Brightness control apparatus for video display appliance

A brightness control apparatus for a video display appliance which keeps the brightness of the picture uniformly over the whole screen by determining the amplification rate of the video signal to be displayed at the corners of the screen relatively higher than that to be displayed at its center. The apparatus includes a parabolic wave generator for generating horizontal and vertical parabolic waves corresponding to horizontal and vertical sync signals separated from the video signal, first and second amplifying sections for amplifying the horizontal and vertical parabolic waves outputted from the parabolic wave generator with predetermined amplification rates, respectively, and a signal superimposing section for superimposing the amplified horizontal and vertical parabolic waves with a DC voltage provided from a variable resistor for adjusting the amplification rate of the video signal and providing a superimposed signal to a video amplifying section as an amplification-rate control signal of the video amplifying section.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to brightness control of a video display 
appliance, and more particularly to a brightness control apparatus for a 
video display appliance which can keep the brightness of a picture 
uniformly over the whole screen and thus provide good picture quality. 
2. Description of the Related Art 
FIG. 1 is a block diagram illustrating the construction of a conventional 
brightness control apparatus for a video display appliance. Referring to 
FIG. 1, the conventional brightness control apparatus includes a video 
amplifying section 1 for amplifying an input RGB (red, green, blue) video 
signal, a variable resistor VR1 for adjusting the amplification rate of 
the video amplifying section 1, and a CRT (cathode ray tube) 2 for 
displaying on its screen the video signal amplified by the amplifying 
section 1. 
The operation of the conventional brightness control apparatus as 
constructed above will be explained. 
When the RGB video signal is inputted to the video amplifying section 1, it 
is amplified with a predetermined amplification rate by the video 
amplifying section 1, and then the amplified video signal is outputted to 
the CRT 2, so that the video signal is displayed on the screen of the CRT 
2. 
The amplification rate of the video amplifying section 1 is determined by 
varying the resistance value of the variable resistor VR1. Specifically, 
as the resistance value of the variable resistor VR1 varies, the voltage 
supplied from the variable resistor VR1 to the video amplifying section 1 
varies, resulting in that the amplification rate of the video amplifying 
section 1 also varies, and this causes the brightness of the video signal 
being displayed on the screen of the CRT 2 to be adjusted accordingly. 
FIG. 2 is a view illustrating the brightness state of the picture displayed 
by the conventional brightness control apparatus. As described above, the 
conventional brightness control apparatus amplifies the input video signal 
with a predetermined amplification rate and outputs the amplified video 
signal to the CRT 2. At this time, a user can adjust the brightness of the 
picture by varying the amplification rate of the video amplifying section 
1 by means of the variable resistor VR1. 
According to the conventional brightness control apparatus, however, the 
picture displayed at the center portion of the screen becomes brighter 
than that at its corner portions due to the difference in deflection angle 
between the center and the corner portions. Specifically, since the 
distance between an electron gun of the CRT 2 and each corner portion of 
the screen is relatively longer than that between the electron gun and the 
center portion of the screen, the picture displayed at each corner portion 
becomes relatively darker than that displayed at the center portion. 
Especially, such a difference in brightness of the screen portions is 
greatly intensified in a CRT having a large-sized screen and thus this 
causes the quality of the picture displayed on the CRT to deteriorate. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to solve the problems involved in 
the related art, and to provide a brightness control apparatus for a video 
display appliance which can keep the brightness of the picture uniformly 
over the whole screen and thus provide a good picture quality by setting 
the amplification rate of the picture signal displayed at the corner 
portions of the screen relatively higher than that displayed at its center 
portion. 
It is another object of the present invention to provide a brightness 
control apparatus for a video display appliance which can be suitably 
adopted to a large-screen CRT that has a serious problem of the brightness 
difference of the screen, to provide good picture quality. 
In order to achieve the above objects, there is provided a brightness 
control apparatus for a video display appliance having a video amplifying 
section for amplifying an input video signal to provide the amplified 
video signal to a CRT, and a variable resistor for adjusting the 
amplification rate of the video amplifying section, the brightness control 
apparatus comprising: 
a parabolic wave generator for generating a horizontal parabolic wave 
corresponding to a horizontal sync signal separated from the video signal, 
and a vertical parabolic wave corresponding to a vertical sync signal 
separated from the video signal; 
a first amplifying section for amplifying the horizontal parabolic wave 
generated from the parabolic wave generator with a first amplification 
rate; 
a second amplifying section for amplifying the vertical parabolic wave 
generated from the parabolic wave generator with a second amplification 
rate; and 
a signal superimposing section for superimposing both the horizontal and 
vertical parabolic waves amplified by the first and second amplifying 
sections, respectively, with a DC voltage outputted from the variable 
resistor to provide a superimposed signal to the video amplifying section.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 3 is a block diagram showing the construction of the brightness 
control apparatus for a video display appliance according to the present 
invention. 
Referring to FIG. 3, the brightness control apparatus according to the 
present invention includes a video amplifying section 1 for amplifying an 
input RGB video signal, a variable resistor VR1 for adjusting the 
amplification rate of the video amplifying section 1, and a CRT 2 for 
displaying on its screen the video signal amplified by the amplifying 
section 1. 
The brightness control apparatus according to the present invention also 
includes a parabolic wave generator 10 for generating a horizontal 
parabolic wave which corresponds to a horizontal sync signal separated 
from the RGB video signal and a vertical parabolic wave which corresponds 
to a vertical sync signal separated from the RGB video signal, a first 
amplifying section 20 for amplifying the horizontal parabolic wave 
outputted from the parabolic wave generator 10 with a first amplification 
rate, a second amplifying section 30 for amplifying the vertical parabolic 
wave outputted from the parabolic wave generator 10 with a second 
amplification rate, and a signal superimposing section 40 for 
superimposing the horizontal and vertical parabolic waves amplified by and 
outputted from the first and second amplifying sections 20 and 30, 
respectively, with a DC voltage outputted from the variable resistor VR1 
to provide the superimposed signal to the video amplifying section 1. 
The first amplifying section 20 includes an operational amplifier OP1 for 
amplifying the horizontal parabolic wave, and a variable resistor VR2, 
coupled to the operational amplifier OP1 for determining the first 
amplification rate of the operational amplifier OP1. 
The second amplifying section 30 includes an operational amplifier OP2 for 
amplifying the vertical parabolic wave, and a variable resistor VR3, 
coupled to the operational amplifier OP2, for determining the second 
amplification rate of the operational amplifier OP2. 
Also, the signal superimposing section 40 comprises a capacitor C3 for 
superimposing the amplified horizontal and vertical parabolic waves 
amplified by the operational amplifying sections 30 and 40, respectively, 
with the DC output voltage of the variable resistor VR1 and providing the 
superimposed signal as an amplification-rate control signal of the video 
amplifying section 1. 
In the drawing, the reference numerals C1 and C2 denote coupling 
capacitors, R1 to R4 denote resistors, and Vref denotes a reference 
voltage inputted to the operational amplifiers OP1 and OP2. 
The operation of the brightness control apparatus according to the present 
invention as constructed above will now be explained with reference to 
FIGS. 3, 4A to 4C. 
First, when the RGB video signal is inputted to the video amplifying 
section 1, it is amplified with the amplification rate determined in 
accordance with the amplification-rate control signal inputted to the 
amplification-rate control terminal CT of the video amplifying section 1, 
and then the amplified video signal is outputted to the CRT 2, so that a 
picture having the brightness which corresponds to the amplification rate 
is displayed on the screen of the CRT 2. 
The parabolic wave generator 10 generates the horizontal parabolic wave 
which corresponds to the horizontal sync signal separated from the RGB 
video signal as shown in FIG. 4A, and outputs the horizontal parabolic 
wave to the inverting (-) terminal of the operational amplifier OP1 in the 
first amplifying section 20. This operational amplifier OP1 amplifies the 
horizontal parabolic wave inputted thereto with the first amplification 
rate determined by the variable resistor VR2, and outputs the amplified 
horizontal parabolic wave to the capacitor C3 of the signal superimposing 
section 40. 
At the same time, the parabolic wave generator 10 generates the vertical 
parabolic wave which corresponds to the vertical sync signal separated 
from the RGB video signal as shown in FIG. 4B, and outputs the vertical 
parabolic wave to the inverting (-) terminal of the operational amplifier 
OP2 in the second amplifying section 30. This operational amplifier OP2 
amplifies the vertical parabolic wave inputted thereto with the 
amplification rate determined by the variable resistor VR3, and outputs 
the amplified vertical parabolic wave to the capacitor C3 of the signal 
superimposing section 40. 
The horizontal and vertical parabolic waves outputted from the operational 
amplifiers OP1 and OP2, respectively, are superimposed and filtered 
through the capacitor C3 so that the DC component of the superimposed 
horizontal and vertical parabolic signal is removed. The filtered 
horizontal and vertical parabolic signal is then superimposed with the DC 
voltage A outputted from the variable resistor VR1, and then the 
superimposed signal is outputted to the amplification-rate control 
terminal CT of the video amplifying section 1. 
FIG. 4C illustrates the superimposed signal wherein the DC voltage A, the 
horizontal and vertical parabolic waves are superimposed together through 
the capacitor C3. Referring to FIG. 4C, the superimposed horizontal and 
vertical parabolic signal, which are composed of the horizontal parabolic 
wave component and the vertical parabolic wave component, has risen from 
the level of the DC voltage A. 
The superimposed voltage waveform of FIG. 4C is inputted to the 
amplification-rate control terminal CT of the video amplifying section 1, 
and thus the video amplifying section 1 amplifies the input video signal 
with the amplification rate which corresponds to the superimposed voltage 
waveform. Specifically, the video signal to be displayed at the corner 
portions of the screen is amplified with the amplification rate which is 
relatively higher than that of the video signal to be displayed at the 
center portion of the screen, resulting in that the brightness of the 
picture becomes uniform over the whole screen of the CRT 2, thereby 
improving the picture quality. At this time, a user may properly adjust 
the variable resistors VR2 and VR3 in the first and second amplifying 
sections 20 and 30 to obtain the optimum picture state. 
From the foregoing, it will be apparent that the brightness control 
apparatus according to the present invention provides the advantages that 
it can keep the brightness of the picture uniformly over the whole screen 
of the CRT and thus provide a good picture quality by determining the 
amplification rate of the video signal displayed at the corner portions of 
the screen relatively higher than that displayed at its center portion. 
Especially, the present invention can be suitably adopted to a CRT having 
a large screen. 
While the present invention has been described and illustrated herein with 
reference to the preferred embodiment thereof, it will be understood by 
those skilled in the art that various changes in form and details may be 
made therein without departing from the spirit and scope of the invention.