Abstract:
A liquid crystal display (LCD) having a matrix of liquid crystal pixels is provided. A plurality of digital-to-analog converters (DACs) are coupled to the LCD matrix through analog voltage switches and are adapted to produce output voltages that are applied to the pixels in the LCD matrix. Through the combination of DACs and analog voltage switches, groups of pixels are pre-written to an average value of the pixels in that group which is fairly close to their final voltage values of each pixel so that the liquid crystal material can begin slewing and settling as early as possible. Then one or more writes to each of the pixels is made of the precise voltage value desired at each of the pixels. Alternate, adjacent odd and even rows of pixels may be written together and then only the even or odd rows are finally written to obtain the desired final voltage values at each of the pixels in the LCD.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to liquid crystal display devices, and more particularly to a system and method for more accurately and quickly writing a frame of video information to a liquid crystal display comprising a plurality of pixels. 
   BACKGROUND OF THE INVENTION 
   Liquid crystal displays (LCDs) are commonly used in devices such as portable televisions, portable computers, control displays, and cellular phones to display information to a user. LCDs act in effect as a light valve, i.e., they allow transmission of light in one state, block the transmission of light in a second state, and some include several intermediate stages for partial transmission. When used as a high resolution information display, as in one application of the present invention, LCDs are typically arranged in a matrix configuration with independently controlled pixels (the smallest segment of the display). Each individual pixel is signaled to selectively transmit or block light from a backlight (transmission mode), from a reflector (reflective mode), or from a combination of the two (transflective mode). 
   A LCD pixel can control the transference for different wavelengths of light. For example, an LCD can have pixels that control the amount of transmission of red, green, and blue light independently. In some LCDs, voltages are applied to different portions of a pixel to control light passing through several portions of dyed glass. In other LCDs, different colors are projected onto the pixel sequentially in time. If the voltage is also changed sequentially in time, different intensities of different colors of light result. By quickly changing the wavelength of light to which the pixel is exposed an observer will see the combination of colors rather than sequential discrete colors. Several monochrome LCDs can also result in a color display. For example, a monochrome red LCD can project its image onto a screen. If a monochrome green and monochrome blue LCD are projected in alignment with the red, the combination will be full color. 
   The monochrome resolution of an LCD can be defined by the number of different levels of light transmission that each pixel can perform in response to a control signal. A second level is different from a first level when the user can tell the difference between the two. An LCD with greater monochrome resolution will look clearer to the user. 
   LCDs are actuated pixel-by-pixel, either one at a time or several simultaneously. A voltage is applied to each pixel and the liquid crystal responds to the voltage by transmitting a corresponding amount of light. In some LCDs an increase in the actuation voltage decreases transmission, while in others it increases transmission. When multiple colors are involved for each pixel, multiple voltages are applied to the pixel at different positions or times depending upon the LCD. Each voltage controls the transmission of a particular color. For example, one pixel can be actuated to allow only blue light to be transmitted while another allows only green. A greater number of different light levels available for each color results in a much greater number of possible combination colors. 
   Converting a complex digital signal that represents an image or video into voltages to be applied to the pixels of an LCD involves circuitry that can limit the monochrome resolution. The signals necessary to drive a single color of an LCD are both digital and analog. It is digital in that each pixel requires a separate selection signal, but it is analog in that an actual voltage is applied to the pixel to determine light transmission. 
   Each pixel in the core array of the LCD is addressed by both a column (vertical) driver and a row (horizontal) driver. The column driver turns on an analog switch that connects an analog voltage representative of the video input (control voltage necessary for the desired liquid crystal twist) to the column, and the row driver turns on a second analog switch that connects the column to the desired pixel. 
   The video inputs to the LCD are analog signals centered around a center reference voltage of typically from about 7.5 to 8.0 volts. This center reference voltage is not a supply or signal from anywhere, rather it is a mathematical entity. This center reference voltage is called “VCOM” and connects to the LCD cover glass electrode which is a transparent conductive coating on the inside face (liquid crystal side) of the cover glass. This transparent conductive coating is typically Indium Tin Oxide (ITO). 
   One frame of video pixels are run at voltages above the center reference voltage (positive inversion) and for the next frame the video pixels are run at voltages below the center reference voltage (negative inversion). This alternating between positive and negative inversions results in a zero net DC bias at each pixel. This removes the “image sticking” phenomena. 
   Writing video voltage values to each pixel in, for example, an 800×600 (SVGA) frame takes about 2 milliseconds using 8 analog channels in parallel operation, with each analog channel given about 25 nanoseconds to apply the appropriate video voltage value to each of its set of pixels of the SVGA frame. Unfortunately, the liquid crystal material itself takes about 3 to 4 milliseconds to settle to within one percent of its final reflectivity. That leaves very little time to flash the light source (for example: light emitting diodes—LED) for the illumination step. For example, using a three color frame image at 80 Hz, each of the color (red-green-blue) frames at 240 Hz, allows only 4.2 milliseconds per frame. Considering the requirements imposed by frame inversion, and the problem of color-breakup with color-sequential images, 80 Hz is about the slowest rate at which to present images. With increased resolution of present and future LCD display video images, a faster and more accurate way of writing pixels is desired 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a system and method for quickly and accurately writing video voltages to the matrix of pixels of a liquid crystal display. 
   In the embodiments of the present invention, a matrix of liquid crystal pixels is provided. A plurality of digital-to-analog converters (DACs) are coupled to the matrix through analog voltage switches and are adapted to produce output voltages that are applied to the pixels in the matrix. Through the combination of DACs and analog voltage switches, groups of the pixels (sub-matrices) of the pixel matrix are pre-written very quickly yet fairly close to their final voltage values so that the liquid crystal material can begin slewing and settling as early as possible. The embodiment then does another one or more writes to each of the pixels for a precise voltage value at each of the pixels. The LED is flashed to illuminate the LCD frame and then the next color frame starts being written. 
   According to the embodiments of the present invention, after one frame of a color-sequence color exposure is finished, and the LED is turned off, the next frame is prewritten to coarse groups (sub-matrices) of, for example but not limited to, 8×8 pixels which are preferably written to an average voltage value of the final values of the pixels in that group. This pre-write to the group of pixels preferably may be written in about one sixty-fourth of a normal write time, or about 30 microseconds. 
   Determination of the average voltage values may be calculated as the pixel value streams enter the control logic of the LCD system. The calculated voltage values may be stored in a memory, such as for example but not limited to, random access memory (RAM) and would require only an additional {fraction (1/64)} of the RAM storage required for storage of the individual pixel voltage values (for an 8×8 group size). It is contemplated and within the spirit and scope of the invention that many other group sizes may be implemented, and any number of DACs may be used. 
   In another embodiment of the invention, a plurality of groups may be written with the average values of the pixels in those groups, all at the same time by using the plurality of DACs. Each DAC could write to a respective group during the same time period. 
   In another embodiment of the invention, the exact pixel voltage values for a frame can be written in two steps, each taking one half the time of writing the frame in the normal line-by-line manner. For example, first the adjacent odd and even rows are written together, using the values for the odd rows. Then a second pass is performed by writing voltage values only to the even rows. The same effect can be accomplished by writing the even row values first, then on the second pass writing values only to the odd rows. 
   A technical advantage of the present invention is that it more quickly and accurately controls the light characteristics of pixels of a liquid crystal display. Another technical advantage of the present invention is that the voltage slew times are decreased so that each pixel may settle to its final voltage value more quickly. Another technical advantage of the present invention is that it allows faster write times for each frame of the LCD. 
   Other technical advantages of the present disclosure will be readily apparent to one skilled in the art from the following figures, descriptions, and claims. Various embodiments of the invention obtain only a subset of the advantages set forth. No one advantage is critical to the invention. For example, one embodiment of the present invention may only provide the advantage of controlling the pixels of a liquid crystal display, while other embodiments may provide several of the specified and apparent advantages. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present disclosure and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
       FIG. 1  is a schematic block diagram of a liquid crystal display system in accordance with embodiments of the present invention; 
       FIG. 2  is a schematic block diagram of a portion of an embodiment of the liquid crystal display of  FIG. 1 ; 
       FIG. 3  is a schematic block diagram of another embodiment of the liquid crystal display of  FIG. 1 ; 
       FIG. 4  is a functional flow diagram of the operation of an embodiment of the present invention; 
       FIG. 5  is a functional flow diagram of the operation of another embodiment of the present invention; 
       FIG. 6  is a functional flow diagram of the operation of the embodiment of  FIG. 4  further comprising memory storage of pixel voltage values and average values; 
       FIG. 7  is a functional flow diagram of the operation of the embodiment of  FIG. 5  further comprising memory storage of pixel voltage values and average values; and 
       FIG. 8  is a more detailed schematic block diagram of the video to pixel translation logic illustrated in FIG.  1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is directed to liquid crystal display devices having circuits for fast writing to pixels a frame of video information. A group of pixels are first precharged to an average value of the final pixel values then the final pixel values are written to each pixel. A combination of more than one final write cycles may be used to further improve the speed and accuracy of writing to the pixels a frame of video. 
   Referring now to the drawings, the details of preferred embodiments of the invention are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix. 
     FIG. 1  illustrates a schematic block diagram of a liquid crystal display system in accordance with the embodiments of the present invention. A high-level block diagram of a system for writing voltage values to pixels of a liquid crystal display (LCD) is generally represented by the numeral  100 . The voltage values being written to the pixels are representative of a frame of video data. The voltage values control the “twist” of the liquid crystal material at each pixel so that when a light is flashed on or through the LCD, the light polarization and ultimately the intensity of the light is controlled by the “twist” of the liquid crystal material each pixel in the LCD. 
   For illustrative purposes, the LCD  100  depicted in  FIG. 1  comprises 64 pixel columns by 48 pixels rows for a total of 3072 individually addressable pixels. The LCD  100  is further divided into 8×8 pixel groups  102 . The combination of row control logic  104  and column control logic  106  are used to select each of the pixels for writing thereto in the LCD  100 , as more fully described hereinbelow. Video to pixel translation logic (hereinafter translation logic)  108  performs the necessary calculations and steps to translate a video frame image  109  into discrete digital values which are sent to digital-to-analog converters (DACs)  110 ,  111 ,  112 ,  113 ,  114 ,  115 ,  116  and  117 , and the pixel location addresses thereof are sent to the row and column control logic  104  and  106 . It is contemplated and within the scope of the present invention that an LCD having any number of rows and columns may benefit from the present invention. In addition, any number of DACs may be used according to embodiments of the present invention. 
   Referring now to  FIG. 2 , a schematic block diagram of a portion of an embodiment of the liquid crystal display system of  FIG. 1  is illustrated. An 8×8 pixel group  102  comprises pixels  200  through  277 , pixel row switches  300  through  377  and pixel column switches  290  through  297 . An LCD operates by applying certain voltage values to each pixel of the LCD. A certain voltage at a pixel causes liquid crystals at that pixel to change their “twist” orientation so that light passing through the LCD or being reflected is thereby affected. The translation logic  108  uses the received video frame information  109  to create appropriate voltage values which are representative of that portion of the video frame at each one of the pixel locations. In addition, the translation logic  108  associates an x-y coordinate (row-column) location for each of these pixel voltage values. 
   The DACs  110 - 117  receive digital representations of the voltage values from the translation logic  108  and convert these digital representations to analog voltage values which must then be applied to each corresponding pixel location. Each of the pixels  200 - 277  has a capacitance  180  associated therewith, and each of the columns has a capacitance  182  associated therewith. The capacitance  180  of each pixel may not all be the same, nor may the capacitance  182  of each column be the same. The column capacitance  182  is greater than the pixel capacitance  180 . An analog voltage value must charge the respective column and pixel capacitances to which it is applied. The output of the DAC is connected to the column and thereby fully charges the capacitance to a desired analog voltage, then the pixel is connected to the column and the pixel capacitance is charged from the voltage on the column. Since the column capacitance is greater than the pixel capacitance, the voltage on the pixel will be substantially same as the voltage on the column. 
   The liquid crystal material also has a finite time constant for orientation by the applied voltage. The voltage applied to each pixel must also be alternately reversed in polarity so that a direct current charge does not develop on the liquid crystal material. All of these factors increase the write time of a pixel necessary for the liquid crystals of the pixels to settle into the desired light modification positions. It is desirable and necessary that the pixel capacitance be charged as quickly as possible so as to maximize the available settling time of the liquid crystal material at each pixel position. 
   All LCDs charge a column to a certain voltage then select a pixel row so that the intersection thereof is the desired pixel to be charged. For example, columns  0 - 7  are charged from the DACs  110 - 117 , respectively, when the column switches  290 - 297  are closed. Pixels  200 - 207  are charged from the columns  0 - 7 , respectively, when the row switches  300 - 307  are closed. A plurality of DACs may be used to simultaneously charge a like number of columns, then a like number of switches in a row may be used to charge a like number of pixels from the charged columns. The column control logic  104  and row control logic  106  control operation of the column switches  290 - 297  and row switches  300 - 377 , respectively, for the pixel group  102 . Other pixel groups  102  are controlled in a similar fashion. 
   An embodiment of the invention determines an average voltage value of an 8×8 pixel group  102 , then this average voltage value is simultaneously written to each one of the pixels  200 - 277  in the pixel group  102 . This may be accomplished by sending the digital representation of the average voltage value to the DACs  110 - 117 , then closing column switches  290 - 297  and row switches  300 - 377 . All of the pixels in the pixel group  102  are thus charged to the average voltage value. Next individual pixels are addressed and charged to each respective pixel voltage value in accordance with the video frame. Slew time is reduced because the final voltage value at each pixel does not have to charge as much as would be the case if, for example, going directly from a positive inversion voltage value to a negative inversion voltage value. 
   Another embodiment of the invention first charges an entire pixel group  102  as described above, then writes adjacent odd and even rows of pixels together, using the voltage values for the odd rows of pixels. Then a second pass is performed by writing voltage values only to the even rows. The same effect can be accomplished by writing the even row values first, then on the second pass writing values only to the odd rows. 
     FIG. 3  illustrates a schematic block diagram of a portion of another embodiment of the liquid crystal display system of FIG.  1 . The DACs  0 - 7  may be used to simultaneously write average voltage values to eight pixel groups  102 . Switches  298   a - 298   h  connect or disconnect the DACs  110 - 117  to common buses of each pixel group  102 . This embodiment further increases the available write and settling times for pixels of the LCD  100 . 
   In the embodiments of the present invention, the video to pixel translation logic  108  is adapted to compute average voltage values for each pixel group  102 , send addressing information to the row control logic  104  and the column control logic  106 . An average voltage value is applied to each of the pixels of the pixel group  102 , and then at least one more pass is made to finalize each pixel voltage. This reduces the write settling times of the pixels and improves the image accuracy in a given time period. 
   Referring now to  FIG. 4 , a functional flow diagram of the operation of an embodiment of the present invention is illustrated. In step  402 , the average voltage values for each group of pixels is calculated. In step  404 , the calculated average voltage values are written to each group of pixels. Then in step  406 , the voltage values for each pixel are written thereto. 
   Referring now to  FIG. 5 , a functional flow diagram of the operation of another embodiment of the present invention is illustrated. In step  502 , the average voltage values for each group of pixels is calculated. In step  504 , the calculated average voltage values are written to each group of pixels. In step  506 , the odd row voltage values are written to each pixel of adjacent odd and even rows. In step  508 , the voltage values are written to each pixel of the even rows. 
   Referring now to  FIG. 6 , a functional flow diagram of the operation of the embodiment of  FIG. 4  further comprising memory storage of pixel voltage values and average values is illustrated. In step  601 , the pixel voltage values are stored in a memory. In step  602 , the average voltage values for each group of pixels is calculated. In step  603  the calculated average voltage values for each group of pixels is stored in a memory. In step  604 , the calculated average voltage values are written to each group of pixels. Then in step  606 , the voltage values for each pixel are written thereto. 
   Referring now to  FIG. 7 , a functional flow diagram of the operation of the embodiment of  FIG. 5  further comprising memory storage of pixel voltage values and average values is illustrated. In step  701 , the pixel voltage values are stored in a memory. In step  702 , the average voltage values for each group of pixels is calculated. In step  703  the calculated average voltage values for each group of pixels is stored in a memory. In step  704 , the stored average voltage values are written to each group of pixels. In step  706 , the odd row stored voltage values are written to each pixel of adjacent odd and even rows. In step  708 , the stored voltage values are written to each pixel of the even rows. 
   Referring to  FIG. 8 , a more detailed schematic block diagram of the video to pixel translation logic is illustrated. The translation logic  108  comprises a video frame pixel to LCD pixel voltage calculation logic and pixel value memory controller  808 , LCD pixel group average voltage calculation logic  810 , LCD pixel address logic  812  and LCD pixel voltage value memory storage  814 . Video frame information  109  is translated into final voltage values for each pixel of the LCD in the video frame pixel to LCD pixel voltage calculation logic and pixel value memory controller  808 , and an average voltage value is found from the pixel final voltage values of each group ( 102 ) of pixels in the LCD pixel group average voltage calculation logic  810 . 
   The average voltage values may be directed to the appropriate DACs for each pixel group  102  and then the final voltage values may be directed to the appropriate DACs for each pixel of the LCD system  100 . In addition, the average voltage values and the final voltage values may be stored in the memory storage  814  for concurrent use, and/or subsequent use in writing the voltage values to the pixel groups and individual pixels. The LCD pixel address logic  812  controls the row control logic  104  and column control logic  106  so that the analog switches connect the appropriate DAC outputs for maximum efficiency in reducing slew time and pixel writing speed. 
   It is contemplated and within the scope of the embodiments of the present invention that the LCD and LCD system may be partially or entirely fabricated on a semiconductor integrated circuit. 
   While the present invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.