Computer graphics processing and selective visual display system – Display driving control circuitry – Intensity or color driving control
Reexamination Certificate
2001-04-20
2003-11-11
Wu, Xiao (Department: 2674)
Computer graphics processing and selective visual display system
Display driving control circuitry
Intensity or color driving control
C345S692000, C345S082000
Reexamination Certificate
active
06646654
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a modulation circuit for generating and outputting a plurality of pulse signals at a predetermined period and an image display and a modulation method using the modulation circuit, more particularly relates to a modulation circuit of a driving signal for a light emitting diode (LED) or an organic electroluminescence (EL) element, and an image display comprising an LED or an organic EL element.
2. Description of the Related Art
Since the invention of the blue LED, LED color displays that use LEDs to form pictures by pixels emitting the three primary colors have been widely and generally fabricated. An LED is highly durable can be used semipermanently, so is optimal for long-term use outdoors. Therefore, LEDs have been extensively used for large-scale displays in stadiums and event sites and for information display panels or advertisements on sides of buildings and inside railway stations. In recent years, along with the increasing luminance and lower prices of blue LEDs, such LED color displays have been spreading rapidly.
FIG. 1
is a view of a drive circuit of an LED forming a pixel of an LED display.
In
FIG. 1
, reference numeral
100
indicates a drive circuit and
200
an LED. In addition, Spx represents a video signal supplied to an individual pixel, and Id a current flowing through the LED
200
, respectively.
The drive circuit
100
outputs a current according to the video signal Spx to the LED
200
, while the LED
200
emits light according to the supplied current. An LED display is comprised of exactly the same number of circuits consisting of the drive circuits
100
and LEDs shown in
FIG. 6
as that of the pixels. By making the LEDs of the pixels emit light with luminances according to the video signals Spx supplied to the pixels, a person viewing the screen can recognize a picture. The video signal Spx supplied to each pixel is generally input to the drive circuit
100
as a digital value of a certain number of bits.
FIG. 2
is a view of the waveform of the current flowing through the LED
200
in FIG.
1
.
In
FIG. 2
, the ordinate indicates the current flowing through the LED
200
by a relative value, while the abscissa indicates time by a relative value. In addition, Ipulse indicates the peak value of the waveform of the pulse-shaped current flowing through the LED, tw the time width of the pulse portion, and T the period of the waveform.
As shown in
FIG. 2
, the current flowing through the LED forming a pixel of an LED display has a periodic pulse-like waveform. The luminance is controlled by pulse width modulation to make the pulse width tw variable.
In principle, the current flowing through the LED is a direct current. It is possible to change the current value in accordance with the video signal Spx to adjust the luminance, but in this case, it is necessary to finely control the current value by the drive circuit. There is the disadvantage that the circuit for this control ends up increasing the number of parts. It is easier to increase the resolution of the time than the resolution of the current value, so in general the pulse width modulation system such as shown by the current waveform of
FIG. 2
is adopted.
Due to the nature of human senses, the luminance of light blinking in a manner staying lit for less than {fraction (1/60)} of a second is perceived to have a constant luminance. Therefore, even an LED is driven by a current of the waveform shown in
FIG. 2
, if the period T of the current is shorter than the aforesaid time, the blinking light from the LED can be made to be perceived by people as light of a constant luminance.
Further, generally, the magnitude of the luminance of an LED perceived by the human senses is proportional to the current flowing through the LED averaged over time. Therefore, the luminance changes in proportion with the duty of the pulse current.
The level of a video signal input to an LED display, however, is normalized in advance to match the luminance characteristics of a cathode ray tube (CRT). If such a video signal is input as it is to an LED, which has different luminance characteristics from a CRT pixel, the following problem arises.
FIG. 3
is a view of the relation of the luminances of an LED and CRT pixel with the level of an input signal.
In
FIG. 3
, the ordinate represents the luminance of an LED or CRT pixel by a relative value, while the abscissa represents the level of the signal input to an LED or a CRT pixel by a relative value. The curves indicated by A and B show the luminance characteristics of a CRT pixel and an LED, respectively.
Note that for the luminance characteristic A of a CRT pixel, the level of the video signal is expressed by voltage, while for the luminance characteristic B of an LED, the level of the video signal is expressed by the current flowing through the LED.
As shown in
FIG. 3
, the luminance of an LED has a linear relationship with the signal level, while the luminance of the CRT pixel has a nonlinear relationship with the signal level. In general, the luminance of a CRT pixel is proportional to the 2.2th power of the voltage level of the video signal. If a current proportional to a video signal normalized to match such a characteristic is directly supplied to an LED, the LED appears brighter than a CRT pixel in the region of low output of light, but appears darker than a CRT pixel in the region of high output of light. Consequently, a picture formed by such pixels has a ratio of luminance of the bright portions and dark portions different from the original picture, so looks unnatural to the viewer.
In order to solve this problem, in an LED display of the related art, a signal corrected to eliminate the influence due to the above luminance characteristic of the video signal is input to the drive circuit
100
as the above video signal Spx. Specifically, for example, when driving an LED of a linear luminance characteristic by a video signal produced to match with CRT pixel emitting light of a luminance proportional to the 2.2th power of the signal level, a signal proportional to the 2.2th power of the video signal is generated to drive the LED.
However, if the bit length of the original video signal is not sufficiently large, the binary data obtained by raising this digitalized image data to the 2.2th power is incapable of expressing fine changes of value in the region where the value of the original video signal is small. In other words, if the bit length of the digitalized video signal is small, the grey scale ends up rough in the low luminance region resulting in an unnatural picture. In order to avoid such a problem, it is necessary to increase the bit length of the video signal. Specifically, in an LED display of the related art, it is necessary to generate a video signal of a length of 12 to 16 bits to reproduce a picture which had been expressed by a video signal of a length of 8 bits in the case of a CRT. If the bit length of the video signal is increased in this way, the bit length of the pulse width modulation circuits for driving the LEDs also has to be increased, so the overall circuit scale becomes larger and the cost and power consumption rise.
Further, the pulse-like waveform shown in
FIG. 2
is generally generated by counting a clock signal serving as a time reference. Increasing the bit length of a video signal means increasing the number of times to count the clock signal by that extent, so when using a clock signal of the same frequency, the period T of pulse width modulation ends up longer. For example, when generating and modulating the pulse width of a 12-bit video signal, 4 bits larger than an 8-bit video signal, and comparing them with the same frequency of the clock signal, the period T of pulse width modulation becomes 16 times that of an 8-bit video signal. Since the period T of pulse width modulation is set using the characteristic of the human senses described above, if this period is too long, “flickering” where the blinking of the light will be perceived by the
Kananen Ronald P.
Rader & Fishman & Grauer, PLLC
Sony Corporation
Wu Xiao
LandOfFree
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