JP2008020633A - Liquid crystal display device and illumination control method for the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve decrease in contrast associated with improvement in visibility of a moving image. <P>SOLUTION: A liquid crystal display device includes a liquid crystal display panel DP that periodically carries out video signal display and non-video signal display, a backlight BL illuminating the liquid crystal display panel DP, and a display control circuit CNT that turns on the backlight BL for the video signal display and turns off the backlight for the non-video signal display. The display control circuit applies a driving voltage to the backlight, the driving voltage temporarily set to a start level higher than the lighting level upon starting the video signal display. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a liquid crystal display device that performs video signal display and black or halftone non-video display for each frame period, and flashes a backlight for the video signal display and the non-video signal display, respectively. The present invention relates to a lighting control method.

  A flat display device typified by a liquid crystal display device is widely used to display an image in a computer, a car navigation system, a television receiver, or the like. A liquid crystal display device generally includes a liquid crystal display panel including a matrix array of a plurality of liquid crystal pixels, a backlight that illuminates the liquid crystal display panel, and a display control circuit that controls the display panel and the backlight. The liquid crystal display panel has a structure in which a liquid crystal layer is sandwiched between an array substrate and a counter substrate. The backlight is a lamp light source such as a cold cathode tube, for example, and is driven by, for example, a pulse width modulated high frequency voltage (see Patent Document 1).

  The array substrate has a plurality of pixel electrodes arranged in a substantially matrix, a plurality of gate lines arranged along a row of the plurality of pixel electrodes, a plurality of source lines arranged along a column of the plurality of pixel electrodes, and a plurality of And a plurality of switching elements arranged in the vicinity of the intersection position of the plurality of gate lines and the plurality of source lines. Each switching element is made of a thin film transistor (TFT), for example, and is turned on when the corresponding gate line is driven to apply the potential of the corresponding source line to the corresponding pixel electrode. A common electrode is provided on the counter substrate so as to face the plurality of pixel electrodes arranged on the array substrate. The pair of pixel electrodes and the common electrode constitute a pixel together with a pixel region which is a part of the liquid crystal layer located between these electrodes, and the liquid crystal molecule arrangement is controlled by an electric field between the pixel electrode and the common electrode in the pixel region. The display control circuit includes a gate driver that drives a plurality of gate lines, a source driver that drives a plurality of source lines, and a controller circuit that controls these gate drivers, source drivers, and backlights.

  In the case where the liquid crystal display device is mainly used for a television receiver that displays a moving image, use of an OCB mode liquid crystal display panel in which liquid crystal molecules exhibit good response has been studied (see Patent Document 2). In this liquid crystal display panel, the liquid crystal molecules are in the splay alignment in which the liquid crystal molecules are almost laid down before the power is turned on by the alignment film rubbed in parallel with each other on the pixel electrode and the common electrode. The liquid crystal display panel performs a display operation after the liquid crystal molecules are changed from the splay alignment to the bend alignment by a relatively strong electric field applied in an initialization process when power is turned on.

  The reason why the liquid crystal molecules are splayed before the power is turned on is that the splayed orientation is more stable than the bend orientation in terms of energy when no liquid crystal driving voltage is applied. Once such a liquid crystal transitions to bend alignment, it reversely transitions back to splay alignment when a voltage application state below the level at which splay alignment energy and bend alignment energy antagonize or when no voltage is applied for a long period of time. It has the property of end up. In the splay alignment, the viewing angle characteristics are significantly different from the bend alignment, resulting in abnormal display.

Conventionally, in order to prevent reverse transition from bend alignment to splay alignment, for example, a driving method in which a large liquid crystal driving voltage is applied to the liquid crystal during a part of a frame period in which an image is displayed has been adopted. In a normally white liquid crystal display panel, since this liquid crystal driving voltage corresponds to a black display voltage, this is called black insertion driving.
JP-A-6-310294 JP 2002-202491 A

  By the way, since the liquid crystal display panel is a hold type display device that holds the display state until the video signal is updated, it is difficult to smoothly show the movement of the object due to the influence of the retinal afterimage generated in the visual perception of the observer in the moving image display. The above-described black insertion drive is effective in improving the visibility of the moving image, which is deteriorated by the observer's vision, because the retinal afterimage is cleared by changing the pixel luminance to a pseudo discrete impulse response waveform. In order to prevent the reverse transition described above, the black insertion rate is normally set to about 20% as a ratio of the black insertion period (non-video signal display period) in one frame period, but this black insertion rate is increased to about 50%. As a result, it is possible to obtain a moving image visibility that is comparable to a CRT and has no sense of incongruity.

  However, the increase in the black insertion rate decreases the brightness of white display averaged over one frame period. As a result, the contrast decreases from about 500 to about 285. Furthermore, since the backlight was always lit including the black insertion period during which no illumination is required, power was wasted.

  Conventionally, the backlight blinking drive has been proposed as a technique for reducing power consumption while reducing the luminance of black display by not illuminating the display panel during the black insertion period. When the backlight is composed of a plurality of lamp light sources that illuminate a plurality of display areas that divide the liquid crystal display panel, each lamp light source is turned off at the start timing of the black insertion period of the corresponding display area, and the black insertion period It is controlled to turn on at the end timing. When a black insertion drive of a liquid crystal display panel is performed with a black insertion rate of 50% and a backlight blinking drive is performed with a duty ratio equal to the black insertion rate (lighting period with respect to one frame period), good video visibility In addition, a contrast of about 390 can be obtained. However, this contrast value was insufficient compared to the value obtained at the black insertion rate = 20%.

  An object of the present invention is to provide a liquid crystal display device and an illumination control method thereof that can improve contrast that decreases with improvement in moving image visibility.

  According to the present invention, a liquid crystal display panel that periodically performs video signal display and non-video signal display, an illumination light source that illuminates the liquid crystal display panel, and an illumination light source that is turned on for video signal display and used for non-video signal display A light source control unit that turns off the illumination light source, and the light source control unit is configured to apply a drive voltage that is temporarily set to a starting level larger than the lighting level to the illumination light source when starting the video signal display. A display device is provided.

  Furthermore, according to the present invention, there is provided an illumination control method for a liquid crystal display device comprising: a liquid crystal display panel that periodically displays video signals and non-video signals; and an illumination light source that illuminates the liquid crystal display panel. The illumination control method of turning on the illumination light source, turning off the illumination light source for non-video signal display, and applying a drive voltage that is temporarily set to a starting level higher than the lighting level to the illumination light source at the start of video signal display Is provided.

  According to the inventor's consideration, since the response speed of the illumination light source is slow when blinking driving of the illumination light source is performed, it takes time for the light intensity of the illumination light source to reach a sufficient value after the drive voltage is applied. Was confirmed. In the above-described liquid crystal display device and illumination control method, the driving voltage of the illumination light source is temporarily set to a starting level that is higher than the lighting level at the start of video signal display. In this way, the light intensity of the illumination light source rises sharply immediately after the drive voltage is applied. Therefore, it is possible to improve the contrast that is lowered when the lighting period of the illumination light source is shortened to improve the visibility of the moving image.

  Hereinafter, a liquid crystal display device according to an embodiment of the present invention will be described with reference to the accompanying drawings.

  FIG. 1 schematically shows a circuit configuration of the liquid crystal display device. The liquid crystal display device includes a liquid crystal display panel DP, a backlight BL that illuminates the display panel DP, and a display control circuit CNT that controls the display panel DP and the backlight BL. The liquid crystal display panel DP has a structure in which a liquid crystal layer 3 is sandwiched between an array substrate 1 and a counter substrate 2 which are a pair of electrode substrates. In the liquid crystal layer 3, for example, normally white liquid crystal molecules are preliminarily transitioned from the splay alignment to the bend alignment, and the reverse transition from the bend alignment to the splay alignment is periodically applied and blocked by a voltage resulting in black display. Liquid crystal material. The display control circuit CNT controls the transmittance of the liquid crystal display panel DP by the liquid crystal driving voltage applied from the array substrate 1 and the counter substrate 2 to the liquid crystal layer 3. The transition from the splay alignment to the bend alignment can be obtained by applying a relatively large electric field to the liquid crystal by a predetermined initialization process performed by the display control circuit CNT when the power is turned on.

  FIG. 2 shows the cross-sectional structure of the liquid crystal display panel DP in detail. The array substrate 1 includes a transparent insulating substrate GL made of a glass plate or the like, a plurality of pixel electrodes PE formed on the transparent insulating substrate GL, and an alignment film AL formed on the pixel electrodes PE. The counter substrate 2 is a transparent insulating substrate GL made of a glass plate or the like, a color filter layer CF formed on the transparent insulating substrate GL, a common electrode CE formed on the color filter layer CF, and on the common electrode CE. It includes an alignment film AL to be formed. The liquid crystal layer 3 is obtained by filling the gap between the counter substrate 2 and the array substrate 1 with a liquid crystal material. The color filter layer CF includes a red coloring layer for red pixels, a green coloring layer for green pixels, a blue coloring layer for blue pixels, and a black coloring (light-shielding) layer for black matrix. In FIG. 2, the liquid crystal molecules are in a splay alignment state. The liquid crystal display panel DP includes a pair of retardation plates RT arranged outside the array substrate 1 and the counter substrate 2, a pair of polarizing plates PL arranged outside the retardation plates RT, and the array substrate 1 side. A light source backlight BL is provided outside the polarizing plate PL. The alignment film AL on the array substrate 1 side and the alignment film AL on the counter substrate 2 side are rubbed in parallel with each other. Thereby, the pretilt angle of the liquid crystal molecules is set to about 10 °.

  In the array substrate 1, a plurality of pixel electrodes PE are arranged in a substantially matrix shape on the transparent insulating substrate GL. In addition, a plurality of gate lines Y (Y1 to Ym) are arranged along the rows of the plurality of pixel electrodes PE, and a plurality of source lines X (X1 to Xn) are arranged along the columns of the plurality of pixel electrodes PE. . A plurality of pixel switching elements W are arranged in the vicinity of the intersection position of the gate line Y and the source line X. Each pixel switching element W is formed of a thin film transistor in which the gate is connected to the gate line Y and the source-drain path is connected between the source line X and the pixel electrode PE, and corresponds to when driven through the corresponding gate line Y. Conduction is established between the source line X and the corresponding pixel electrode PE.

  Each pixel electrode PE and common electrode CE is made of a transparent electrode material such as ITO, for example, and is covered with an alignment film AL and controlled to a liquid crystal molecular arrangement corresponding to the electric field from the pixel electrode PE and common electrode CE. A liquid crystal pixel PX is configured together with a pixel region that is a part of the pixel area.

  Each of the plurality of liquid crystal pixels PX has a liquid crystal capacitance CLC between the pixel electrode PE and the common electrode CE. The plurality of auxiliary capacitance lines C1 to Cm are each capacitively coupled to the pixel electrode PE of the liquid crystal pixel PX in the corresponding row to form an auxiliary capacitance Cs. The auxiliary capacitor Cs has a sufficiently large capacitance value with respect to the parasitic capacitance of the pixel switching element W.

  The display control circuit CNT includes a gate driver YD that sequentially drives the plurality of gate lines Y1 to Ym so that the plurality of switching elements W are conducted in units of rows, and a period in which the switching elements W in each row are conducted by driving the corresponding gate lines Y. , A source driver XD for outputting the pixel voltage Vs to the plurality of source lines X1 to Xn, a backlight driver LD for driving the backlight BL, a driving voltage generating circuit 4 for generating a driving voltage for the display panel DP, and A controller circuit 5 that controls the gate driver YD, the source driver XD, and the backlight driver (inverter) LD is provided.

  The driving voltage generation circuit 4 generates a compensation voltage generation circuit 6 that generates a compensation voltage Ve applied to the auxiliary capacitance line C through the gate driver YD, and a predetermined number of gradation reference voltages VREF used by the source driver XD. And a common voltage generating circuit 8 for generating a common voltage Vcom applied to the common electrode CT. The controller circuit 5 includes a vertical timing control circuit 11 that generates a control signal CTY for the gate driver YD based on the synchronization signal SYNC input from the external signal source SS, and a source based on the synchronization signal SYNC input from the external signal source SS. A horizontal timing control circuit 12 that generates a control signal CTX for the driver XD, a video processing circuit 13 that performs conversion for black insertion driving to add a black signal (non-video signal) to a video signal input from the external signal source SS, and An inverter control circuit 14 that controls the backlight drive unit (inverter) LD based on the control signal CTX output from the vertical timing control circuit 11 and the black insertion rate setting signal BKS from the external signal source SS is included. The video signal and the black signal include a plurality of pixel data for the liquid crystal pixels PX (horizontal pixel lines) in each row, and are updated every frame period (V = vertical scanning period). The control signal CTY is supplied to the gate driver YD, and the control signal CTX is supplied from the video processing circuit 13 to the source driver XD together with the pixel data DO of the conversion result. The control signal CTY is used for vertical timing control of the gate driver YD required for driving the plurality of gate lines Y1 to Ym, and the control signal CTX is used for horizontal timing of the source driver XD required for driving the plurality of source lines. Used for control.

  In black insertion driving, black insertion writing and video signal writing are performed in units of a predetermined number of horizontal pixel lines in each frame period. For this reason, the gate driver YD drives a plurality of gate lines Y1 to Ym in parallel for a black insertion writing (non-video signal writing) in parallel and a plurality of gate lines Y1 to Ym for video signal writing. Are controlled by a control signal CTY so as to sequentially drive a predetermined number. Further, the source driver XD converts pixel data DO for the liquid crystal pixels PX in each row output in series from the video processing circuit 13 as a conversion result into a pixel voltage using the gradation reference voltage VREF, and a plurality of source lines are converted by these pixel voltages. The control signals CTX are controlled so as to drive X1 to Xn in parallel and periodically invert the polarities of these pixel voltages. The pixel voltage is a voltage applied to the pixel electrode PE with reference to the common voltage Vcom of the common electrode CE. Here, the black insertion period is from black insertion writing of the first horizontal pixel line to video signal writing of the first horizontal pixel line, and the ratio of the black insertion period to one frame period, that is, the black insertion rate is set to about 50%. Is done.

  FIG. 3 shows the relationship between the backlight BL and the liquid crystal display panel DP shown in FIG. The display screen DS shown in FIG. 3 includes a plurality of liquid crystal pixels PX arranged in a matrix. The backlight BL includes, for example, k lamp light sources LP arranged at a predetermined pitch in parallel with the rows of the plurality of liquid crystal pixels PX on the back surface of the display panel DP. These lamp light sources LP mainly illuminate a plurality of display areas in which the screen DS is equally divided in the vertical direction. Each of these lamp light sources LP is composed of one cold cathode tube (CCFL) which is a discharge lamp. In the actual display panel DP, the screen DS is divided into, for example, 24 display areas, and each display area is set to include liquid crystal pixels PX for approximately 25 rows (lines). In this case, each of the 24 cold-cathode tubes illuminates about 25 rows of liquid crystal pixels PX as an illumination target. Each lamp light source LP may be composed of a plurality of cold cathode tubes.

  The inverter control circuit 14 controls the backlight driver LD so that each of the plurality of lamp light sources LP blinks in accordance with the video signal display period and the black insertion period of the corresponding display area (specifically, the central horizontal pixel line). To do. As a result, the plurality of lamp light sources LP are sequentially turned on and off at a predetermined time difference. For example, when a black insertion rate of 50% is set by the black insertion rate setting signal BKS, the video signal display period and the black insertion period are each set to a length of 50% with respect to one frame period, and the backlight driving is performed. The section LD turns off the lamp light source LP at the start timing of the black insertion period (= end timing of the video signal display period), and turns on the lamp light source LP at the end timing of the black insertion period (= start timing of the video signal display period). .

  FIG. 4 shows, as a comparative example, the response waveform of the lamp light source LP obtained when the backlight drive unit LD outputs a drive voltage in the conventional format. The backlight driver LD outputs a high-frequency voltage having a waveform as shown in FIG. 4 as a driving voltage for the lamp light source LP. The lamp light source LP is turned on when the driving voltage rises to the lighting level Von, and is turned off when the driving voltage falls to the turn-off level Voff. The drive voltage is maintained at the lighting level Von for the length of the video signal display period, and is maintained at the extinguishing level Voff for the length of the black insertion period. However, as shown in FIG. 4, the light intensity of the lamp light source LP changes with a great delay from the ideal response waveform corresponding to the waveform of the drive voltage. The effective illumination light amount is obtained by integrating the light intensity over the video signal display period, but the integrated value is significantly lower than that of an ideal response waveform.

  For this reason, the backlight drive unit LD is configured to output a high-frequency voltage having a waveform as shown in FIG. 5 as a drive voltage of the lamp light source LP. The drive timing is the same as in the comparative example shown in FIG. That is, the lamp light source LP is turned on when the driving voltage rises to the lighting level Von, and is turned off when the driving voltage falls to the extinguishing level Voff. The drive voltage is maintained at the lighting level Von for the length of the video signal display period, and is maintained at the extinguishing level Voff for the length of the black insertion period. On the other hand, since the driving voltage is temporarily set to a starting level Vh that is higher than the lighting level Von at the start of video signal display, the light intensity of the lamp light source LP corresponds to the waveform of the driving voltage as shown in FIG. Transition to an ideal response waveform without significant delay. In FIG. 5, the frequency of the high frequency voltage is set to 39 kHz, the start level Vh is set to 120% of the lighting level Von, and the start level period Th is set to 24% of the video signal display period. As a result, the effective illumination light quantity is not significantly reduced as compared with an ideal response waveform.

  As described above, according to the present embodiment, since the driving voltage of the lamp light source LP is temporarily set to the starting level Vh that is higher than the lighting level Von when the video signal display is started, the light intensity of the lamp light source LP is driven. It starts to rise sharply immediately after the voltage is applied. Therefore, it is possible to improve the contrast which is lowered when the lighting period of the lamp light source LP is shortened for improving the visibility of the moving image. When the contrast was actually measured, a value of 490 was obtained. This value is almost equivalent to the value obtained with a black insertion rate of 20%.

  In addition, this invention is not limited to the above-mentioned embodiment, It can deform | transform variously in the range which does not deviate from the summary.

  In the above-described embodiment, the driving voltage of the lamp light source LP is temporarily set to the starting level Vh that is temporarily higher than the lighting level Von at the start of the video signal display. It is also effective to slightly advance within a range that does not exceed the response time required for the transition of the rising of the light intensity shown in FIG. 3, and as a result, a better contrast can be obtained.

  In the above-described embodiment, the OCB mode liquid crystal display panel DP is used. However, the present invention can also be applied to other liquid crystal display panels that periodically perform video signal display and non-video signal display. However, the liquid crystal mode with a fast response speed such as the OCB mode is more effective.

It is a figure which shows schematically the circuit structure of the liquid crystal display device which concerns on one Embodiment of this invention. It is a figure which shows the cross-section of the liquid crystal display panel shown in FIG. It is a figure which shows the relationship between the backlight shown in FIG. 1, and a display panel. It is a figure which shows the response waveform of the lamp light source obtained when a drive voltage is output with the conventional format from the backlight drive part shown in FIG. 1 as a comparative example. It is a figure which shows the response waveform of the lamp light source which applied the drive voltage actually output from the backlight drive part shown in FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Array substrate, 2 ... Opposite substrate, 3 ... Liquid crystal layer, 4 ... Drive voltage generation circuit, 5 ... Controller circuit, 6 ... Compensation voltage generation circuit, 7 ... Tone reference voltage generation circuit, 8 ... Common voltage generation circuit , 11 ... Vertical timing control circuit, 12 ... Horizontal timing control circuit, 13 ... Image data conversion circuit, 14 ... Inverter control circuit, BL ... Back light, LP ... Back light source, DP ... Liquid crystal display panel, PE ... Pixel electrode, CE ... Common electrode, CLC ... Liquid crystal capacitor, Cs ... Auxiliary capacitor, PX ... Liquid crystal pixel, W ... Switching element, Y ... Gate line, X ... Source line, CNT ... Display control circuit, LD ... Backlight drive unit, YD ... Gate driver, XD ... Source driver.

Claims (8)

  A liquid crystal display panel that periodically performs video signal display and non-video signal display, an illumination light source that illuminates the liquid crystal display panel, and the illumination light source that is lit to display the video signal and the illumination that displays the non-video signal A light source control unit that turns off the light source, and the light source control unit is configured to apply a driving voltage, which is temporarily set to a starting level higher than a lighting level, to the illumination light source when starting the video signal display. A liquid crystal display device.   The illumination light source includes a plurality of lamp light sources that respectively illuminate a plurality of display areas dividing the liquid crystal display panel, and the light source control unit is configured to sequentially turn on the plurality of lamp light sources at a predetermined time difference. The liquid crystal display device according to claim 1.   The liquid crystal display device according to claim 1, wherein the liquid crystal display panel has a structure in which a liquid crystal layer is sandwiched between a pair of substrates and liquid crystal molecules are bend-aligned in the liquid crystal layer.   The liquid crystal display device according to claim 1, wherein the non-video signal display is black or a halftone display close to black.   An illumination control method for a liquid crystal display device comprising: a liquid crystal display panel that periodically displays video signals and non-video signals; and an illumination light source that illuminates the liquid crystal display panel, wherein the illumination light source is used for displaying the video signal. The illumination light source is turned off for displaying the non-video signal, and a driving voltage that is temporarily set to a starting level larger than the lighting level is applied to the illumination light source at the start of the video signal display. Lighting control method.   6. The illumination control method according to claim 5, wherein a plurality of lamp light sources that respectively illuminate a plurality of display areas obtained by dividing the liquid crystal display panel as the illumination light source are sequentially turned on at a predetermined time difference.   The illumination control method according to claim 5, wherein the liquid crystal display panel has a structure in which a liquid crystal layer is sandwiched between a pair of substrates and liquid crystal molecules are bend-aligned in the liquid crystal layer.   6. The illumination control method according to claim 5, wherein the non-video signal display is black or halftone display close to black.

Images