Technical Field

The present invention relates to an improved system for generating indexing signals for a beam indexing type of color cathode ray tube (CRT) .

Background Art In view of the growing development of high definition television (HDTV) , there is a need to develop a color CRT that can generate high resolution and high brilliance images for direct viewing and for single tube projection HDTV systems.

The current method used to generate color images in a color CRT for consumer TV employs three electron guns, a shadow mask, and a screen consisting of arrays of red, blue, and green phosphor dots arranged such that each electron gun will direct electrons to dots of a specific color. In this way, separate primary color images are generated which, when combined, produce the color picture. The obvious disadvan- tage of this scheme is in the use of three electron guns and in the shadow mask which typically absorbs eighty percent of the electrons from the guns, severely limiting the maximum possible brilliance of the image. Attempting to increase the brilliance of the image by raising the intensity of the electron beams results in the generation of x-rays and in localized heating of the shadow mask metal causing expansion and a loss of accurate targeting of electrons on the phos¬ phor dots. Basically, the shadow mask type of CRT cannot be

refined to meet the resolution and brilliance requirements of HDTV.

U.S. Patent No. 3,081,414 to Goodman discloses a color CRT that eliminates the three gun/shadow mask principle and which can be used for direct viewing or in a single tube projection system. However, the Goodman system is not suitable for HDTV because the cathode ray tubes using the Goodman principle cannot be made in larger sizes, i.e., larger than an eight by eight inch (20.3 by 20.3 cm) screen size, and individual primary color intensities of the image cannot be adjusted by external electronic controls to pro¬ duce an accurately balanced color picture or a true black and white picture.

A major problem in previous beam indexing system, such as that disclosed in the Goodman patent, is that the modula¬ tion of the electron beam by the video signals caused phase errors in the indexing signals and color fringing of the picture displayed on the screen, in effect reducing the resolution and brilliance of the picture displayed by the CRT. In addition, in those Goodman type systems which use ultraviolet or x-ray emissive strips on the screen or on the back of a metalized layer, the radiation pulses must be detected by an electron-multiplier or other photosensitive device capable of detecting radiation on all parts of the screen equally, without regard to the size of the screen.

On larger tube sizes, this is virtually impossible to accom¬ plish.

Disclosure of the Invention What is desired, therefore, is to provide a system using the Goodman principles but wherein the CRT can be made in larger sizes, the primary color intensities can be accu¬ rately adjusted, and the picture displayed on the CRT has increased resolution and brilliance.

The present invention provides an apparatus for gener- ating indexing signals for synchronizing the video infor a

tion emitted by a single gun color cathode ray tube (CRT) , the CRT having a screen consisting of alternating red, blue and green vertical phosphor strips in groups of three, with strips of conductive or photovoltaic material overlaying each phosphor strip, or placed between adjacent phosphor strips. The same color strips are connected in parallel in three arrays, and connected to three individual trigger buses. The conductive lines are connected to a positive bias voltage such that when the electron beam scans the screen in a conventional manner, trigger pulses are generat¬ ed in the conductive strips and are fed to a trigger pulse amplifier and then to a multiplexer and used as control signals. The multiplexer in turn gates the color video signals from the workstation or television set such that the corresponding video signal for that color is modulating the beam as the electron beam strikes a phosphor strip.

The present invention thus provides an improved single gun color CRT indexing system with the CRT screen size being expandable from that available in the prior art, a picture of increased resolution and brilliance due to the enhanced synchronism between the trigger pulses and the modulating chroma video signals, and a system whereby the individual primary color intensities of the screen image are adjustable to produce an accurately balanced color picture or a true black and white picture.

Brief Description of the Drawings For a better understanding of the invention as well as other objects and features thereof, reference is made to the following description which is to be read in conjunction with the accompanying drawings wherein:

FIG. 1 is a partial block diagram showing both the optical and electrical components comprising the system of the present invention;

FIG. 2 is an end view of the conductive and phosphor strips forming the color/indexing signal generating portion

of the system shown in FIG. 1;

FIG. 3 illustrates a technique to compensate for trigger pulse delays;

FIG. 4 is a simplified block diagram of a serial system for generating three indexing pulses from a single conduc¬ tive or photovoltaic strip for a group of three colors; FIG. 5 is a simplified block diagram of a parallel system for generating three indexing trigger pulses from a single conductive or photovoltaic strip for a group of three colors;

FIG. 6 is a simplified block diagram of a hybrid paral¬ lel serial system for generating three indexing trigger pulses from a single conductive or photovoltaic strip for a group of three colors; and FIG. 7 is a simplified block diagram for reducing capacitive effects by subgrouping conductive strips and using isolation amplifiers.

Modes for Carrying Out the Invention FIG. 1 is a simplified block diagram of the novel triggering pulse system of the present invention. A color cathode ray tube (CRT) 10 comprises a single electron gun (not shown) , a CRT grid or cathode 12 and a screen 14 (shown schematically in phantom outline in FIGS. 5 and 6), the screen 14 comprising a series of vertical conductive strips 16, 18...32 overlying corresponding phosphor strips 34,

36...50 (conductive strips 28, 30 and 32 are shown partially broken away to illustrate underlying phosphor strip 46, 48 and 50, respectively). Phosphor strips 34, 40 and 46 com¬ prise a phosphor material which emits red light when scanned by the electron beam generated by the CRT gun, strips 36, 42 and 48 comprising a phosphor material which emits blue light when scanned by the electron beam generated by the CRT gun and strips 38, 44 and 50 comprising a phosphor material which emits green light when scanned by the electron beam generated by the CRT gun. It should be realized that only a

small portion of the screen is illustrated, there being approximately 1,500 lines and 8,000 strips, including black matrix strips, in a typical screen. The scanning is accom¬ plished in the conventional manner, i.e., the electron beam scans a horizontal line across the face of screen 14 and is then positioned to scan another line and so on until the entire screen face is scanned. The phosphors comprising strips 34, 36...50 are conventional and will not be described herein since they do not form a part of the present invention. The conductive strips 16, 18...32 preferably comprise aluminum, although other metals trans¬ parent to an electron beam can be utilized, and are placed on the surfaces of the adjacent phosphors by conventional vacuum deposition techniques. It should be noted that photovoltaic material, the photovoltaic material generating an electrical signal in response to an incident electron beam, could be utilized in lieu of the metallized strips. The conductive strips are connected in parallel forming three arrays, the first array comprising strips 16, 22 and 28 having each strip of the array connected to bus 52, the second array comprising strips 18, 24 and 30, having each strip of the array connected to bus 54, and the third array comprising strips 20, 26 and 32 having each strip of the array connected to bus 56. Buses 52, 54 and 56 couple the sequence of pulses generated by each conductive strip when scanned by the electron beam to trigger amplifiers 60. Each bus is con¬ nected to a source of positive bias voltage E, typically 300 volts, to enhance the generation of the pulse signals in the case of a weak screen. Trigger amplifiers 60 limit the amplitude of the generated pulses on the corresponding buses, pulse width modulators 62 control the pulse width, and pulse delay circuits 64 adjust the pulse delay, to form the indexing control pulses for electron multiplexer 66. The multiplexer 66 in turn gates the chroma (video) signals

such that as the electron beam contacts a phosphor strip, the corresponding video signal is coupled to CRT grid 12 to modulate the beam in a synchronized manner.

FIG. 2 is an end view of CRT screen 14 illustrating the conductive strips overlying a portion of the adjacent phos¬ phor strip. Control of the width of the trigger pulse by external adjustments enables the intensity of the colors to be controlled. In particular, if the width of the trigger pulse for the blue chroma (color) signal, for example, is controlled such that the modulated chroma signal is on for the entire width of the blue phosphor strip, the intensity of the resultant blue color will be approximately twice as great than would be the case if the beam was on for a time period corresponding to a one-half of the width of the blue phosphor strip.

The present invention thus limits the indexing pulses to a constant amplitude, and establishes a minimum level of intensity for the beam. The independent channels for con¬ trolling the three video signals allows the relative inten- sities of the colors to be varied to produce the proper mix for a true ^x white' screen. The adjustment would typically be accomplished following the time the CRT is installed.

FIG. 3 schematically illustrates a technique for com¬ pensating for the varying trigger delays caused by the signal transit times between the conductive strips variously placed in the screen and the trigger take-off point. A typical workstation CRT screen has 1500 horizontal lines forming the height of the screen; in order to prevent the picture formed on the screen from being slanted, each conductive strip is slanted in the direction of the beam scan. The slant, if not otherwise compensated for, is caused by the inherent propagation transit times of the electrons stimulated at the top conductive strip so that, assuming a vertical orientation, the electron stimulation from the first horizontal pixel would arrive at the trigger

take off point prior to the arrival of the electron stimula¬ tion from the same first horizontal pixel at the bottom of the conductive strip. The slanting of the conductive strips (the slant angle being dependent on slant size) as shown in the screen depiction illustrated in FIG. 3, provides the appropriate compensation by advancing the arrival of the trigger pulses, the advancement (provided by the slant angle) increasing as the electron beam scans vertically downward. The left edge of the screen is represented by reference numeral 69, the trigger take off point (approxi¬ mate mid-point of the screen and point where bus is connect¬ ed to follow-on circuitry) by reference numeral 71, and the right edge of the screen by reference numeral 73 (bus 52 is one of the three buses used) . Although the slant feature has been described with reference to the conductive strip/phosphor arrangement shown in FIGS. 1 and 2, it can also be used with the conductive strip/phosphor arrangement described hereafter with reference to FIGS. 4-6. FIG. 4 shows another embodiment of the present inven¬ tion wherein three indexing trigger pulses are generated from one conductive (or photovoltaic) strip for each group or array of three colors. Specifically, conductive strips 70, 72 and 74 separate arrays 76, 78 and 80, respectively, each array comprising three strips of red (82) , blue (84) and green (86) phosphors. In the simplified serial arrange¬ ment illustrated, as the electron beam from the CRT gun strikes conductive strip 70, a pulse is generated on bus 90, the pulse being amplified and shaped by trigger amplifier 92. The output from amplifier 92, corresponding to the emission from red phosphor strip 82, is coupled via lead 94 to multiplexer 66. Thereafter, and in a sequence delayed by the delays inherent in an electronic circuit, pulse genera¬ tor 96 is energized by amplifier 92 and generates a signal on lead 98 which is coupled to multiplexer 66. Finally, the

delayed output from pulse generator 96 energizes pulse generator 100 which generates a signal on lead 102 which is coupled to multiplexer 66. This identical sequence is repeated for each horizontal line in screen 14 as the elec- tron beam strikes conductive strip 72 and the phosphors in array 78, and then conductive strip 74 and the phosphors in array 80.

FIG. 5 is a more detailed block diagram of a serial circuit arrangement for generating three indexing trigger pulses from a single conductive or photovoltaic strip per color phosphor using the conductive strip/phosphor arrange¬ ment shown in FIG. 4. The output from the first biased conductive or photovoltaic strip is coupled to pulse ampli¬ fier 110, the output of which is coupled to pulse shaper and impedance transformer 112. The output of transformer 112 is applied, in parallel, to pulse delay adjustment devices 114, 116 and 118, the outputs of which are applied to pulse width adjustment devices 120, 122 and 124, respectively. The outputs of devices 120, 122 and 124, corresponding to red, blue and green triggers, respectively, are coupled to multiplexer 66, the output of which then is applied to CRT grid 12 to synchronize the modulating red chroma, blue chroma and green chroma video signals with the position of the corresponding color phosphor strips on CRT screen 14. In this embodiment, the proper sequence is realized by adjusting delays 114, 116 and 118 such that the single pulse activates the multiplexer 66 in the correct sequence, the pulse width adjustment devices 120, 122 and 124 adjusting the chroma intensity by varying the "on" time of the chroma video signal.

FIG. 6 shows another embodiment for generating three indexing trigger pulses from a single conductive or photo¬ voltaic strip for a single array. In particular, the output from screen 14 is applied to pulse amplifier 130, the output of which is coupled to pulse shaper and impedance transform-

er 132 via lead 131. The output of pulse shaper 132 is connected via lead 133 to pulse delay adjuster 134, the output of which is connected to pulse width adjuster 136 via lead 137 and pulse delay adjuster 138 via lead 139. The output of pulse width adjuster 136, corresponding to the red chroma trigger, is connected to multiplexer 66 via lead 146. The output of pulse delay adjuster 138 is connected to pulse width adjuster 142 and pulse delay adjuster 143 via leads 144 and 145, respectively, the output of pulse width adjust- er 142, corresponding to the blue chroma trigger, being connected to multiplexer 66 via lead 149. The output of pulse delay adjuster 143 is connected via lead 148 to pulse width adjuster 147, the output thereof being connected to multiplexer 66 via lead 151. As noted with respect to FIG. 5, pulse width adjustment devices 136, 142 and 147 provide a simplified technique for accurately controlling the inten¬ sity of the chroma video signals and thus providing a true white color picture.

Referring now to FIG. 7, a circuit arrangement for minimizing the effects of capacitive coupling between con¬ ductive (metal) strips is there illustrated. Electrical pulses created in a biased conductive strip when contacted by the electron beam may be capacitively coupled into adja¬ cent conductive strips on either side. These false pulses might interfere with the operation of the multiplexer. As described hereinafter, isolation amplifiers are utilized to reject the lower energy pulses created by capacitive cou¬ pling, and pass the much higher energy pulses resulting from the electron beam contacting a conductive strip. The cir- cuit arrangement, although described with reference to the conductive strip/phosphor configuration shown in FIGS. 1 and 2, can also be adapted for use with the array configuration shown in FIGS. 4-6.

As illustrated, conductive strips 200 and 202 are connected to isolation amplifier 204 and conductive strips

206 and 208 are connected to isolation amplifier 210. The outputs of isolation amplifiers 204 and 210 are connected to triggering amplifier 212, amplifier 212 corresponding to amplifier 60 shown in FIG. 1 and generating the red trigger signal. In a similar manner, conductive strips 214 and 216 are connected to isolation amplifier 218 and conductive strips 220 and 222 are connected to isolation amplifier 224, the outputs of isolation amplifiers 218 and 224 being con¬ nected to amplifier 226 corresponding to amplifier 60 of FIG. 1 and generating the blue trigger signal. Finally, conductive strips 230 and 232 are connected to isolation amplifier 234 and conductive strips 236 and 238 are connect¬ ed to isolation amplifier 240, the outputs of isolation amplifiers 234 and 240 being connected to amplifier 242, with the latter corresponding to amplifier 60 of FIG. 1 and generating the green trigger signal. In essence, the isola¬ tion amplifiers isolate the high frequency capacitive loads of the conductive strips from the other electrical circuit components. The grouping of conductive strips as illustrat- ed also serves to reduce the capacitance between conductive strips.

Industrial Applicability The present invention thus provides a significant improvement over prior art line indexing CRT devices by providing a system for generating electrical line indexing pulses which enables large screen CRT's to be utilized, and allows the primary color intensities of the image to be simply and inexpensively adjusted by external controls to produce an accurately balanced color picture. The system also provides a technique for compensating for transit propagation delays by slanting the conductive or photovoltaic strips, the system also providing a technique for compensating for capacitive effects associated with the conductive strips used to generate the index triggering pulse signals.