Abstract:
Disclosed is a display device having a beam index type color CRT that includes an index signal controller for controlling input and output of index signals, an index memory for temporally saving the index signals in units corresponding to of a screened image of a scene on a screen panel, and a switch for switching the index signals that is under the control of the index signal controller.

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a beam index type color cathode ray tube (CRT), and more particularly, to an improved beam index type color CRT capable of displaying images much more clearly by preventing a deterioration of the contrast of displayed images. 
     (b) Description of the Related Art 
     A shadow mask cathode ray tube (CRT) is generally employed as an image displaying unit for televisions, computer monitors, etc. because of its low cost and relative clear image quality. 
     However, the shadow mask CRT exhibits low beam current efficiencies because a large portion of the beam current is blocked from striking the phosphor and is absorbed by the shadow mask. Such beam current absorbed by the shadow mask forms high current areas on the shadow mask, which causes thermal expansion on the shadow mask, resulting in deterioration of color definition of the CRT. 
     Also, because the shadow mask CRT is susceptible to effects from the earth&#39;s magnetic field, an inner shield for intercepting the earth&#39;s magnetic field must be provided in the CRT. 
     To overcome these shortcomings of the shadow mask type CRT, beam index type color CRTs without a shadow mask and inner shield have been developed. 
     FIG. 1 shows a conventional beam index type color CRT. As shown in FIG. 1, the beam index type color CRT includes a panel assembly  10  consisting of a screen panel  20  (in which phosphor is deposited) and a funnel  30 , an electron gun  11  installed inside the funnel for emitting electron beams, a deflection yoke  12  mounted around a neck of the funnel  30  for deflecting the electron beams from the electron gun  11 , an index beam (an ultra-violet ray) condenser plate  13  mounted on the curved part of the funnel  30 , and photo sensor  14  for detecting and multiplying the index beam. 
     FIG. 2 shows an example of a conventional beam index type color CRT. As shown in FIG. 2, the beam index type color CRT includes a photo sensor  14  which detects the index beam which is generated from the beam index type color CRT upon impingement of the electron beam on index stripes thereof and responsively generates an index signal (an electric signal) depending on an amount of the detected index beam, a band pass filter (BPF)  20  for filtering a predetermined frequency of the index signals, a limiter  21  for limiting a predetermined amplitude of the index signals filtered by the filter  20 , a phase-locked loop (PLL)  22  for synchronizing an index signal to color selecting signals, a start-set controller  23  for setting an index starting signal in accordance with the index signal filtered by the band pass filter  20 , a ring counter  24  for counting each cycle of the filtered index signals, a color signal switch  25  for generating a color signal according to a index signal counted by ring counter  24 , a run-in area controller  26  for controlling a run-in area on the screen panel, an index processing circuit  27  for synchronizing the color signal with the index signal and producing a synchronized signal, and an amplifier  28  for amplifying the synchronized signal and sending the amplified signal to the electron gun. 
     As represented in FIGS. 3 and 4, in the conventional beam index type color CRT, the inner surface of the screen panel  31  has parallel triads of red, green, and blue color phosphor stripes  33  vertically arranged with a black layer  32  between the adjacent color phosphor stripes  33 . Index stripes  35  are provided on a transparent aluminum layer  34  which functions as a beam reflecting layer deposited on the triads of red, green, and blue (RGB) color phosphor stripes  33  and black layer  32 . 
     On the one hand, in order to obtain the index signal as well as color signal, the index stripes must be constantly exposed to the electron beam in the beam index type CRT. 
     That is, the electron beam scans the screen panel even when the generation of color signals are not required for a dark image construction, such as when displaying a night scene. 
     Since the electron beam must constantly scans the screen panel for generating an index signal, even when a color signal is not required for a portion of the display panel, the constantly scanning electron beam activates the red, green, and blue phosphors of the portion such that unintended light emissions of the phosphors lowers the contrast of a beam Index type color CRT. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in an effort to solve the problems of the prior art. 
     It is an object of the present invention to provide an improved beam index type color CRT capable of obtaining a clear image by preventing the contrast of the image from being deteriorated, This is accomplished by the scanning the screen panel with an electron beam that is modulated on the basis of index data obtained from a previous image. 
     To achieve the above object, the inner shield of the present invention comprises an index signal controller for controlling input and output of the index signals, an index memory for temporally saving the index signals in units of a screen panel scene, and a switch for switching the index signals that is controlled by the index signal controller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention: 
     FIG. 1 is a sectional side view of a conventional beam index type color CRT; 
     FIG. 2 is a functional block diagram of a conventional beam index type color CRT display system; 
     FIG. 3 is a cross-sectional view of a screen panel of the CRT in FIG. 1.; 
     FIG. 4 is an enlarged view of detail A of FIG. 3; and 
     FIG. 5 is a functional block diagram of a beam index type color CRT display system according to the preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will be described hereinafter with reference to the accompanying drawings. 
     FIG. 5 is a functional block diagram of a beam index type color CRT display system according to the preferred embodiment of the present invention. 
     As shown in FIG. 5, the beam index type color CRT display system of the present invention comprises a photo sensor  60  which detects an index beam which is generated from the beam index type color CRT upon impingement of an electron beam on index stripes thereof and responsively generates an index signal (an electric signal) depending on an amount of the detected index beam, a band pass filter (BPF)  61  for filtering a predetermined frequency of index signal, a limiter  62  for limiting a predetermined amplitude of the index signal filtered by the filter  61 , a phase-locked loop (PLL)  63  for synchronizing the frequency and phase of the index signal generated the photo sensor  60  to that of color selecting signals, a start setting controller  64  for setting a index starting signal in accordance with the index signal filtered by the band pass filter  61 , a ring counter  65  for counting each cycle of the filtered index signal, a color signal switch  66  for generating a color signal according to the index signal counted by ring counter  65 , a run-in controller  67  for controlling a run-in area on the screen panel, an index signal controller  68  for synchronizing the color signal and the index signal and producing a synchronized signal, an index memory  69  for temporally saving index signals from the index signal controller  68 , a switch  70  which is controlled by the index signal controller  68 , and an amplifier  71  for amplifying the synchronized signal and sending the amplified signal to the electron gun. 
     A beam index type CRT according to the preferred embodiment of the present invention will now be described with reference to the accompanying drawings. 
     An electron beam emitted from the electron gun scans the phosphor screen panel  20  and first impinges on the index stripes  35 , then pass through aluminum layer  34  to activate the color phosphors. When the electron beam impinge upon the index stripes, the index stripes emit index beams which are detected by the photo sensor  60 . The photo sensor  60  generates an index signal corresponding to the detected index beam and sends the index signal to the phase-locked loop  63  via the band pass filter  61  and the limiter  62 . The index signal is filtered in frequency by the band pass filter  61  and limited in amplitude by the limiter  62 . The phase-locked loop  63  synchronizes a phase of the index signal to that of a color gating signal and sends the color gating signal to the ring counter  65 . 
     Also, the start set controller  64  generates an index start signal on the basis of the index signal coming from the band pass filter  61  and sends the index start signal to the phase-locked loop  63  and ring counter  65 . The ring counter  65  counts each cycle of the filtered index signal and when this count reaches a predetermined value, the counter  65  generates an output signal and sends the signal to the RGB switch  66 . 
     The run-in controller  67  generates a run-in area signal on the basis of the index signal from the band pass filter  61  and the run-in area signal to the index signal controller  68 . The index signal controller  68  synchronizes color signals from the RGB switch  66  and the run-in area signal from the run-in controller  67  such that the RGB switch  66  sends a synchronized color signal to the electron gun via the switch  70  and the amplifier  71 . Thus, the electron gun emits an electron beam according to the synchronized color signal from the RGB switch  66 . 
     Also, the index signal controller  68  detects the index signals filtered by the band pass filter  61  and temporally saves all the index signals indexing the whole screen panel into an index memory  69 . The index signals from one entire screened image corresponding to a scene are saved (or for more than one screen at the beginning of the operation of the CRT), and then updated every cycle of index signals that correspond to a scene. That is, when the index signals are saved in the index memory  69 , the index signal controller  68  remembers the positions of the index stripes such that when constructing a following scene on the screen, the electron beam is precisely impinged on the corresponding color phosphors between the index stripes, resulting in enhancing the contrast of an image on the screen. 
     As described above, in the beam index type CRT of the present invention, since the index signals that correspond to a scene are temporally saved and used for constructing a subsequent scene, the electron beam is precisely impinges on the desired color phosphors, resulting in the prevention of screen image contrast from being deteriorated by a constant black level index signal. 
     While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment. On the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.