Computer graphics processing and selective visual display system – Display driving control circuitry – Intensity or color driving control
Reexamination Certificate
2001-12-27
2004-05-11
Chang, Kent (Department: 2673)
Computer graphics processing and selective visual display system
Display driving control circuitry
Intensity or color driving control
C345S082000, C345S083000
Reexamination Certificate
active
06734875
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a fullcolor LED display system displaying gradation-rich, multicolor images by combining, for example, LED lamps of three primary colors of RGB (red, green and blue). Particularly, the invention relates to a system of pulse-width modulation method for lighting and activating an LED lamp by an activating pulse having been pulse-width modulated based on gradation data for each color.
BACKGROUND ART
Basic Structure of Fullcolor LED Display System
Following the development of high-luminance blue LEDs (light-emitting diodes), fullcolor LED display systems, combining the three primary colors RGB, are beginning to become popular. An example of a specification of a typical device is as below. A display screen is in a large size of 2.4 meters in height and 3.4 meters in width. A total of 61,440 pixel lamps of 480 lines vertically and 128 dots laterally are arrayed in this screen. Each of the pixel lamps is an LED-multicolor-assembled lamp in which respective LEDs in the three primary colors RGB are densely gathered. Pixel data for driving one pixel consists of a total of 24 bits, that is 8 bits respectively for each RGB. The displaying gradation for each of the colors RGB is 256 tones respectively, and thus, a fullcolor expression of 16,777,216 colors is made possible.
In this type of fullcolor LED display system, it is possible to use, as its video source, an NTSC video signal used in a regular television broadcasting system or a VTR. An NTSC video signal having been input to a display-control device is A/D converted, and is converted and processed into digital signals of a total of 24 bits of 8 bits respectively for RGB. Image data for one screen, containing (61,440×24) bits corresponding to the 61,440 pixel lamps, is buffered in a frame memory. From this frame memory, image data of 24 bits for a single pixel is respectively distributed to a activating circuit of each pixel lamp, and is latched to a register in the activating circuit.
In the pixel-lamp activating circuit, the red LEDs are activated and lit at a tone corresponding to the 8 bits of red data latched in the register. Similarly, the green LEDs are activated and lit at a tone corresponding to the 8 bits of green data, and the blue LEDs are activated and lit at a tone corresponding to the 8 bits of blue data.
Gradation Control With Pulse-Width Modulation Method
Such a gradation control is generally conducted by a known pulse-width modulation method. A clock pulse of a sufficiently-high constant frequency is continuously generated; a (2
8
)=8-bit counter is incremented by the clock pulse; and an 8-bit count value of the counter is repetitively changed at a constant period Ts from all “0” to all “1”. By comparing, with a digital comparator, the magnitude between this 8-bit calculated value and the 8-bit gradation data latched in the register of the activating circuit, an activating pulse with a pulse width Tw corresponding to the 8-bit gradation data and with the above-mentioned period Ts is output from the comparator. The pixel-lamp activating circuit feeds a constant current through the LED and lights it for a time period of the pulse width Tw of the activating pulse. This pulse lighting is repeated at period Ts.
That is, the pulse width Tw of the activating pulse with a period Ts is determined proportional to the binary value of the 8-bit gradation data, and a displaying luminance corresponding to the 8-bit gradation data is obtained by pulse-lighting the LED with a constant current for time Tw during period Ts.
Gamma Correction of TV Signals
Even nowadays, the mainstream television-image display devices are CRT television sets. Since the RGB three-colored fluorescent materials of the CRT television sets do not illuminate in proportion to the voltage of the input video signal, the relation between the input signal and the optical output is nonlinear. As well known, such a characteristic is referred to as GAMMA. If the nonlinearity (gamma) of the CRT is corrected at each television set, the television set becomes complicated and expensive. Thus, in the current television method, signals having been gamma-corrected at the sending side are broadcasted. The actual gamma value becomes a quite different value according to measuring conditions and measuring methods. In the NTSC method, gamma correction is conducted assuming that the gamma value of the image-display device is 2.2.
However, in an LED display system, the relation between the input signal and the optical output is approximately linear, and is not nonlinear as of a gamma of a CRT television set. The relation is not completely nonlinear, but the characteristic is significantly different from the gamma of a CRT.
If a gamma-corrected NTSC video signal is taken as a video source of an LED display system, it would be necessary to carry out an inverse-gamma correction with means of some kind and carry out gradation control according to the approximately-linear characteristic of the LED, if a high-quality image displaying were to be realized.
Gradation Control by Nonlinear Pulse-Width Modulation
In a Japanese Patent Application Laid-open Publication (No. 7-306659) issued in 1995, a technique as follows was disclosed concerning a multicolor LED display unit:
(1) An LED display unit (screen) is formed by orderly arraying a multitude of LEDs in the three primary colors RGB. An LED lighting circuit for lighting each of the LEDs and adjusting the lighting color and brightness thereof is installed to the unit.
(2) The LED lighting circuit comprises: a pulse-width modulation circuit which outputs an activating pulse corresponding to an inputted gradation data; and an LED activating circuit which lights the LED with the activating pulse from the pulse-width modulation circuit.
(3) The pulse-width modulation circuit comprises: a nonlinear counter in which the relation between time and a count value takes a nonlinear action; and a digital comparator which compares the magnitude between the count value of the nonlinear counter and the gradation data stored to a buffer memory to generate the aforementioned activating pulse.
(4) The nonlinear counter comprises: a pulse generator which generates a count pulse of 16 types, each having a different period; a selection circuit which selects one type of count pulse out of the 16 types; a binary counter which counts the count pulse having been selected by the aforementioned circuit; and a decoder circuit which generates a selection signal for selecting the 16 types of count pulses from the higher-order 4 bits of the binary counter.
(5) When the count value of the binary counter is small, the selection circuit has selected a count pulse having a short period according to the selection signal from the decoder circuit, and thus, the count value of the binary counter increases rapidly. When the count value of the binary counter becomes large, the selection signal from the decoder circuit changes, and the selection circuit selects a count pulse having a long period, and thus, the count value of the binary counter increases slowly.
(6) Gradation data is successively sent from an external device, such as a display controller, to the LED display system, and is temporarily stored in a memory. The gradation data stored in the memory is input to the digital comparator via the buffer memory. The pulse width Tw of the activating pulse which is output from the digital comparator is nonlinearly modulated in view of the gradation data; in a range where the gradation data is small, the rate of change of the pulse width Tw is small, and as the gradation data becomes large, the rate of change of the pulse width Tw becomes large.
In the conventional multicolor LED display unit as described-above, by adopting gradation control according to nonlinear-pulse-width modulation, in the case where a gamma-corrected NTSC video signal is taken as a video source, it is possible to carry out an inverse-gamma correction of a line-graph like approximation which matches the approximately linear characteristic of the
Ohishi Masatoshi
Tokimoto Toyotaro
Avix Inc.
Chang Kent
Collard & Roe P.C.
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