Abstract:
An image display device includes a display section in which (m×n) display pixels are arrayed in a matrix form and located at intersections of m row wirings and n column wirings, a length L of the row being greater or equal to a length K of the column. Particularly, the display device further includes a scanning driver which sequentially selects the column wirings, and a driving circuit which supplies a drive signal to each of the row wirings based on a video signal synchronously with selection made by the scanning driver.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-188379 filed Jun. 21, 2001, the entire contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an image display device of driving a matrix array of display pixels, and more particularly to the image display device called a field emission display (FED), for example.  
           [0004]    2. Description of the Related Art  
           [0005]    An FED is known as an image display device of a structure in which row wirings (row lines) and column wirings (column lines) are arranged to intersect from each other, with surface conduction type electron emission elements (display pixels) disposed at intersections thereof. Generally, the FED comprises a scanning section for sequentially scanning and selecting the row lines in units of lines and a driving section for supplying drive signals corresponding to a video signal to the column lines in units of lines (for example, Jpn. Pat. Appln. KOKAI Publication No. 9-297556).  
           [0006]    In an image display device using surface conduction type electron emission elements, each display pixel is formed by combining three surface conduction type electron emission elements and red (R), green (G) and blue (B) phosphors which illuminate upon irradiation of electron beams from the three electron emission elements.  
           [0007]    An image is displayed by the pixels driven in a manner that the row lines (H 1  to Hq) are sequentially scanned and selected by the scanning driver, while pulse width modulation signals (drive signals) are supplied from the drive circuit to the column lines (G 1  to Gp).  
           [0008]    Generally, the screen of the image display device has an aspect ratio that the horizontal length is larger than the vertical length (4:3 or 16:9). Thus, the number of display pixels connected to each row line is greater than that of display pixels connected to each column line.  
           [0009]    The image display device as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-297556 includes a simple matrix wired array of surface conduction type electron emission elements. In the matrix wired array, different potentials are applied to the elements due to the voltage drop across the wiring resistance. That is, the potential actually applied to each element is lowered in proportion to a wiring length between the drive voltage supply terminal and the element, so that an uneven distribution occurs in the potentials of the elements.  
           [0010]    Particularly, a horizontally elongated display device whose screen has an aspect ratio of 16:9 (horizontal length to vertical length) has entered the main stream. With this screen, an increased number of display pixels are arranged along the row line (scanning line), making it more likely to suffer from the above problem.  
           [0011]    For example, where display pixels are arrayed in a ratio of 1280:720 (=columns:rows), it comes that 1280×3 (RGB) surface conduction type electron emission elements are provided for each row line (scanning line). Due to this, the influence of the wiring resistance becomes more serious and correspondingly, uneven distribution occurs in the luminous intensity of the elements arranged in the row direction, thereby deteriorating the quality of the image display device considerably. For example, a difference of at least 2 to 3V in potential occurs between both ends of the row line.  
         BRIEF SUMMARY OF THE INVENTION  
         [0012]    Accordingly, an object of the present invention is to provide an image display device that can display a high-quality image, while suppressing unevenness in brightness caused due to the voltage drop across the wiring resistance.  
           [0013]    To achieve the above object, the present invention provides an image display device comprising a display section in which (m×n) display pixels are arrayed in a matrix form and located at intersections of m row wirings and n column wirings, a length L of the row being greater or equal to a length K of the column; a scanning section which sequentially selects the column wirings; and a driving section which supplies a drive signal to each of the row wirings based on a video signal synchronously with selection made by the scanning section.  
           [0014]    Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0015]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiment of the invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention.  
         [0016]    [0016]FIG. 1 is a schematic view showing a first embodiment of the present invention;  
         [0017]    [0017]FIG. 2 is a schematic view showing a second embodiment of the present invention;  
         [0018]    [0018]FIG. 3 is a timing chart showing drive timings of the column lines shown in FIG. 2;  
         [0019]    [0019]FIG. 4 is a diagram showing a concrete example of a scanning direction changer in an image processing circuit;  
         [0020]    [0020]FIGS. 5A and 5B are explanatory diagrams of the structure of a display pixel using a surface conduction type electron emission element;  
         [0021]    [0021]FIG. 6 is a diagram showing an example of a driving circuit in the display device of the embodiment; and  
         [0022]    [0022]FIG. 7 is a schematic view showing a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.  
         [0024]    [0024]FIG. 1 shows a first embodiment of the present invention. It is assumed that this image display device has for example, pixels of 1280:720 (horizontal:vertical). For color display, the number of display pixels is 1280:720 (horizontal:vertical)×3 (corresponding to R, G and B). Thus, 720×3 (corresponding to R, G and B) display pixels are connected to a row line (scanning line).  
         [0025]    The row line is driven by a scanning driver  101 . The scanning driver  101  sequentially scans the row lines (A 1  to An) in units of lines. On the other hand, the column line is connected to a driving circuit  102 . The driving circuit  102  supplies those drive signals to the column lines (B 1  to Bm), which are pulse-width modulated in units of column lines. The pulse width is modulated depending upon brightness such that the maximum value falls within the drive period of a single column line. Therefore, the pulse width becomes small for a pixel of a low brightness, and large for a pixel of a high brightness. In this manner, an image is displayed in a display section.  
         [0026]    Reference numeral  308  denotes a video processing circuit. This video processing circuit  308  incorporates a scanning direction changer  81  which temporarily stores a video signal input as pixel data for one field or one frame into a buffer memory in units of rows and retrieves the pixel data in units of columns so that the scanning direction is changed.  
         [0027]    Synchronously with pixel data retrieval from the buffer memory in the video processing circuit  81 , a timing generating circuit  307  sets up ON-timing of each column line for the scanning driver  101  and sample and output timings of the retrieved data for the driving circuit  102 .  
         [0028]    In the display section  400 , the pixel colors of R, G, B, R, G, B, R, G, B, . . . are assigned to the pixels arranged along each vertical (column) line instead of each horizontal (row) line.  
         [0029]    In the above-mentioned display device, the scanning driver  101  sequentially selects and drives the column lines A 1 , A 2 , A 3 , . . . during the period of one field or one frame. Similarly, the drive circuit  102  outputs the video signal of rows to the row lines B 1 , B 2 , B 3 , . . . during the period of one field or one frame. Consequently, an image is displayed in the display section  400 .  
         [0030]    According to the device of the first embodiment, the number of the display pixels assigned to a wiring to be scanned is 720×3 which is smaller than the conventional example of 1280×3, so that the value of wiring resistance is reduced by this decreased amount, thereby unevenness in brightness is eliminated.  
         [0031]    As compared to the conventional example, the number of lines to be scanned by the scanning driver  101  during the period of one field or one frame is increased from 720 to 1280. This is an increase of 78%, which is sufficiently within an operating capacity range.  
         [0032]    The aspect ratio of the horizontal length to the vertical length is 9:16 and the length of the column line to be driven by the scanning driver  101  is about 56% the length of the row line to be conventionally driven by the Y driver. This means that the wiring resistance of the column line is about 56% of the conventional wiring. As a result, the difference in brightness for each column line is reduced from the conventional level.  
         [0033]    The present invention is not restricted to the above-described embodiment.  
         [0034]    [0034]FIG. 2 shows a second embodiment of the present invention. Here, in a display section  400 , display pixels are arrayed horizontally and vertically as a matrix of 1280×3 (RGB):720. The display pixels connected to the column lines (C 1  to Ck) are connected to the row lines (E 1  to Ej), while the display pixels connected to the column lines (D 1  to Dk) are connected to the row lines (F 1  to Fj). That is, according to this embodiment, the column lines of the display section  400  are divided to two blocks formed in the left and right regions. Then, the column lines (C 1  to Ck) and row lines (E 1  to Ej) are used to drive the display pixels in the left region, while the column lines (D 1  to Dk) and row lines (F 1  to Fj) are used to drive the display pixels in the right region.  
         [0035]    Therefore, the scanning driver  201  and the driving circuit  203  are assigned to the column and row lines in the right region while the scanning driver  202  and the driving circuit  204  are assigned to the column and row lines in the left region.  
         [0036]    The timing generating circuit  307  provides timing signals to the driving circuits  203 ,  204  and the scanning drivers  201 ,  202 . The video signal for the left region is supplied from the video processing circuit to the driving circuit  203 , and the video signal for the right region is supplied from the video processing circuit  308  to the driving circuit  204 .  
         [0037]    [0037]FIG. 3 shows the drive timings of the scanning lines (column lines) for explaining the operation of the display device shown in FIG. 2. The column lines in the left region are scanned one by one in the order of C 1 , C 2 , C 3 , . . . Ck, and synchronously with this, the column lines in the right region are scanned one by one in the order of D 1 , D 2 , D 3 , . . . Dk.  
         [0038]    In the above-described embodiment, each scanning driver  201 ,  202  only has to scan 1280×3/(2×3) lines during the period of one frame. These lines may be divided additionally.  
         [0039]    According to this embodiment, the number of the display pixels assigned to a wiring to be scanned is decreased to 720 to reduce the value of wiring resistance. Thus, unevenness in brightness is eliminated. Further, since two scanning lines are concurrently driven, the operation frequency of the scanning driver can be lowered.  
         [0040]    Although according to this embodiment, the column lines to be scanned are divided to two blocks formed in the left and right regions, two blocks of column lines may be arranged alternately in an order of C 1 , D 1 , C 2 , D 2 , . . . Ck, Dk, for example.  
         [0041]    Particularly, in an electron emission element, as a current of several mA flows to emit electrons, when the number of the display pixels is increased, the electron emission element is more easily affected by the wiring resistance. Thus, the display device of this embodiment is configured to reduce the influence of the wiring resistance.  
         [0042]    [0042]FIG. 4 shows a concrete example of the scanning direction changer. Usually, an NTSC type video signal is produced to sequentially drive display pixels arranged in the row (lateral or horizontal) direction. After the video signal is converted to digital pixel data through an A/D converter  503 , pixel data for one frame are written into a frame memory  501  in units of rows and when all the pixel data for one frame have been stored, the pixel data are read out from the frame memory  501  in units of columns to sequentially drive the display pixels arranged in the column (longitudinal or vertical) direction. While the pixel data for one frame are read out from the frame memory  501 , the pixel data for the next frame data are written into another frame memory  502  in units of rows. The pixel data for the next frame are read out from the frame memory  501  in units of columns, while writing to the frame memory  501  is performed in the same manner as described above. After the pixel data are written into a selected one of the frame memories  501 ,  502  in the order corresponding to the display pixels arranged in the row direction, these pixel data are read out from the selected frame memory in the order corresponding to the display pixels arranged in the column direction. As a result, the scanning direction is changed.  
         [0043]    Although in the drawings of the above-described embodiments, display pixels are arrayed in a striped form, the present invention is applicable to the case where the display pixels are arrayed in a delta form. Further, the present invention is not restricted to a type in which the scanning driver is disposed on one side of the wirings and connected thereto, it is also applicable to the type in which the scanning driver is disposed on both sides of the wirings and connected thereto.  
         [0044]    Next, the structure of the display pixel in the display section  400  will be described.  
         [0045]    Referring to FIGS. 5A and 5B, reference numeral  411  denotes a substrate, reference numerals  412  and  413  denote element electrodes, reference numeral  414  denotes a conductive thin film, reference numeral  415  denotes an electron emission member formed by an electro forming process, and reference numeral  416  denotes a thin film formed by an electro activation process.  
         [0046]    As the substrate  411 , for example, various kinds of glass substrates such as quartz glass and blue glass, various kinds of ceramic substrate such as alumina or a substrate produced by overlaying an insulating layer such as SiO 2  on the above described various kinds of substrates are available.  
         [0047]    The element electrodes  412  and  413  are provided on the substrate  411  in parallel to the substrate surface and formed of conductive material properly selected from metals such as Ni, Cr, AuTiMo, W, Pt, Ti, Cu, Pd and Ag, alloy of these metals, metallic oxide such as In 2 O 3 -SnO 2 , semiconductor such as polysilicon, and the like. Although the electrode can be formed easily by using a film forming technology such as vacuum deposition with patterning technology such as photolithography and etching, it is permissible to employ other methods (for example, printing).  
         [0048]    The shape of the element electrodes  412  and  413  is designed appropriately corresponding to the application purpose of a given electron emission element. Generally, an electrode interval L is designed by selecting an appropriate value from a range of several hundred Angstrom to several hundred micrometers and particularly a range of several micrometers to several tens of micrometers is preferable for application to the imaging device. Further, as a thickness d of the element electrode, an appropriate value is selected from a range of several hundred Angstroms to several micrometers.  
         [0049]    A particle film is used for the conductive thin film  414 . This film is a film containing a great number of particles as composition element (including island-like aggregates). If the particle film is investigated microscopically, usually, a structure in which individual particles are disposed separately, disposed adjacent to each other, or, overlap each other, is observed.  
         [0050]    The diameter of the particles used for the particle film is within a range of several Angstroms to several thousand Angstroms and more preferably, it is within a range of 10 Angstroms to 200 Angstroms. The thickness of the particle film is set up appropriately considering several conditions described below. The conditions include a condition necessary for connecting to the element electrode  412  or  413  electrically, a condition necessary for executing the electro forming favorably, a condition necessary for setting electric resistance of the particle film itself to an appropriate value and the like.  
         [0051]    Although the particle diameter is in a range of several Angstroms to several thousand Angstroms, more preferably it is within a range of 10 Angstroms to 500 Angstroms.  
         [0052]    The material which can be used to form the particle film includes for example, metals such as Pd, Pt, Ru, Ag, Au, Ti, In, Cu, Cr, Fe, Zn, Sn, Ta, W and Pb, oxides such as PdO, SnO 2 , In 2 O 3 , PbO and Sb 2 O 3 , borides such as HfB 2 , ZrB 2 , LaB 6 , CeB 6 , YB 4  and GdB 4 , carbides such as TiC, ZrC, HfC, TaC, SiC and WC, nitrides such as TiN, ZrN and HfN, semiconductor such as Si and Ge, carbon and the like and any one is selected from these materials appropriately.  
         [0053]    Although as described above, the conductive thin film  414  is formed with a particle film, its sheet resistance value is designed to be in a range of 10 3  to 10 7  (Ohm/sq).  
         [0054]    Because the conductive thin film  414  and the element electrodes  412 ,  413  are desired to be electrically connected, they are so structured as to partially overlap. As for the overlapping manner, although in FIG. 5, the substrate, element electrode and conductive thin film are overlaid in order from the bottom, it is permissible to overlay the substrate, conductive thin film and element electrode in this order from the bottom as needed.  
         [0055]    The electron emission member  415  is a crack-shaped part formed in the conductive thin film  414  and has a higher resistance than the surrounding conductive thin film in terms of electricity. The crack-shaped part is formed by carrying out the electro forming process, which will be described later, on the conductive thin film  414 . In some case, particles of several Angstroms to several hundred Angstroms in diameter are disposed within the crack-shaped part. Because the position and configuration of an actual electron emission member are difficult to represent accurately, FIGS. 5A and 5B show them schematically.  
         [0056]    The thin film  416  is a thin film composed of carbon or carbon compound and covers the electron emission member  415  and its surroundings. The thin film  416  is formed by carrying out the electro activation process, described later, after the electro forming process.  
         [0057]    The thin film  416  is composed of any one of monocrystal graphite, polycrystal graphite and non-crystal carbon or mixture thereof and although the film thickness is set to 500 Angstroms or less, it is more preferably 300 Angstroms or less.  
         [0058]    In the aforementioned electron emission element (display pixel), if a voltage is applied between the element electrodes  412  and  413 , electrons are emitted from the electron emission member  415 . Consequently, electrons emitted from the electron emission member  415  collide with a phosphor dot  442  coated on a glass substrate  441  as shown in FIG. 5A so as to obtain illumination. A voltage is supplied to the phosphor dot  442  through a transparent electrode. The element electrodes  412  and  413  are connected to the column wiring and row wiring described before, respectively.  
         [0059]    [0059]FIG. 6 shows the display section  400  schematically while the scanning driver  101  and the driving circuit  102  are represented in a simple way. In the display section  400 , reference numeral  450  denotes display pixels. The driving circuit  102  has a register  604  for fetching pixel data and a latch circuit  605  for latching the pixel data in this register  601  by the unit of columns.  
         [0060]    Each pixel data in the latch circuit  605  is inputted to a data comparator  606 . Data-comparing portions in the data comparator  606  are prepared in the same number as respective pixels. Each data-comparing portion compares count data from a counter  607  with pixel data. Then, if there is less count data than pixel data, a high level is outputted and if there is more count data than pixel data, a low level is outputted.  
         [0061]    The output of the data-comparing portion is supplied to a gate circuit  608  each having a corresponding gate portion. When a high level is given from a corresponding comparing portion, each gate portion of the gate circuit outputs a voltage of 4V and supplies it to a corresponding row wiring. When a low level is given from a corresponding comparing portion, it outputs a voltage of 0V and supplies it to a corresponding row wiring.  
         [0062]    The data comparator  605  and the gate circuit  608  compose a pulse width modulator. The width of a pulse outputted by the pulse width modulator is variable depending upon the value of the pixel data. If the value of the pixel data is large, the pulse width increases. If the value of the pixel data is small, the pulse width decreases. Consequently, in the display pixel which is supplied with a pulse from the gate circuit  608 , the brightness varies according to the pulse width, that is, gradation is presented.  
         [0063]    The counter  607  is reset by reset pulse R 1  so as to count clock CK 2 . The time between the first reset pulse R 1  and the second one is the value T 1 , which is obtained by dividing the time of the vertical period by the number of display pixels on a single row wiring. However, if the scanning driver  101  selects column wirings by the unit of plural (u) column wirings, the value T 1  is divided by u. The scanning driver  101  is a register which sequentially selects the column wirings and is reset with a reset pulse R 2 . Time between the first reset pulse R 2  and the second one is equal to the time of the vertical period.  
         [0064]    The embodiments of the present invention are not restricted to the above-described examples.  
         [0065]    [0065]FIG. 7 shows yet another embodiment of the embodiment shown in FIG. 2. In a display section  400 , display pixels are arrayed laterally and longitudinally as a matrix of 1280×3(RGB):720. Each red display pixel is connected to one of three column lines.  
         [0066]    On the side of a scanning driver  201 , for example, column lines C 1 , C 4 , C 7 , . . . are connected to red display pixels of each row and the red display pixels of each column are connected to row lines E 1 R-EjR. Column lines C 2 , C 5 , C 8 , . . . are connected to green display pixels of each-row and the green display pixels of each column are connected to row lines E 1 G-EjG. Further, column lines C 3 , C 6 , C 9 , . . . are connected to blue display pixels of each row and the blue display pixels of each column are connected to row lines E 1 B-EjB.  
         [0067]    On the side of a scanning driver  202 , column lines D 1 , D 4 , D 7 , . . . are connected to red display pixels of each row and the red display pixels of each column are connected to row lines F 1 R-FjR. Column lines D 2 , D 5 , D 8 , . . . are connected to green display pixels of each row and the green display pixels of each column are connected to row lines F 1 G-FjG. Further, column lines D 3 , D 6 , D 9 , . . . are connected to blue display pixels of each row and the blue display pixels of each column are connected to row lines F 1 B-FjB.  
         [0068]    With this configuration, the respective scanning drivers  201  and  202  only have to scan (1280×3)/(2×3) lines during the period of one frame. ×3 means that the RGB are scanned at the same time. Therefore, the frequency is determined such that 640 pulses are output during the period of one frame. At this frequency, a more sufficient operating capacity is secured.  
         [0069]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.