Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-12-23
2002-08-06
Liang, Regina (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S099000, C345S102000
Reexamination Certificate
active
06429839
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) apparatus which is employed as a display apparatus incorporated in an information processing apparatus, an audio-video apparatus, an advertising indicator, etc. Furthermore, the present invention also relates to an electronic device for providing a control signal to the LCD apparatus.
2. Description of the Related Art
In recent years, personal computers and word processors have become widespread. As a display apparatus incorporated therein, a light-weight, thin LCD apparatus which may be driven by a battery has become dominant in the market replacing a CRT display apparatus which consumes a large amount of power and space.
A typical LCD apparatus employs a cold cathode ray tube as a light source for illuminating the display panel from the back, and the light source is driven by a light source driving circuit. The method for controlling the brightness in the display panel can generally be classified into two types of methods. One is a current-based light modulation method wherein the luminance is controlled by varying a current amplitude of the cold cathode ray tube. The other is a chopping light modulation method wherein the light source is alternately turned ON/OFF at a high speed based on a light modulation signal, and the luminance is virtually controlled by changing the duty ratio which is a ratio between an ON period and an OFF period of the light source.
The chopping method can accurately control the luminance over a wide brightness range. However, the frequency of flashing of the light used for the light modulation is apt to cause an interference with the driving frequency of the liquid crystal display device. As a result of the interference, flicker in the display and/or moving stripes are perceived on the screen.
In order to remove such drawbacks, various methods have been devised. Japanese Laid-Open Publication No. 4-143722 (“Prior Art 1.”) discloses a backlight and a control method thereof. As shown in
FIG. 12
, a light modulation signal generation circuit
21
is provided in a stage preceding a light source driving circuit
26
for driving a cold cathode ray tube
2
. The frequency of the light modulation signal of the light source driving circuit
26
is adjusted so that the shift-number and a shift direction factor of light/dark portions on the screen are slightly offset.
Japanese Laid-Open Publication No. 3-64895 (“Prior Art 2”) discloses another control method for a backlight. A control device shown in
FIG. 13
includes a duty control circuit
15
and a monostable multivibrator
11
coupled via an OR circuit
12
to a light source driving circuit
16
for driving a fluorescent lamp
2
which acts as a backlight of a liquid crystal display panel
1
. The fluorescent lamp
2
is turned ON/OFF based on a rectangular wave signal whose duty cycle is variable. The rectangular wave signal is synchronized with a signal obtained by dividing a frequency of an image synchronization signal for a liquid crystal screen by an integer n (n>0).
According to Prior Art 1 (Japanese Laid-Open Publication No. 4-143722), a light modulation signal frequency Fb and a driving frequency Ff of an LCD apparatus are set independent of each other. Therefore, even when a value suitable for the light modulation signal frequency Fb is determined by way of calculation, there remains a difficulty in regulating the driving frequency Ff and the light modulation signal frequency Fb with high accuracy while maintaining a desirable relationship therebetween. Furthermore, even when these frequencies are initially set to appropriate values, respectively, aging of the LCD device, temperature variation, etc., may cause a frequency-shift, whereby flicker in the display is perceived on the screen.
Prior Art 2 (Japanese Laid-Open Publication No. 3-64895) discloses a method for synchronizing the light modulation signal with the liquid crystal driving signal as shown in FIG.
13
. However, regarding the frequency of the light modulation signal, Prior Art 2 only describes “a signal corresponding to an image synchronization signal whose frequency is divided by an integer n equal to or greater than zero.” Prior Art 2 fails to disclose specifically what type of signal the image synchronization signal is, and what number is optimal for the integer n. Therefore, even when a light modulation signal is obtained by suitably dividing the frequency of a horizontal synchronization signal having a driving cycle of one horizontal line or a display data latch pulse, which are used as liquid crystal driving signals, flicker in the display may be generated in many cases.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, a liquid crystal display apparatus includes a liquid crystal display device: a light source for illuminating the liquid crystal display device; a LCD device driving circuit for providing a driving voltage to the liquid crystal display device based on a display data signal and a plurality of liquid crystal driving signals including a display data latch signal; a dividing circuit for dividing a frequency of the display data latch signal by a factor of N so as to obtain a period which is N times as large as that of the display data latch signal, where N is an integer greater than zero; a duty control circuit for changing an ON duty ratio of the frequency-divided signal by using the frequency-divided signal as a reference frequency; and a light source driving circuit for turning the light source ON/OFF based on a signal from the duty control circuit having the ON duty ratio set in the duty control circuit, wherein, where a driving duty is (1/D), and a remainder of D divided by the integer N is A, each of the integer N and the value D is set to an integer greater than zero which satisfies the following expression:
−1≦{(
N/A
)−2}≦1.
According to another aspect of the present invention, a liquid crystal display apparatus includes a liquid crystal display device; a light source for illuminating the liquid crystal display device; a LCD device driving circuit for providing a driving voltage to the liquid crystal display device based on a display data signal and a plurality of liquid crystal driving signals; a dividing circuit for dividing a frequency of a horizontal synchronization signal having a period of one horizontal period, to obtain a period which is N times as large as that of the a horizontal synchronization signal, where N is an integer greater than zero; a duty control circuit for changing an ON duty ratio of the frequency-divided signal by using the frequency-divided signal as a reference frequency; and a light source driving circuit for turning the light source ON/OFF based on a signal f rom the duty control circuit having the ON duty ratio set in the duty control circuit, wherein where a period of the vertical synchronization signal is M times as large as a period of the horizontal synchronization signal, and a remainder of M divided by N is A, each of the integer N and the value M is set to an integer greater than zero which satisfies the following expression:
−1≦{(
N/A
)−2}≦1.
According to still another aspect of the present invention, a liquid crystal display apparatus includes a liquid crystal display device; a light source for illuminating the liquid crystal display device; a LCD device driving circuit for providing a driving voltage to the liquid crystal display device based on a display data signal and a plurality of liquid crystal driving signals including a display data latch signal; a dividing circuit for dividing a frequency of the display data latch signal or a horizontal synchronization signal having a cycle of one horizontal period, to obtain a period which is M times as large as that of the display data latch signal or the horizontal synchronization signal, where M is an integer greater than zero; a duty control circuit for changing an ON duty ratio of the frequency-divided signal by using t
Conlin David C.
Dike Bronstein, Roberts & Cushman IP Group, Edwards & Angell, LL
Liang Regina
Pledger Timothy Carter
Sharp Kabushiki Kaisha
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