Display system

The life time of a CRT is extended by decreasing the temperature of the cathode by 7-30% from the normal operating temperature and by applying a constant voltage signal to a cathode of the CRT which is shifted from a first voltage level for displaying the lowest luminance of the image toward a second voltage level for displaying the highest luminance of the image. The shift is of the order of 5-25%, so that a constant anode current in the range of 2-14 uA flows in the CRT.

BACKGROUND OF THE INVENTION 
FIELD OF THE INVENTION 
This invention relates to a display system including a data processing 
device, such as a personal computer and a display device including a 
cathode ray tube (CRT). 
BACKGROUND ART 
Multimedia technology has recently been developed. In this technology, 
dynamically moving images of high contrast and high luminance must be 
displayed. The CRT is the best display device for displaying such images 
since it has rapid response time and the capability of displaying high 
contrast and high luminance images. However, a strong electron beam or a 
high anode current is required to display such images. The life time of 
the CRT is shortened by use of a strong electron beam. The life time is 
defined as the time of use until the luminance of the CRT is degraded to a 
predetermined level. One parameter which determines the life time is 
cathode emission, i.e. the capability of emitting an electron beam. The 
electrons are emitted from the oxide cathode by a reduction or deoxidation 
process occurring at the oxide cathode. The higher the degree or rate of 
the electron emission from the cathode or the degree of cathode emission 
is, the shorter is the life time of the CRT. To display a high luminance 
image or high load image requiring that substantially all of the dots are 
displayed at high luminance, a large amount of electron emission is 
required, so that a prior CRT of the type used for a television set or 
personal computer, having a life time of about 12000 hours, is not 
suitable for a CRT display device used with multimedia technology. 
To reduce the cathode emission, it has been proposed to switch a displayed 
image from a normal image to an image requiring a low load at the cathode 
when a personal computer detects a no key-inputting operation during a 
predetermined time period. To this end, the personal computer has the 
capability of generating both the video signals of the normal image and 
video signals of the low load image, and supplies the video signals of the 
low toad image to CRT display device when it detects no key-inputting 
operation for the predetermined time period. However, with this approach, 
a normal operating voltage is applied to the heater element of the CRT, so 
that the cathode of the CRT is sufficiently activated for chemical 
reduction to occur, whereby cathode emission proceeds. 
To stop the cathode emission, it has been proposed to turn the heater 
element off. When the voltage applied to the heater element is removed, 
the heater becomes cold. This approach has the following problems. It 
takes about 10-20 seconds to restore the heaters element to the normal 
operating condition. In addition a drift of convergence and a variation of 
color balance occurs during a transient period before the electrodes in 
the CRT come to a thermal equilibrium condition. The drift of convergence 
continues for about 20 minutes and the variation of color balance 
continues for about one minute. By turning the heater element off while 
operating voltages are applied to the remaining electrodes, such as the 
cathode, first grid, and second grid, the cathode enters a semiconducting 
state, so that the cathode in the semiconducting state is broken down 
under the existing electrostatic field. Undesired ions, originating from 
organic material in the CRT, are accelerated to the cathode, and the 
surface of the cathode reacts with the ions; whereby the cathode emission 
capability is degraded. 
SUMMARY OF THE INVENTION 
It is a principal object of the invention to provide a display system 
wherein lifetime of the display is not unduly decreased when an image of 
high luminance is displayed. 
A display system according to the present invention comprises: a data 
processing means for generating video including means for detecting that 
no input operations have occurred and for generating a control signal 
representing a no input operation state when no input operations have 
occurred for a predetermined time; and a display means including means 
connected to the data processing means for responding to absence of the 
control signal, to amplify the video signals to generate amplified video 
signals which vary between a first voltage level representing the lowest 
luminance of an image and a second voltage level representing a highest 
luminance of the image, and for causing the control signal to generate a 
constant voltage signal at a voltage level shifted from the first voltage 
level toward the second voltage level, a cathode ray tube including a 
cathode for receiving the amplified video signals or the constant voltage 
signal, and voltage supply means for responding to absence of the control 
signal to apply a normal operating voltage to the heater element of the 
cathode ray tube, and for responding to the presence of the control signal 
to apply a voltage which is lower than the normal operating voltage. 
The means connected to the data processing means comprises: an amplifier 
means for receiving the video signals and capable of switching gain 
between a normal operating gain to generate the amplified video signals 
which varies between the first voltage level and the second voltage level 
and zero gain to generate the constant voltage signal; a gain control 
means for responding to the absence of the control signal to switch said 
gain of the amplifier means to the normal operating gain, and for 
responding to the control signal to switch the gain of the amplifier means 
to zero gain; and means for responding to the control signal to shift the 
constant voltage signal from the first voltage level toward the second 
voltage level. The constant voltage signal is shifted from the first 
voltage by 5-25% and the voltage applied to the heater element is 
decreased to reduce the temperature of the cathode from its normal 
operating temperature by 7-30%. 
A display system according to the present invention comprises a data 
processing means for generating blue, green and red video signals and for 
detecting a no key-input operation to generate a control signal 
representing the no key-input operation; first means connected to the data 
processing means to receive the blue video signals for responding to 
absence of the control signal to amplify the blue video signals to 
generate amplified blue video signals which vary between a first voltage 
level representing the lowest luminance of an image and a second voltage 
level representing the highest luminance of the image, and for responding 
to the control signal to generate a constant voltage blue signal at a 
voltage level shifted from the first voltage level toward the second 
voltage level; second means connected to the data processing means to 
receive the green video signals for responding to the absence of the 
control signal to amplify the green video signals to generate amplified 
green video signals which vary between the first voltage level and the 
second voltage level, and for responding to the control signal to generate 
a constant voltage green signal at the first voltage level; third means 
connected to the data processing means to receive the red video signals 
for responding to absence of the control signal to amplify the red video 
signals to generate amplified video signals which vary between the first 
voltage level and the second voltage level, and for responding to the 
control signal to generate a constant voltage red signal at the first 
voltage level, a cathode ray tube including three cathodes for receiving 
the amplified blue, green, and red video signals, respectively, or the 
constant voltage blue, green, and red signals, respectively; and voltage 
supply means for responding to absence of the control signals to apply a 
normal operating voltage to a heater element of the cathode ray tube, and 
for responding to the control signal to apply a voltage which is lower 
than the normal operating voltage to the heater element. 
The first means comprises an amplifier means for receiving the blue video 
signals and capable of switching gain between a normal operating gain to 
generate the amplified video signals which vary between the first voltage 
level and the second voltage level and zero gain to generate the constant 
voltage signal; a gain control means for responding to absence of the 
control signal to switch the gain of the amplifier means to the normal 
operating gain, and for responding to presence of the control signal to 
switch the gain of the amplifier means to zero gain; and means for 
responding to the control signal to shift the constant voltage signal from 
the first voltage level toward the second voltage level. The constant 
voltage blue signal is shifted from the first voltage level by 5-25%, and 
the voltage applied to the heater element is decreased to reduce the 
temperature of the cathode from its normal operating temperature by 7-30%.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention is based upon the discovery made by the inventors that the 
life time of a CRT is remarkably extended by decreasing the temperature of 
the cathode by 7-30% from the normal operating temperature for displaying 
the image, and by applying a constant voltage signal to a cathode of the 
CRT which is shifted from a first voltage level for displaying the lowest 
luminance of the image toward a second voltage level of the video signals 
for displaying the highest luminance of the image by 5-25%, so that a 
constant anode current in the range of 2-14 uA flows in the CRT. 
The 5-25% represents the percentage difference of the amplitude between the 
first and second voltage levels. In this condition, a very light blanket 
color is displayed on the display surface of the CRT. This mode of the CRT 
is called an image display suppress mode, while the mode in which the 
video signals vary between the first and second voltage levels (which are 
applied to the cathode of the CRT) is called a normal display mode. 
The decrease of the temperature of the cathode lowers the cathode emission, 
but it causes the undesired reaction of the ions with the cathode surface. 
To solve this problem a constant anode current in the range of 2-14 uA is 
caused to flow when the temperature of the cathode is decreased to prevent 
the undesired reaction at the cathode surface. It has been experimentally 
found that the life time of the CRT is extended to 40000-44000 hours in 
the above temperature and current ranges. Particularly, in the case when 
the decrease of the cathode temperature is 10% and the value of the 
constant anode current is 9-10 uA, the life time of the CRT is extended to 
44000 hours. 
Referring to FIG. 1, a display device 2 is connected to a data processing 
device, such as a personal computer 1. The display device 2 includes a 
video amplifying circuit 3, a power supply circuit 4, a horizontal 
deflection circuit 5, a vertical deflection circuit 6, a timer circuit 7, 
a biasing circuit 8 and a CRT 9. 
The power supply circuit 4, the horizontal deflection circuit 5, the 
vertical deflection circuit 6 and the CRT 9 are circuits well known in the 
art. 
The power supply circuit 4 receives AC voltage and supplies various 
voltages to the circuits of the display device 2. The horizontal 
deflection circuit 5 receives horizontal synchronization pulses from the 
personal computer 1 through a line 25, and controls horizontal deflection 
operation of the CRT 9. The vertical deflection circuit 6 receives 
vertical synchronization pulses from the personal computer 1 through a 
line 26, and controls vertical deflection operation of the CRT 9. The CRT 
9 includes a heater element 27, a cathode 22 receiving blue (B) video 
signal, a cathode 23 receiving green (G) video signal, a cathode 24 
receiving red (R) video signal, a first grid 28 suppressing the electrons 
and a second grid 29 accelerating the electrons, a focus grid 30 for 
operating to focus the electron beam, a horizontal deflection coil 31, a 
vertical deflection coil 32 and a metal back 33. 
The video amplifying circuit 3, amplifies the blue, green and red video 
signals received from the personal computer 1 via a line 21, and supplies 
the three color video signals to the cathodes 22, 23 and 24, respectively, 
in a normal display mode in which characters or images from the personal 
computer 1 are displayed on the display surface of the CRT 9. The video 
amplifying circuit 3 performs the operation of the present invention 
described hereinafter in response to the control signal indicating the no 
key-input operation during a predetermined time period supplied from the 
personal computer 1. 
The detailed circuit configuration of the video amplifying circuit 3 for 
performing the operation in accordance with the present invention is shown 
in FIG. 2. A block 37 receives the blue video signal from the personal 
computer 1 at an input terminal 21B. Block 37 includes a pre-amplifier 40, 
an adder circuit 44, a main amplifier 41, a differential amplifier 42, an 
open collector circuit 43 and resistors 46, 47, 48, 49 and 50. An output, 
i.e. the blue video signal, of the main amplifier 41 is supplied to the 
cathode 22 of the CRT 9. The blocks 38 and 39 have the same circuit 
configuration as that of the block 37, except that the blocks 38 and 39 do 
not include the open collector circuit 43 and the end 49A of the resistor 
49 is connected to the reference level. The block 38 amplifies the green 
video signal and supplies it to the cathode 23 of the CRT 9, and the block 
39 amplifies the red video signal and supplies it to the cathode 24 of the 
CRT 9. 
The waveforms 61-68 at the nodes (A), (B), (C) and (D) in FIG. 2 are shown 
in FIG. 3A, FIG. 3B, FIG. 3C and FIG. 3D respectively. The waveforms 61, 
62, 63 and 64 are the waveforms in the normal display mode for displaying 
the characters or image on the display surface of the CRT 9. The video 
amplifying circuit 3 also includes a gain control circuit 69 and an input 
terminal 73. 
Gain control voltages on lines 34, 35 and 36 are controlled by the gain 
control circuit 69 in response to the control signal supplied from the 
personal computer 1, indicating that there is no input operation during a 
predetermined time period such as, for example 30 minutes. Usually an 
input operation means that a key on a keyboard associated with personal 
computer 1 has been pressed to provide input to the personal computer. 
However, more generally, the input may take many forms, such as that from 
a touch screen, a light pen, a handwriting pad or any other input device. 
The control signal is supplied to the input terminal 73 through a line 20 
in FIG. 1. That is, the personal computer has a timer which measures the 
time period and generates a signal of low level when it detects continuous 
input operations (by an operator) and generates a control signal of high 
level when it detects no input operations during the predetermined time 
period. When the control signal of low level is supplied to the input 
terminal 73, a transistor 71 is turned off, so that a collector to a 
transistor 71 is maintained at a positive voltage. 
In this state, a contrast control voltage divider 74, a voltage divider 75 
for adjusting the red color, a voltage divider 76 for adjusting the green 
color and a voltage divider 77 for adjusting the blue color are 
adjustable, whereby the gain control voltages on the lines 34, 35 and 36 
are supplied to the pre-amplifier 40 in the blocks 37, 38 and 39, 
respectively. The pre-amplifiers 40 in the block 37, 38 and 39 amplify the 
video signal 61 received at the input terminals 21B, 21G and 21R to 
generate the wave form 62 shown in FIG. 3B. 
The circuit in block 37 (including the differential amplifier 42 and the 
open collector circuit 43) generates a voltage V1 (FIG. 3C) on a line 80 
in response to the control signal of low level applied to the input 
terminal 73 during a clamp signal at a terminal 89 generated by the 
horizontal deflection circuit 5 during the horizontal retrace period. The 
circuit of block 37 also generates a voltage V2 which is higher than the 
voltage v1 on line 80 in response to the control signal of high level. 
More particularly, when the control signal of low level indicating that 
the operator is continuing key-input operations is applied to the input 
terminal 73, the end 49A of the resistor 49 is connected to the reference 
voltage or ground potential by the open collector circuit 43. 
The circuit configuration of the open collector circuit 43 is shown in FIG. 
4. When a control signal of low level is applied to the input terminal 73, 
a transistor 90 is turned off, and a transistor 91 is turned on, so that 
the line 49A is connected to the reference voltage. When a control signal 
of high level is applied to the input terminal 73, transistor 90 is turned 
on, and the transistor 91 is turned off; that is a base current does not 
flow through the base and the emitter of the transistor 91, so that the 
voltage on line 49A or the collector of transistor 91 is free to float. 
Since the line 49A is connected to the reference voltage in the normal 
display mode, the voltage level on a line 79 (FIG. 2) is determined by 
resistors 48, 49 and 50 and a positive voltage +V. The differential 
amplifier 42 generates the voltage V1 shown in FIG. 3 in response to the 
difference between the voltage on the lines 78 and 79. 
The biasing circuit 8 (FIG. 1) receives a voltage from the power supply 4 
and the horizontal deflection circuit 5 to generate various voltages for 
the heater element 27, and the first, second and focus grids 28, 29 and 
30. 
Since the preferred embodiment of the invention applies one of two voltage 
levels to the heater element 27 in response to the level of the control 
signal, a portion of the biasing circuit 8 for generating the heater 
voltage is shown in FIG. 5. The circuit includes a voltage regulator 81, 
diodes 82 and 83 and a transistor 84. The voltage regulator 81 generates a 
higher voltage than a voltage at a connecting node 88 by a predetermined 
voltage, such as 5V. 
The voltage on the node 88 is switched between two levels, such as 0V and 
1.2V in response to the level of the control signal applied to an input 
terminal 85. A voltage drop across the diodes 82 and 83 is 1.2V. When the 
level of the control signal is low, the transistor 84 is turned off, so 
that the voltage on node 88 is at 1.2V, whereby the voltage regulator 81 
generates a heater voltage of 6.2V on the output terminal 87, as shown in 
FIG. 6. The heater voltage of 6.2V fully activates the heater element 27 
of the CRT9, shown in FIG. 1, so that the temperature of the cathodes 22, 
23, and 24 is maintained at the normal operating temperature, such as 1100 
degrees K, and the CRT9 is operated in the normal display mode. 
When the level of the control signal is high, the transistor 84 is turned 
on, so that the voltage on node 88 is at 0V, whereby the voltage regulator 
81 generates a heater voltage of 5.0V on the output terminal 87, as shown 
in the FIG. 6. By decreasing the heater voltage to 5.0V, the temperature 
of the cathodes 22, 23 and 24 is decreased to 1000 degrees K. That is, the 
temperature of the cathodes 22, 23 and 24 is decreased from the normal 
operating temperature by 9%. 
Describing the operation of the circuit shown in FIG. 2 in the normal 
display mode with reference to the waveforms 61, 62 63 and 64, in which 
the control signal of low level is supplied from the personal computer 1 
to the display device 2 through the line 20, the gain of the 
pre-amplifiers 40 of the blocks 37, 38 and 39 is maintained at the normal 
operating gain, and the heater voltage is maintained at the normal 
operating voltage, i.e. 6.2V, so that the temperature of the cathodes 22, 
23 and 24 is maintained at the normal operating temperature, i.e. 1100 
degrees K, and the differential amplifier 42 generates the lower voltage 
V1. The personal computer 1 supplies three kinds of video signals, i.e. 
the blue video signals, green video signals and red video signals, to the 
video amplifying circuit 3 via the line 21. The blocks 37, 38, 39 receive 
the blue, green and red video signals, respectively. The blue video 
signals represented by the waveform 61 in FIG. 3A is supplied to the 
pre-amplifier 40 of block 37. The voltage level 61A is a voltage level 
representing the highest luminance of the blue image and the voltage level 
61B is a voltage level representing the lowest luminance of the blue 
image. 
The pre-amplifier 40 amplifies the waveform 61 to generate the waveform 62, 
the lowest voltage level 62A of which is clamped at the voltage V1 by the 
adding circuit 44, as shown in FIG. 3C. The main amplifier 41 amplifies 
the waveform 63 to generate the waveform 64 with the polarity being 
reversed. That is, the main amplifier 41 inverts and amplifies the 
waveform 63, so that the voltage V1 between the reference voltage of 0V 
and the lowest voltage level 63A representing the lowest luminance of the 
image is amplified to the voltage V3 between the reference voltage +B, 
e.g. +70V, and the voltage level 64A, e.g. 65V, representing the lowest 
luminance of the image, and the amplitude of the waveform 63 is amplified 
to the amplitude of the waveform 64 between the voltage level 64A and the 
voltage level 64B, e.g. 25V. The waveform 64 is supplied to the cathode 22 
of the CRT 9 of FIG. 1. When the voltage level 64A is applied to the 
cathode 22, no electrons are emitted from the cathode 22, so that no blue 
image is displayed on the display surface of the CRT 9. This is called the 
lowest luminance of the image. When the voltage level 64B is applied to 
the cathode 22, the cathode fully emits electrons, so that a blue image of 
the highest luminance is displayed. The same operation as above is 
performed by the blocks 38 and 39, whereby the color image is displayed on 
the display surface of the CRT 9. 
Next, describing the operation during the image display suppress mode in 
accordance with the present invention, when the control signal of high 
level is supplied from the personal computer 1 to the display device 2 
through the line 20, the gain of the pre-amplifiers 40 in blocks 37, 38 
and 39 is decreased to zero level, and the heater voltage is decreased to 
5.0V, so that the temperature of the cathodes 22, 23 and 24 is decreased 
to 1000 degrees K, and the output voltage of the differential amplifier 42 
is increased to V2, while the differential amplifiers 42 in the blocks 38 
and 39 generate the voltage V1, respectively. Therefore, the 
pre-amplifiers 40 in all the blocks 37, 38 and 39 receive the blue, green 
and red video signals, as shown by the waveform 65 in FIG. 3, and generate 
an output signal of a constant level of zero amplitude (OV), as shown by 
waveform 66. 
It is noted that block 37 has the function of switching the output voltage 
of the differential amplifier 42 to the voltage V2 in the image display 
suppress mode, but the blocks 38 and 39 do not have the above switching 
function, that is, the differential amplifier 42 in the blocks 38 and 39 
generate the voltage V1 in both the normal display mode and image display 
suppress mode. Therefore, waveform 66 in block 37 is clamped to voltage 
V2, as shown by the waveform 67, while waveforms 66 in blocks 38 and 39 
are clamped to the voltage V1, as shown by waveform 67A. Waveform 67, 
representing the blue signal, is supplied to the main amplifier 41 in 
block 37, which amplifies the voltage V2 between the reference voltage 0V 
and the voltage level of waveform 67 to the voltage V4 between the 
reference voltage level +B and waveform 68. The waveform 67A, representing 
the green or red signal, is supplied to the main amplifier 41 in blocks 38 
and 39, which amplifies the voltage V1 to the voltage V3. 
The waveform 68 of the blue image is applied to the cathode 22 of the CRT9 
and the waveforms 68A of the green and the red images are applied to the 
cathodes 23 and 24, respectively. 
It is noted that the voltage level of the waveform 68 of the blue image is 
shifted from the voltage levels of the waveforms 68A of the green and red 
images by the voltage .DELTA.V, that is the constant voltage level of the 
waveform 68 of the blue image is shifted by the voltage .DELTA.V from the 
voltage level of the waveform 68A representing the lowest luminance of the 
image towards the voltage level 62B representing the highest luminance of 
the image. 
When the waveforms 68A of the green and red images are applied to the 
cathodes 23 and 24 of the CRT 9, respectively, no electrons are emitted 
from cathodes 23 and 24, so that no green and red images are displayed. 
When the waveform 68 of the blue image is applied to the cathode 22 of the 
CRT 9, a small number of electrons are emitted from the cathode 22, so 
that a weak electron beam represented by a weak anode current in the range 
of 2-14 uA flows in the CRT, whereby a very light blanket blue color is 
displayed on the display surface of the CRT 9. 
In this manner, in the image display suppress mode, the temperature of the 
cathodes 22, 23 and 24 of the CRT 9 is decreased from the normal operating 
temperature, e.g. 1100 degrees K, by 7-30%, and the constant voltage level 
of the blue signal is shifted from the voltage level representing the 
lowest luminance of the blue image by 5-25%, whereby a weak electron beam 
represented by an anode current in the range of 2-14 uA flows in the CRT9 
to display the very light blanket blue color. As described above, the 
decrease of the temperature of the cathode lowers the cathode emission, 
but it causes the undesired reaction of the ions with the cathode surface. 
To eliminate the undesired reaction at the cathode, a constant anode 
current or electron beam of 2-14 uA is caused to flow. 
The reason for selecting the cathode 22 for the color blue is that the 
degree to which blue stimulates the human eyes is relatively low. 
As described above, the display device 2 switches its operational mode 
between the normal display mode and the image display suppress mode in 
response to the level of the control signal. 
Further, the timer circuit 7 shown in FIG. 1 is provided to detect a lapse 
of a predetermined time period, for example, one hour after the receipt of 
the control signal of high level on line 20 to turn the power supply 
circuit 4 off. If the level of the control signal is restored to the low 
level during the predetermined time period, the timer circuit 7 is reset. 
The purpose of the timer circuit 7 is to turn all the circuits in the 
display device 2 off in the following situation. That is, sometimes, the 
operator tends to forget to turn the display device 2 off when he/she 
finishes office work or he/she leaves his desk for a long period of time. 
Due to the operation of timer circuit 7, the cathode emission if stopped, 
and power is saved. 
Thus as described herein, the present invention extends the life time of a 
CRT to 40000-44000 hours.