1. Field of the Invention
The present invention relates to a dither method for converting color display information in a liquid crystal display device, and more particularly, to a circuit in which a greater number of colors is represented using a smaller number of color levels.
2. Discussion of the Related Art
The CRT (Cathode Ray Tube) is the most common display device for reproducing color display information. The CRT uses electron guns to display a red color, a green color and a blue color. The greater the screen size, the thicker the CRT must be, because the CRT device can reproduce an image only if a distance between the electron guns and the screen of the CRT is sufficient. Therefore, the CRT is not a proper device for portable display applications.
In recent years, many flat display device alternatives to the CRT have been developed. Among them, a liquid crystal display (LCD) device has become particularly popular. A conventional LCD includes, as shown in FIG. 1, a controller IC (integrated circuit) 13, a scan line driver IC 11, a signal line driver IC 10, and thin film transistors (or TFTs) 16 arranged in an array. A plurality of scan lines 15 are connected to outputs of the scan line driver IC 11, and a plurality of signal lines 14 are connected to outputs of the signal line driver IC 10. The thin film transistors 16 corresponding to an array of pixels 17 are arrayed at intersections of the scan lines 15 and the signal lines 14. A gate electrode of each TFT 16 is connected to the scan line 15, a source electrode of the TFT 16 is connected to the signal line 14, and a drain electrode of the TFT 16 is connected to a pixel electrode. When a voltage is applied to the gate electrode of the TFT 16, the source electrode of the TFT 16 and the drain electrode of the TFT 16 are electrically connected. When there is no voltage on the gate electrode, the source and the drain electrodes of the TFT 16 are electrically isolated.
A conventional method for reproducing an image on an LCD screen is as follows. Image information is converted into a signal voltage by the controller IC 13, and the signal voltage is held at the signal line driver IC 10. The signal line driver IC 10 applies the signal voltage to the signal line 14 in response to a scan signal. For example, when the scan line driver IC 11 applies the scan voltage to the first scan line 15 based on a predetermined frequency signal, the TFTs 16 connected to the first scan line 15 are turned on. The signal voltages of a first line of the image information also are applied to electrodes of a first line of the pixels 17 of the pixel array. When the scan line driver IC 11 applies the scan voltage to the second scan line 15, the signal line driver IC 10 outputs a second line of the image information, which is applied to a second line of electrodes of the pixels 17 of the pixel array. Similarly, voltages representing other lines of the image information are applied to other lines of the pixels 17 of the pixel array. Thus, the image information is reproduced on the LCD device.
In order to reproduce color image information, the image information is divided into color information including red, green and blue (R, G and B) color elements. The color elements are displayed on one pixel of the LCD screen. These techniques are well known in the field of manufacturing of color LCDs.
A conventional method for reproducing the color information on a color LCD is as follows. FIG. 2 shows a conventional controller IC of the color LCD device. The conventional controller IC includes a ROM (Read Only Memory) table 21 having color data bits that are sent to the signal lines according to a horizontal sync signal H.sub.s and a vertical sync signal V.sub.s ; a latch 22 for receiving input image data according to the clock signal Ck and sending an address signal to the ROM table 21; and a Frame Rate Controller (FRC) 20 for outputting a signal for determining a dot position and a frame page of the color data bits from the ROM 21.
The input color data, which includes L bits from a video processing unit such as a VGA card, is sent to the latch 22 on the clock signal Ck. At the latch 22, the input color data is translated to an address bit representing an address of the color data in the ROM 21. The FRC 20 determines the scan line 15, where the dot belongs, according to the horizontal sync signal H.sub.s, and determines the frame page of the color data according to the vertical sync signal V.sub.s. That is, the input color data is used for the address data of the ROM 21, which outputs output color data. The output color data from the ROM 21 is applied to the signal line driver IC 10. The output color data determines the voltage level for driving the liquid crystal. The color image is reproduced on the LCD based on the driving voltage level of the liquid crystal.
The number of primary colors is determined by a number of bits L in the output color data. If the number of bits L is 3, then the color elements, R, G and B, have 3-bit color level. Therefore, the number of colors of one pixel is 2.sup.9. That is, 512 colors can be reproduced. Hereafter, "true color" refers to color dots R, G and B having 8-bit color levels, so the number of possible colors on one pixel 17 is 2.sup.24 =16,777,216. A true color display can therefore reproduce 16.7 million colors.
In the controller IC 13, the number of bits of the input color data is 8 bits, so the 8-bit input color data is true color. However, the output color data is not 8 bits. Because an 8-bit driver IC is very expensive, a total price of LCD would also be high. Generally, the price of a driver IC for 3-bit data or 6-bit data is $5 or $9 respectively, and that of an 8-bit driver IC is between $25 and $40. Furthermore, manufacturing the LCD panel is complicated if the output data bus line is 8 bits, as compared to using a data bus line with fewer than 8 bits. Thus, a great deal of research and development is directed towards reproducing true color using less than 8 bits.
Additionally, a conventional controller IC 13 uses a ROM table for reproducing the color information. The ROM is also very expensive. Even though the output color data may be 6 bits, the frames for reproducing true color must have different color levels. Thus, more ROM is needed, and the manufacturing cost of the LCD increases.