US2794064A - Color kinescope switching-grid capacitance compensation - Google Patents

Description

May 28, 1957 N. RYNN 2,794,064

COLOR KINESCOPE SWITCHING-GRID CAPACITANCE COMPENSATION FiledDec. 18, 1951 2 Sheets-Sheet l [(TTORNEY COLOR KINESQGPE SWlTCHlNG-GRH) CAPACITANCE COMPENSATION Nathan Rynn, Princeton, N. 3., assignor to Radio Corporation of America, a corporation of Delaware This invention relates to improvements in apparatus for reproducing images in natural color with a cathode ray tube having a single beam.

Such tubes may be operated so that the beam is successively modulated with signals representing the intensities of diiferent selected component colors. Means are provided for directing the beam to a phosphor surface that emits light of a color corresponding to the selected component color represented by the signals applied to modulate the beam at any given instant. Various forms of cathode ray tubes may be employed but in many of them the electrode or electrodes employed to switch the beam from one color phosphor or another have a large amount of inherent distributed capacity. This inherent capacity may exist between the electrodes themselves or may be between the electrodes and ground. Therefore, if the frequency with which the beam is switched from one color phosphor to another is of the order of megacycles, a large amount of power is required to drive the color switching electrode or electrodes.

Accordingly, it is the primary object of this invention to provide an improved means whereby the amount of power required to drive the color switching electrode or electrodes is substantially reduced.

Briefly, this objective may be achieved by causing the color switching voltage to appear across an inductance and by coupling this inductance to the color switching electrode or electrodes in such manner that a substantial portion of the distributed capacity associated with them is placed in parallel with this inductance. The value of the inductance is'so selected that the parallel circuit formed by it and the distributed capacity, as well as any other capacitances and circuit components present, is resonant at the color switching frequency. In this way, the inherent capacitance of the color switching electrode does not act as a load upon the driving source, but is a functional part of the driving source.

The manner in which this objective is obtained will be more clearly understood after a detailed consideration of the drawings in which:

Figure 1 illustrates the manner in which this invention may be employed in a tube wherein the color switching electrode is in the form of a grid.

I Figure 2 illustrates the operation of the particular color tube shown in Figure 1; and

Figure 3 illustrates another form of color tube that may be employed wherein the color switching electrode is in the form of an electron mirror that reflects the beam of electrons hack to a proper phosphor in accordance with the voltage applied to the mirror.

The transmitted signal may represent the color information and the intensity of a scanned scene in a variety of ways. A receiver 11 derives video signals from the transmitted wave that represents the intensity of each of the selected component colors employed. It will be appar cut that this reeciver must be constructed in accordance with the type of signal transmitted. If the transmitted 2,794,064 Patented May 28, 1957 ors employed, then, as will be understood by those skilled in the art, the receiver 11 must be provided with means for successively commutating each of the color signals. If, on the other hand, the transmitted signal is such as to successively represent the intensity of the primary colors employed, then the receiver 11 merely detects the signal in the ordinary Way. In the embodiment shown in Figure 1, it will be assumed that the transmitted signal itself successively represents the intensity of the primary colors as the scene to be televised is being scanned. The transmitted signal also includes a synchronizing component which may be employed, in a manner to be described, to control the rate at which the image reproducing means switches from one primary color to another. If the rate of change of the primary colors is large in comparison with the line scanning frequency,

signal simultaneously represents each of the primary colthe synchronizing component may take the form of a burst of alternating current energy that is placed on the back porch of the line blanking pulse. This burst of color synchronizing frequency may be separated from the signal supplied by the receiver 11 by a source 12 in a manner described in the RCA Bulletins on Color Television and UHF, October 1949 to July 1950. Although the particular manner in which this burst of synchronizing energy is separated does not form a part of this invention and although one way of performing this separation may be found in the above publication, a method of performing the separation will now be described in the interest of clarity.

A monostable multivibrator may be triggered by either the leading or trailing edge of the horizontal sync pulses that are separated out in any well known fashion by the synchronizing circuits of the receiver 11. The components of the monostable multivibrator may be so adjusted that it puts out a pulse during the time the burst is present in the output of the receiver. The output of the receiver including both the synchronizing components normally employed in television, the burst of color synchronizing energy and the video components may be applied to a normally closed gate circuit of any well known type. The pulse supplied by the monostable multivibrator may then be applied to the gate circuit so as to render it capable of passing signals only during the time that the burst of color synchronizing energy is present. The burst may then be used to control the phase of a color switching oscillator in any known way. For example, it may be injected into the tuned circuit of the oscillator.

- The output of the color switching oscillator is then applied to any well known type of phase shifter 13 that is capable of gradually shifting the phase of the oscillations from 0 to approximately 140. After passing through a buffer amplifier 14 and the phase shifter 13, the oscillations are applied to a phase shifter or splitter 15 that supplies outputs separated in phase by to output terminals 16, 17 and 18. It can be seen therefore that the phase of the color synchronizing burst may be rotated through 360 by different outputs of the phase shifter 15 with the movable switch 19 and by adjusting the phase shifter 13.

The movable switch 19 is connected to an amplifier 20 that is in turn coupled to a grid of an amplifier tube 21. A parallel circuit comprised in part of a condenser 22 and an inductance 23 is connected in series with a plate decoupling resistor 24 betweeen the plate 25 of the amplifier tube 21 and a source of B+ potential. A bypass condenser 26 is connected to the, junction between the plate decoupling resistor 24 and the parallel combination of the condenser 22 and the inductance 23 so as to effectively place this junction at ground for potentials having the frequency of thecolor switching frequency. The tube 21 therefore may be considered as a means for producing a continuous voltage wave having the frequency and phase of the burst across the inductance 23. This voltage appears at the plate 25 and is coupled by a condenser 27 to the color switching electrode or electrodes of the cathode ray tube'employed to reproduce the images in color.

The particular color reproducing tube shown in Figure 1 may be described as follows. A series of horizontal strips of phosphors is mounted at one end of the tube. These strips may have various widthsbut as indicated by the letters in the drawing each adjacent strip produces a different primary color. Different sequences of the different color strips may be possible. An electron gun comprised of a cathode 28, a control grid 29and an accelerating anode 3t) serves to direct a beam of electrons toward the phosphor strips. A grid comprised .of a series of parallel horizontalgrid 'wires is mounted between the gun and the phosphor strips. In the particular arrangement shown it will be seen "that each grid wire is parallel to the center of either a blue or red phosphor strip and that the green phosphor strips lie mid-way between adjacent grid wires. 'In between'the grid and the phosphor strips is placed a conducting surface 31. A potentiometer 32 provides a convenientsource for setting the direct current potentials of the grid wires and of the conducting surface ,at the desired values. The more positive end of the potentiometer is connected to the conducting surface 31. The grid wires centered on the red phosphor strips are electrically connected together and this set of grid wires is connected to a movable contact 33 of the potentiometer. A condenser 34 serves to connect this set of grid wires to ground at frequencies of the order of the color switching frequency. It will be noted that another'potentiometer 35 is connected in parallel with the portion of the potentiometer 32 to which the movable contact 33 is applied. A movable contact 36 of the potentiometer 35 is connected to the set of grid wires that are centered on the blue phosphor strips. It will therefore be noted that after the electrons pass through the grid wires they are accelerated because the conducting surface 31 is more positive than the grid wires.

The color switching frequency supplied by the amplifier 21 is coupled to the set of grid wires-opposite the blue phosphors and accordingly the amplifier 21 and its associated circuits may be termed a means for introducing a voltage variation between the two sets of grid wires at the color switching frequency. When thisvoltage passes through its axis, the electrons pass straight through the grid and impinge upon the green strips of phosphor. I

When, however, the voltage is such as to make the set of grid wires opposite the blue phosphor strips positive with respect to the set of grid wires opposite the red phosphor strips, the electrons are directed towards the blue phosphor strips and away from the green and red phosphor strips. In a similar fashion, when the color switching voltage supplied by the amplifier 21 makes the set of grid wires opposite the blue phosphor strips negative with respect to'the set of grid wires opposite the red phosphor strips, the electrons are directed towards the red phosphor strips and away from the blue and green phosphor strips. The reason for this is that the electrostatic fields set up by the grid wires and the conducting surface 31 form cylindrical lenses betweenadjacent grid wires and the change in the relative potentials of adjacent grid wires causes the axis of the lens to tilt one way or the other.

In'the drawing of Figure l, the number of phosphor strips and consequently the number of grid wires has been greatly reduced in order that the structure may be clearly delineated, but it will be understood that in a practical embodiment of this tube many more grid wires and phosphor strips will be employed. Therefore, in the practical case, the grid wires are closer together and the distributed capacity between one set of grid wires and the other is greatly increased. For purposes of simplicity all of the distributed capacity between the grid wires and between the grid wires and ground is indicated by the dotted capacitor 37. These capacities are effec tively in parallel with the condenser 22 and the inductance 23 that are in the plate circuit of the amplifier 21. Ordinarily these distributed capacitances indicated by the dotted condenser 37 would load down the amplifier 21 at the color switching frequency. However, if in accordance with this invention, the value of the inductance 23 and the condenser 22 are properly selected, the parallel circuit formed by them and the distributed capacities may be tuned to parallel resonance at the color switching frequency. It will be understood by those skilled in the art that the condenser 22 might be dispensed with depending upon the size of distributed capacitance 37. The important factor is that the distributed capacitances are placed in parallel with the R. F. load circuit of the amplifier 21 and the R. F. load circuit is so adjusted as to produce parallel resonance at the color switching frequency.

If the color switching frequency is of sinusoidal form, the sections of the electron beams passing between the grid wires trace sinusoidal paths on the phosphor strips as indicated in Figure 2. Owing to the fact that the sections of the beam passing the grid wires are of finite size it is necessary that the beam be keyed whenever these sectional beams are wholly within the confines of a single phosphor strip. This can be accomplished as follows. The output of the phase shifter 15 was applied to the amplifier 20 and to the color switching grid as previously described. This same output is also applied via phase shifter 38 and a buffer amplifier 39 to a frequency tripler 40. The third harmonic of the color switching frequency 3Fs is then amplified by amplifier 41 and coupled viathe transformer 42 and the series resonant circuit 43 that is tuned to the 3Fs harmonic frequency to a junction 44. The output of 'the receiver 11 is also applied to this junction and the junction 44 is coupled to the beam intensity control grid 29 via an inductance 45. The distributed capacity of the control grid 29 to ground and other elements of the tube is illustrated by dotted condenser 46. As explained in my copending application having the Serial Number 262,272 and filed onDecernber 18, 1951, now Patent No. 2,745,037, granted-May 8, .1956, the inductance 45 is of such value as to produce series resonance at the third harmonic frequency with the distributed capacitances 46. The reasons for this will not be further described here as it is the subject of the last mentioned application. The grid 29 is biased to cut off by connection to the movable arm .of the potentiometer 47 and the amplitude of the keying frequency 3Fs that is supplied by the amplifier 41 is just sufilcient to raise the grid 29 to cut-off potential during a small portion of its positive peaks in the absence of any video signals. The video signals supplied by the receiver 11 are of such polarity as to increase the intensity of the beam in-accordance with'their amplitude.

The overall operation of the apparatus shown in Figure 1 will'be discussed in connection with Figure 2. As previously stated, the sections of the electron beam passing between adjacent grid wires strike the green phosphor strips that arecentered between each adjacent pair of grid wires when the switching voltage applied to-one set of the grid wires passes through its axis. Furthermore, when the one set of grid wires'is positive with respect to the other the beam isattracted to the phosphor behind the positive grid wires. The timing of the keying signals applied to the grid '29 in the manner just described is controlled by adjusting the phase shifter 38. If this time is'not properly controlled, it can be seen that the sequence with which the different primary colors are produced would reverse itself in each successive group of primary colors. This use of a sine wave for color switching purposes is described in greater detail in U. S. patent application Serial No. 134,453, filed December 22, 1949, in the name of Paul K. Weimer. If the phase shifter 38 is properly adjusted, the same sequence of reproduction of the primary colors may be produced across the whole line of the raster. In the particular arrangement shown, a positive crest of the keying wave 3Fs occur at 60, 180 and 300 of the color switching wave. The sinusoidal waves shown in Figure 2 are of the frequency corresponding to the color switching wave F5 and with the above relationship of the keying waves 3Fs, the beam is brought to the threshold of the cut-off when it reaches any one of the vertical dotted lines 50, 51, 52 and 53. It will 'be realized of course that only a small horizontal portion of the phosphor strips is shown. When the beam is turned on at a position indicated by the dotted line 50, all the sectional beams passing through the grid structure strike blue phosphor strips and when the beam is turned on at a time indicated by the dotted line 51 all of the sectional beams passing through the grid structure strike the green phosphor strips. In a similar way, when the beam reaches the positions 52 and 53, the red and blue strips are respectively struck by the sectional beams passing through the grid.

Figure 3 illustrates another type of single beam tube for reproducing images in color wherein a color switching Wave may be employed to direct the beam to the proper color emitting phosphors in synchronism vw'th the sequential representation of these colors by the incoming signal. This particular tube is described in detail in a U. S. patent application bearing Serial No. 134,453, filed December 22, 1949, in the name of Paul K. Weimer, now Patent No. 2,650,264, granted August 25, 1953. For purposes of convenience however a brief description of the operation of this tube is herein presented. The beam of electrons is directed by a gun 54 towards an apertured plate 55 that is at with respect to the central position of the beam. The apertures in the plate are parallel slits 56 and they may be curved or straight depending on the particular design of the tube. On the side of the plate 55, remote from the electron gun 54 and in between each of the parallel slits 56 are parallel strips 57, 58 and 59 of phosphor that respectively reproduce red, green and blue primary colored light when struck by the beam of electrons. The electrons that pass through the slits 56 are repelled back to one or another of the phosphor strips depending on the potential applied to an electron mirror 60. There is a substantial distributed capacitance between the electron mirror 60 and other elements in the tube as represented by dotted capacitance 61. This capacitance is shown as existing between the electron mirror 60 and ground, but it will be realized by those skilled in the art that the capacitances that exist between the electron mirror 60 and the plate 55 may eventually be referred to ground as far as A. C. potentials are concerned. The tube shown in Figure 3 may be connected into the circuit of Figure 1 as follows: the electron mirror 60 be connected to the coupling capacitor 27 and the grid 62 of the tube in Figure 3 and would be connected to the inductance 45 of Figure 1. The values of the condenser 22 and the inductance 23 from the RF. load circuit of the amplifier 21 are adjusted so that the parallel circuit formed by them. land the distributed capacitance 61 resonates at the color switching frequency. Whereas two different types of single beam color reproducing tubes have been shown and described, it will be realized by those skilled in the art that this same problem may exist in other types of color tubes employing a single beam of electrons and that the problem can be solved in accordance with the principles of this invention.

What is claimed is:

1. Apparatus for reproducing images in color in response to a signal that successively represents in repeated sequence the individual intensities of the primary colors employed comprising in combination a cathode ray tube, a plurality of groups of strips of phosphors, each strip within a group being adapted to emit light of a different component color when struck by a beam of electrons, a conducting surface mounted adjacent to said groups of phosphor strips, a cathode, an electron gun adapted to project a beam of electrons from said cathode toward said phosphor strips, a grid structure mounted between said gun and said phosphor strips, said grid being comprised of wires that are substantially parallel to said phosphor strips, means for maintaining said grid wires at a direct current potential that is positive with respect to the potential of the cathode, means for maintaining said conducting surface at a direct current potential that is positive with respect to said grid wires so that electrostatic cylindrical lenses are formed by said grid wires and said surface, a source of voltage waves having a frequency at which the signal changes from one primary color representation to another, an inductance coupled to said source in such manner that the voltage waves supplied by said source are applied across it, means for coupling said inductance to said grid wires in such manner that the voltage appearing across said inductance are applied between one set of grid wires and another, said coupling means being such that the inherent capacity between said sets of grid wires is in parallel with said inductance, the value of said inductance being such that the parallel combination of it and the inherent capacity is resonant to the frequency of the voltage waves supplied by said source, a grid for controlling the intensity of said beam of electrons, means for applying said signals :to said grid, means for causing said beam to scan a raster, means for biasing said grid so as to cut oif said beam of electrons and means for overcoming said bias a plurality of times during each cycle of the voltage waves applied between said sets of grid wires.

2. Apparatus for reproducing images in color comprising in combination a cathode ray tube having a screen comprised of a plurality of phopsphors that emit light of difierent colors when struck by a beam of electrons, an electron gun adapted to direct an electron beam toward said screen, means for causing said electron beam to scan a raster on said screen, a color switching grid operating at a predetermined frequency comprised of at least two sets of conducting members, there being high inherent capacity between at least one set of grid members and another causing appreciable power loss at said predetermined frequency, means for energizing said grid at said frequency, said means including an inductance connected between at least one set of said grid members and another, the value of said inductance being such that it and the inherent capacity between said two sets of members form a circuit resonant at said predetermined frequency, said means for energizing said grid further comprising a circuit for impressing a switching voltage of said predetermined frequency across said inductance.

3. Apparatus for producing images in color comprising in combination a kinescope, a screen in said kinescope comprised of phosphors that emit light of a different selected component color when struck by a beam of electrons, means for directing a beam of electrons toward said screen, a color switching electrode adapted to direct said beam of electrons to phosphors that emit light of selected component colors in accordance with the voltage on the electrode, said electrode having an inherent capacity, a grid for controlling the intensity of said beam of electrons, a receiver for detecting signals including video components and synchronizing components, the synchronizing components including a color synchro nizing component, means for applying said video components to said grid, means for causing said beam to scan a raster on said screen, and means for controlling said scanning means in accordance with some of said synchronizing components, a source of voltage waves of color switching frequency, means for synchronizing said source withtthe color synchronizing components of the output ofsaid receiver, an amplifier having atleast a plate, a grid-and a cathode, rneans for coupling the Waves of color switching frequency to said grid, a source of fixed potential, an inductance connected between the plate of said tube and the positive side of said source of fixed potential, means for coupling said cathode to the negative side of said source of fixed potential, a condenser connected between said plate and the color switching electrode of said kinescope, said inductance having such value as to form a circuit including the inherent capacitance of-sai-d electrode that is resonant at said color switching frequency. a

4. Apparatus for reproducing images in color comprising in combination a cathode ray tube having a screen comprised of a plurality of phosphors that emit light of different colors When struck by a beam of electrons, an electron gun adapted to direct an electron beam toward said screen, means for causing said electron beam to scan a raster on said screen, 'a color switching grid operating at a predetermined frequency comprised of at least two sets of conducting members, there being high inherent capacity between at least one set of grid members and another causing appreciable power loss at said predeterinined frequency, means for energizing said grid at sa'id'frequency, said means comprising an inductance connected between at "least one set of said grid members and References Cited in the file of this patent UNITED STATES PATENTS 7 2,461,515 Bronwell Feb. 15, 1949: 2,518,200 Szikl'ai et al. Aug. .8, 1950 2,529,485 Chew Nov. 14, 1950 2,577,368 Schultz a Dec. 14, 1951 2,705,257 Lawrence Mar. 29, 1955 OTHER, REFERENCES Radio Engineers Handbook by F. E. Terman, pages 469-470, published 1943.

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