM Robert E. Smith, Richardson, Tex., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Apr. 10, 1967, Ser. No. 629,433 Int. Cl. H04n 9/12, 9/22 US. Cl. 1785.4 8 Claims ABSTRACT OF THE DISCLOSURE This invention relates to color display systems and more particularly to apparatus for applying different electron accelerating voltages to a phosphor viewing screen.

Various prior art color display systems have been proposed in which the color of light produced by the phosphor screen of a kinescope is varied by varying the energy or velocity of the electrons impinging upon the screen. One proposed method of rendering prosphors differently responsive to electrons of different energies is to coat or overlay certain of the different color phosphors with a barrier layer so that only electrons having energies of at least a predetermined level will excite the phosphor. To obtain a multicolor display by this method, it is then necessary to apply different accelerating voltages to the phosphor screen as the screen is scanned by an electron beam. The different voltages must be applied in proper sequence and at a relatively rapid rate, e.g., at a line-sequential rate.

Among the several objects of the present invention may be noted the provision of apparatus for applying different accelerating voltages to a phosphor screen in a color display system; the provision of such apparatus which can change voltage levels at a relatively rapid rate, e.g., at a line-sequential rate; the provision of such apparatus which is highly reliable; and the provision of such apparatus which is relatively simple and inexpensive. Other objects and features will be in part apparent and in part pointed out hereinafter.

Briefly, apparatus according to the present invention is useful in applying different electron accelerating voltages to a phosphor viewing screen of the type which emits light of different colors when struck by electrons of different energies. The apparatus includes a source providing direct current at a relatively high voltage between a positive and a negative terminal and a vacuum tube having a high voltage blocking capability. The tube has an anode, a cathode and a grid, conduction between the anode and the cathode being variable as a function of the grid-cathode voltage. The anode of the tube is connected to the positive terminal of the source through a voltage dropping impedance and is connected also to the viewing screen. The cathode of the tube is connected to the negative terminal of the source through the collector-emitter circuit of a transistor. The apparatus includes also means for varying conduction between the collector and emitter of the transistor thereby to vary also conduction between the anode and cathode of the tube. Accordingly, different electron accelerating voltages are applied to the screen by the 3,522,368 Patented July 28, 1970 Fee anode of the tube for accelerating electrons to at least two different energies for producing light of respective colors.

In the accompanying drawings in which one of various possible embodiments of the invention is illustrated,

FIG. 1 is a diagrammatic illustration of a color display system including apparatus according to this invention for applying different electron accelerating voltages to the phosphor viewing screen of a kinescope; and

FIG. 2 is a schematic circuit diagram illustrating in greater detail the apparatus for providing different electron accelerating voltages.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

f Referring now to FIG. 1, there is indicated at 11 a color kinescope of a type with which the present invention is useful. Kinescope 11 includes a conventional glass envelope 13 having a screen portion 15 and a neck portion 17. Coated on the inner surface of the screen portion 15 is a phosphor screen or layer 19 which includes phosphors which emit light of different colors when struck by electrons of different energies. Phosphor screen 19 may, for example, be constituted by a mixture of three different kinds of phosphor particles a first of which emits red light when energized by elec trons having energies above a first, relatively low predetermined level; a second of which emits cyan light when energized by electrons having energies above a second or intermediate level; and a third of which emits blue light when energized by electrons having energies above a third, relatively high predetermined level. As

,'screen 19 is subjected to impinging electrons of increasing energies, the three kinds of phosphors are cumulatively energized so that the screen emits red light when struck by electrons at the relatively low level; warm, substantially achromatic light when struck by electrons at the intermediate energy level; and cool, substantially achromatic light when struck by electrons at the relatively high energy level. Such unsaturated color image displays are described in greater detail in application 450,705, 'filed Apr. 26, 1965, now abandoned in favor of application 6l4,362, now Pat. No. 3,337,229. Images presented by such displays appear to have a relatively wide range of hues subjectively having a greater saturation than that which is actually present in the colorimetric sense. Methods of preparing phosphors useful in making such a screen are disclosed in application Ser. No. 459,- 582, filed May 28, 1965, now Pat. No. 3,408,223.

Over phosphor screen 19 is deposited a film 21 of aluminum which is conductive and yet is also thin enough to be substantially electron permeable. By means of film 21, suitable electron accelerating voltages may be applied to the phosphor screen 19.

Within the neck portion 17 of envelope 13 there is mounted a conventional electron gun 23 for emitting a beam of electrons directed toward phosphor screen 19. Gun 23 includes a cathode 25 for providing the electrons which are formed by the gun into a beam and a grid 27 for modulating the beam current or number of electrons emitted by the gun. For the purpose of the example described herein, it is assumed that this color display system is operated in a line-sequential mode. For this purpose, a three-color, line-sequential video signal is applied to grid 27 by means described hereinafter for varying the electron beam current. The video signal thus controls the instantaneous brightness of the light produced by the beam on phosphor screen 19. It should be understood, however, that other modes of presentation, such as field-sequential, may also be employed by appropriately varying the different voltage switching rates described hereinafter and by applying a correspondingly switched video signal to grid 27.

Electrons emitted from gun 23 are accelerated by the voltage applied to screen 19 and pass through the magnetic influence of a deflection yoke 29. Yoke 29 is energized in conventional manner to deflect the beam of electrons over the screen 19 in a scanning raster comprising a series of generally parallel horizontal lines.

Video signals representing color records of the red, green and blue components of the composite full color image to be displayed are applied, through respective leads 35, 37 and 39, to gate circuits indicated at 41 which are operative to pass selected ones of the signals. Gate circuits 41 are operated under the control of a sync signal, provided through a lead indicated at 45, to pass selected ones of the video signals in sequence to an output lead 47. The sync signal provided at lead 45 may, for example, be constituted by the conventional NTSC horizontal synchronization pulse so that the composite video signal provided at lead 47 changes from one color to the next on successive lines, thereby providing a line-sequential video signal in conventional manner.

The gate circuits 41 also operate to supply to an output lead 51 a series of DC. voltage levels in predetermined sequence constituting a staircase waveform as indicated graphically adjacent to the lead 51. The sequence in which the different voltage levels are applied to lead 51 is synchronized with the application of the different video signals to lead 47 so that the same voltage level is always present at the lead 51 when a given one of the video signals is being applied to the lead 47.

The staircase voltage waveform provided to lead 51 is amplified and superimposed upon a high voltage DC. bias by the amplifier and bias source circuits indicated generally at 55. The resultant high voltage stepped waveform is then applied to screen 19 through a lead 57 thereby to provide a sequence of different accelerating voltages for electrons emitted by the gun 23. Electrons emitted by gun 23 during the different time intervals corresponding to the three different voltage levels of the staircase waveform are thus accelerated to different energy levels before reaching phosphor screen 19. The respective voltages and energy levels are chosen in relation to the thresholds or level-sensitive characteristics of the phosphors which make up screen 19 so that the lower energy electrons excite only the red phosphor; the intermediate energy electrons excite both the red and cyan phosphors thereby causing warm, substantially achromatic light to be emitted; and the high energy electrons cause all of the phosphors, including the blue, to be energized thereby causing cool, substantially achromatic light to be emitted.

As the stepped voltage waveform applied to screen 19 is synchronized in relation to the sequence in which video signals representing different colors are applied to gun 23, the beam current is thus modulated to reproduce the various image components represented by the different video signals in respective colors. As noted previously, the changes between successive colors are assumed to take place at a line-sequential rate.

The circuitry which provides the different electronic accelerating voltages to screen 19 is shown in greater detail in FIG. 2. DC. at a suitably high voltage for electron accelerating purposes is provided by a bias source as indicated at 58 having a positive supply terminal 59 and a negative supply terminal 60 which is grounded. For modulating this voltage to provide different voltage accelerating levels there is provided a vacuum tube 63 having substantial high voltage blocking capability. Tube 63 comprises an anode 65, a cathode 67 and a grid 69; As is understood by those skilled in the art, conduction between the anode and cathode of tube 63 varies as a function of the grid-cathode voltage. The anode 65 is connected to the positive terminal of the high voltage bias source 58 through a voltage dropping impedance which,

in the example illustrated, comprises a resistor R1. An inductor may also be used. Anode is also connected to lead 57 which, as described previously, is connected to the screen 19. The electron accelerating voltage provided at screen 19 will thus vary as a function of the voltage drop produced across resistor R1 by current drawn by conduction through the tube 63.

As is understood by those skilled in the art, a vacuum tube such as that indicated at 63 exhibits a capacitive coupling between the anodes and the grid thereof so that voltage changes at the anode are coupled to the grid. This capacitive coupling substantially reduces the switching time which may be provided by such a tube particularly if conduction in the tube is controlled from the grid.

To obtain rapid switching between voltage levels in the apparatus illustrated in spite of this capacitive load ing and the capacitive load of screen 19, conduction through tube 63 is controlled from its cathode by an NPN transistor as indicated at Q1. The tube grid 69 is connected to ground through a resistor R2 which maintains the grid nominally at DC. ground potential and a capacitor C1 which shunts all AC. or switching signals to maintain the grid at A.C. ground also.

The tube cathode 67 is connected to ground, and thus also to the negative terminal of source 58, through the collector-emitter circuit of transistor Q1, the collector of the transistor being connected to the cathode and the emitter to ground. The relatively low level staircase voltage provided at lead 51 by the gate circuits 41 is coupled, through a capacitor C2, to the base electrode of the transistor Q1 for controlling conduction in its collector emitter circuit. The base terminal is normally biased to ground potential by a resistor R3 connecting the base to ground.

In operation, the low voltage signal coupled to the base terminal causes conduction through the collector-emitter circuit of transistor Q1 to vary in a stepwise fashion and conduction through the transistor is coupled to the cathode of tube 63 causing conduction between the anode and the cathode of the tube to vary similarly despite the interelectrode capacitance of the tube and the capacitance of the load. The electron accelerating voltage applied to the screen 19 is thus rapidly switched between the different levels thereby to produce light of different colors at the screen in proper synchronization with the video signals applied to gun 23 so that the color record signals are displayed in respective distinct colors over the entire width of the screen 19.

In view of the above it will be seen that the several objects of the invention are achieved and other advantageous results attained.

What is claimed is:

1. In a color display system having a phosphor viewing screen which emits light of difierent colors when struck by electrons of different energies; apparatus for applying different electron accelerating voltages to said screen comprising:

a source having a positive terminal and a negative terminal for providing direct current at a relatively high voltage;

a vacuum tube having a substantial high voltage blocking capability, said tube having an anode, a cathode and a grid, conduction between said anode and cath ode being variable as a function of the grid-cathode voltage;

a voltage dropping impedance connecting said anode to said positive terminal;

means for connecting said anode to said screen;

means connecting said grid to said positive terminal,

a transistor having a collector, an emitter and a base for controlling conduction between said collector and said emitter;

means for connecting said cathode to said negative terminal through the collector-emitter circuit of said transistor; and

means for varying the voltage at said base between a plurality of levels to vary the conduction between said collector and said emitter and thereby also vary conduction between said anode and said cathode whereby different electron accelerating voltages are applied to said screen for accelerating electrons to at least two different energies.

2. Apparatus as set forth in claim 1 wherein said means for connecting said grid to said positive terminal includes a resistor for establishing the DC bias on said grid and a capacitor connected across said resistor for shunting A.C. signals away from said grid.

3. Apparatus as set forth in claim 1 wherein said voltage dropping impedance is a resistor.

4. Apparatus as set forth in claim 1 wherein said transistor is of the NPN conductivity type and has its collector connected to the cathode of said tube.

5. Apparatus as set forth in claim 1 including means for applying a signal having a stepped waveform to the base of said transistor thereby to vary conduction between its collector and emitter.

6. A line-sequential color display system comprising:

a phosphor viewing screen which emits light of differ-- ent colors when struck by electrons of difierent energies;

electron gun means for generating a beam of electrons directed at said screen;

deflection means for scanning said beam over said screen in a succession of generally parallel line sweeps;

means for modulating the beam current on successive line sweeps in accordance with respective color records;

a source having a positive terminal and a negative terminal for providing direct current at a relatively high voltage;

a vacuum tube having a substantial high voltage blocking capability, said tube having an anode, a cathode and a grid, conduction between said anode and cathode being variable as a function of the grid-cathode voltage;

a voltage dropping impedance connecting said anode to said positive terminal;

means for connecting said anode to said screen;

an NPN transistor having a collector, an emitter and a base for controlling conduction between said collector and said emitter;

means connecting said grid to said positive terminal;

means for connecting said collector to said cathode and for connecting said emitter to said negative terminal; and

means for varying conduction between said collector and said emitter on successive line sweeps in synchronism with the modulation of said beam current thereby to vary also conduction between the anode and cathode of said tube whereby a stepped voltage waveform is applied at high voltage to said screen for accelerating electrons to different energies corresponding to said different colors on successive line sweeps thereby to display each of said records in light of a respective color.

7. A line-sequential color display system as set forth in claim 6 wherein said screen includes a first phosphor which emits substantially red light when energized by electrons having energies above a first, relatively low predetermined level, a second phosphor which emits substantially cyan light when energized by electrons having energies above a second level which is higher than said first level, and a third phosphor which emits substantially blue light when energized by electrons having energies above a third level which is higher than said second level.

8. A line-sequential color display system as set forth in claim 7 including means for applying a signal having a three-level stepped waveform to the base of said transistor thereby to vary conduction in the collector-emitter circuit of said transistor and the anode-cathode circuit of said tube whereby three dilferent accelerating voltages are applied in sequence to said screen.

References Cited UNITED STATES PATENTS 3,312,781 4/1967 Land. 3,330,990 7/ 1967 Guillette. 3,396,233 8/1968 Kagan. 3,413,410 11/1968 Farmer.

RICHARD MURRARY, Primary Examiner

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