Abstract:
A cathode ray tube video display comprises a cathode ray tube (CRT) for displaying a video signal (Vin) coupled thereto. A cathode ray tube electron beam blanker ( 201 ) is coupled to the cathode ray tube (CRT) and is responsive to a blanking trigger signal (GK) for blanking an electron beam (e) within the cathode ray tube (CRT). A blanking controller ( 200 ) is coupled to the cathode ray tube electron beam blanker ( 201 ) and generates the blanking trigger signal (Gk) for controlling electron beam blanking, wherein the electron beam (e) is blanked for different durations in accordance with one of a hot and cold start condition.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Cathode ray tube displays frequently employ circuit arrangements that protect the display phosphor from damage or the presentation of spurious imagery during events such as scanning loss or deflection power supply termination or failure. Typically these protection circuits detect or anticipate the occurrence of the event and upon detection activate circuitry which can, for example, apply a bias to the cathode ray tube grid sufficient to blank or cut off electron beam within the tube. In a tri-color cathode ray tube a metal mask is typically employed to ensure that the individual color phosphors are only illuminated by electrons emitted from the corresponding gun. Consequently this mask intercepts a significant percentage of the electron beam current projected toward the phosphor display surface. Thus, the likelihood of phosphor damage or emission fatigue resulting from an undeflected beam is significantly reduced, however, to eliminate spurious turnoff imagery, displays employing such tubes often use circuitry to bias or kick the CRT grid to a blanking or cutoff potential to prevent the display of these unwanted turnoff artifacts.  
           [0002]    It is known in some cathode ray tube display devices, often selectably operable at a plurality of scanning frequencies, to employ a tube power supply that is electronically separate from the deflection generator. Clearly in such a display, phosphor protection circuitry is required because the supply of power to the tube is not driven, energized or derived from a deflection waveform, hence phosphor damage can result from deflection failure with sustained CRT power.  
           [0003]    In a cathode ray tube projection display, phosphor protection circuitry is most critical because each tube has a single monochrome phosphor and consequently lacks the internal metal mask of the tri-color display. In addition a projection tube is small, for example 7 inches in diameter, when compared with a 32 inch tri-color display tube, thus the ratio of picture power to tube size is significantly greater in a projection tube than in a direct view tube. Hence the beam current density in the projection tube causes significantly greater power to be dissipated by the phosphor. Thus without protection circuitry, any turn on or turn off flash or image, centered on the tube face can result in phosphor damage. The phosphor can suffer localized emission fatigue which can permanently reduce light output and thereby retain an image of the flash. If the transient event is severe the phosphor can be burnt precluding further light emission from that phosphor location.  
         SUMMARY OF THE INVENTION  
         [0004]    In an advantageous arrangement a cathode ray tube video display comprises a cathode ray tube for displaying a video signal coupled thereto. A cathode ray tube electron beam blanker is coupled to the cathode ray tube and is responsive to a blanking trigger signal for blanking an electron beam within the cathode ray tube. A blanking controller is coupled to said cathode ray tube electron beam blanker and generates the blanking trigger signal for controlling electron beam blanking, wherein the electron beam is blanked for different durations in accordance with one of a hot and cold start condition.  
           [0005]    In a further advantageous arrangement a cathode ray tube video display comprises a cathode ray tube for displaying a video signal coupled thereto. A cathode ray tube electron beam blanker is coupled to the cathode ray tube and is responsive to a blanking trigger signal for blanking an electron beam within the cathode ray tube. A blanking controller is coupled to the cathode ray tube electron beam blanker and generates the blanking trigger signal for controlling electron beam blanking, wherein the electron beam is blanked for different durations in accordance with one of a turn on and turn off condition. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 depicts a phosphor protection arrangement with scan and power loss detection.  
         [0007]    [0007]FIG. 2 depicts an inventive phosphor protection arrangement.  
         [0008]    [0008]FIG. 3 shows trigger waveforms generated by the inventive phosphor protection arrangement of FIG. 2.  
         [0009]    [0009]FIG. 4 depicts an exemplary CRT grid bias arrangement.  
     
    
     DETAILED DESCRIPTION  
       [0010]    Protection circuits are known which detect the loss of scanning waveforms or a power supply, such an exemplary tri-color tube circuit arrangement is depicted in FIG. 1. However, the arrangement of FIG. 1 is equally applicable to a CRT projection arrangement where individual grid kick circuits, connected to the control grid of each display tube, are driven from a single power and scan failure arrangement.  
         [0011]    In FIG. 1, an automatic kine bias (AKB) circuit is supplied with a vertical scan signal as part of a vertical scan loss detection circuit. When the vertical scan is lost, the tube bias is slowly changed reducing the beam current until the beam is cutoff. However, this method has a slow response time but does prevent damage to the picture tube. In FIG. 1, 12V power sensing is provided by transistor Q 3  and diode D 1 . The combination of diode D 1  and capacitor C 3  at the emitter of transistor Q 3  prevent the 12V supply from dropping quickly when the 12V supply is lost, for example at turn off or supply failure. After turnoff the 12V supply coupled the base of the transistor Q 3  decays towards ground via resistor R 1  causing transistor Q 3  to turn on thereby activating a grid kick circuit, such as the exemplary arrangement of FIG. 4.  
         [0012]    Although the circuit depicted in FIG. 1 provides detection of both vertical scan failure and 12V power loss, it fails to provide protection against damaging turn on flashes. In addition because the AKB circuit exhibits a long response time when subject to scan failure, the AKB arrangement allows the collapsing scan to be displayed before the CRT is blanked.  
         [0013]    There are significant differences in beam blanking requirements at the time of display power up and power down. Cathode ray tube phosphor protection is required under the following conditions: 
         [0014]    Power down:  
         [0015]    blank the beam responsive to user controlled power down,  
         [0016]    blank the beam upon deflection power supply failure,  
         [0017]    blank the beam upon deflection failure,  
         [0018]    Power up:  
         [0019]    blank the beam at power up cold,  
         [0020]    blank the beam at power up hot. 
         [0021]    At power down two visible artifacts are presented, namely a flash or scan perturbation of short duration but potentially high brightness followed by a slowly decaying glow representative of the last image. Clearly power down beam blanking is required to be fast acting, to prevent a visible flash, and in addition blanking must be sustained for a sufficient period to suppress any image after glow.  
         [0022]    At display power up visible artifacts are presented but these can vary in duration and intensity dependent on the operational condition of the display at turn off, and the elapsed time since last energized. For example, a power off power on cycle time of less than a few minutes can be termed a hot start because the tube heaters have not fully cooled and are thus capable of rapid electron emission, unlike tube heaters quiescent for an hour or more. Thus it can be appreciated that rapid power on detection is required, particularly with a hot start condition. Furthermore, to avoid user irritation, an unnecessarily long beam blanking period must be avoided to prevent any additional delay in image presentation. However, with cold tube heaters, the so called cold start, turn on display artifacts can be delayed in occurrence relative to the hot start condition thus requiring a slightly extended beam blanking period.  
         [0023]    The recognition of the differing requirements for beam blanking at power up and power down, namely quick blanking and unblanking at turn on with quick sustained blanking at turn off, resulted in the advantageous arrangement of FIG. 2. At turn on, beam blanking must obscure any turn on display artifacts but must terminate prior to video signal availability at the kinescope cathode(s). With any power down scenario, beam blanking must turn on quickly and sustain a blanked condition for a considerably longer period than that required at turn on. Stated differently, at turn on the beam is quickly blanked then unblanking to avoid extending any on screen picture absence, however, at power down the beam is rapidly blanked and sustained to preclude any on screen image visibility.  
         [0024]    To prevent the display of the turn on flash, the advantageous arrangement  200  of FIG. 2 is used to activate a grid kick or grid biasing circuit which blanks the CRT beam (e). Although the arrangement of FIG. 2 illustrates a tri-color tube, inventive detector  200  is equally applicable to a CRT projection arrangement where individual grid kick circuits, connected to the control grid of each display tube, are driven from a single power and scan failure arrangement. The arrangement of detector  200  also detects failure of vertical deflection and turn off events, such as power fail and user turn off, all of which cause beam blanking to be activated. In addition vertical scan failure is rapidly detected when compared with the arrangement of circuit of FIG. 1.  
         [0025]    In inventive circuit  200  of FIG. 2, a 5 volt vertical rate pulse, occurring during the vertical blanking interval is coupled to a vertical scan loss and 12V power sensing circuit. The vertical pulse is amplified by transistor Q 1  and results in transistor Q 2  being continuously saturated by the presence of the vertical pulse. The collector of transistor Q 2  is connected via resistor R 5  to the base of transistor Q 3  which is turned off when transistor Q 2  is saturated. When the vertical pulse is lost, transistor Q 2  rapidly turns off and the voltage at the collector of transistor Q 2  which is also across the parallel combination of capacitor C 2  and resistor R 4  starts to decay towards ground. This decaying voltage is coupled to the base of transistor Q 3  which turns on, generating signal Gk at the collector of transistor Q 3 . The grid kick trigger signal Gk is depicted by pulses B and D of exemplary FIG. 3, and is coupled to activate the exemplary grid kick circuit of FIG. 4. Pulses B and D of exemplary FIG. 3 have long decay times which is largely due to the time constant of capacitor C 2  and resistor R 4 .  
         [0026]    If the 12V supply to block  200  is lost, transistors Q 1 , Q 2 , and diode D 1  turn off and the voltage across capacitor C 2  is discharged towards ground by resistor R 4 . The voltage across capacitor C 2  is coupled via resistor R 5  to the base of transistor Q 3  and when the voltage drops to about 10.4 volts causes transistor Q 3  to turn on, generating signal Gk at the collector and activating the grid kick circuit. With the loss of the 12V supply, capacitor C 3  is disconnected from the supply by diode D 1  and provides power to the emitter of transistor Q 3  to sustain conduction.  
         [0027]    Display turn on is depicted by exemplary Gk pulse waveform A of FIG. 3. Capacitor C 2  is connected between the collector of transistor Q 2  and ground and was discharged by resistor R 4  following display turn off. Thus at turn on capacitor C 2  is slowly charged via resistor R 5  and the base of transistor Q 3 , towards the 12 volt supply coupled to the anode of diode D 1 . This charging current sustains conduction in transistor Q 3  for time period τ 1  during which the grid kick circuit is activated. The duration, or rather the termination of Gk pulse A is determined by the conductive state of transistor Q 2 , which following establishment of the vertical pulse input to transistor Q 1 , transistor Q 2  assumes a saturated state. With transistor Q 2  saturated, capacitor C 2  is rapidly charged via the collector of transistor Q 2  to the 12 volt supply less the saturation voltage Vcesat. As the voltage across capacitor C 2  rises it reaches a value nominally two PN junctions below the 12 volt supply (about 10.6 volts) at which point grid kick trigger transistor Q 3  cuts off terminating beam cut off by the grid kick circuit.  
         [0028]    As previously mentioned, at turn on grid kick pulse A is terminated by the appearance of vertical pulse Vp which causes transistor Q 2  to saturate and discharge capacitor C 2 , and terminate pulse A. The desirability of facilitating different durations for start up grid blanking was discussed previously, but in simple terms, a hot start with a tricolor display tube is adequately protected by inventive circuit  200  shown in FIG. 2. However, with a cold start condition in a tricolor display tube, inventive circuit  200  provides phosphor burn protection but may not totally prevent the display of spurious images as the CRT and supplies stabilize. To provide different controllable durations for start up blanking the advantageous delay element depicted in block  250  is added in place of the broken line which supplies +12 volts to the collector of transistor Q 2 . Operation of delay element  250  is as follows. When started under cold start conditions capacitor C 4  is discharged and thus holds transistor Q 4  off and prevents current supply to transistor Q 2 . Capacitor C 4  is charged via resistor R 7  towards the positive supply and at some point the voltage across capacitor C 4  is sufficient to allow transistor Q 4  to conduct and supply current to transistor Q 2  which then operates as described previously. Capacitor C 4  continues to charged towards the +12 volt supply causing transistor Q 4  to saturated and thereby present a low impedance to the current supply to transistor Q 2 . Thus the values selected for capacitor C 4  and resistor R 7  have an effect in determining the duration of turn on pulse A. Diode D 2  provides a discharge path for capacitor C 4  which ensures that start up blanking pulse A has the same duration for both hot and cold starts.  
         [0029]    Under hot start conditions with a tricolor display tube the duration of blanking pulse A is advantageously shortened by the removal or omission of diode D 2  which allows residual charge remaining in capacitor C 4  to provide an earlier turn on for series pass transistor Q 4 . Thus in a warm tricolor display tube the turn on blanking is shortened in proportion to the quiescent time of the display. However in a projection cathode ray tube display the presence of diode D 2  ensures that capacitor C 4  is fully discharged regardless of hot or cold start conditions.  
         [0030]    To provide protection against kinescope arcing capacitor C 1  is connected between the collector and emitter of transistor Q 2 . Thus tube blanking at display turn on is achieved by keeping the base of transistor Q 3  low as the power supplies assume their operating values. In the inventive turn on/turn off trigger generator  200  of FIG. 2, the grid kick circuit is triggered causing the beam to be cut off for different periods of time depending on whether the beam blanking is due to display turn on or a variety of turn off or failure events such as power supply or scan failure.  
         [0031]    [0031]FIG. 3 shows an exemplary trigger voltage Gk generated at the collector of transistor Q 3  and applied to trigger the grid kick circuitry during exemplary on off cycling of the display. It can be seen that trigger pulse A is of significantly shorter duration than pulse B or D which have durations in the order of five or more seconds. Pulse C represents a condition where the display is restarted shortly following a power off event, and shows that pulse C has a duration substantially similar to pulse A.  
         [0032]    An exemplary grid kick or grid bias control shown in FIG. 4. During normal operation transistor  61  of the exemplary grid bias arrangement is biased off by control signal Gk. Under this condition capacitor  65  is charged by a current from resistor  63  and charges to a potential equal to the supply voltage Vs minus the potential at the output (node  70 ) of the potential divider. The values of resistors  71  and  72  in potential divider are selected to provide a specific output voltage Vo at circuit node  70 , since the selection of the potential divider output voltage has a significant influence on the peak brightness of displayed image.  
         [0033]    When triggered by signal Gk, transistor  61  turns on clamping the collector side of capacitor  65  to ground. However, capacitor  65  was charged during the normal operation to a potential equal to the supply voltage Vs (for example 200 volts) minus the potential divider output voltage (25 volts). Accordingly, when the collector side of capacitor  65  is clamped to ground responsive to control signal Gk, the other side of the capacitor assumes a negative potential of about 175 volts. This potential is applied to the grid G of the CRT and provides negative grid cut off thereby protecting the kinescope phosphor from spot burn. Under this condition diode  73  in the potential divider is reverse biased and resistor  72  and Zener diode  75  are both isolated from capacitor  65  thus resistor  71  provides the only discharge path for the capacitor  65 .