Patent Grant
7893908
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2006-132364 filed in Japan Patent Office on May 11, 2006, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display device that employs a liquid crystal panel, of which a liquid crystal projector is representative, and more particularly, to a liquid crystal display device in which a common voltage is supplied to common electrodes to which a plurality of liquid crystal cells are connected in common.
2. Description of the Related Art
As a countermeasure against deterioration of liquid crystals in a liquid crystal display device, an alternating current (AC) drive is implemented in which the polarity of voltage that is applied to the liquid crystals reverses at a prescribed cycle. Such alternating current drives may include, for example, a dot-reversal drive, line-reversal drive, and frame-reversal drive. A liquid crystal display device is driven by one or a combination of a plurality of these drives.
A thin film transistor (TFT) is provided in the portion of intersection between image data line L1 and gate line L2. CLC is the capacitance of the liquid crystal cell, CS is the added capacitance, and CGD is the parasitic capacitance between the gate and drain of the TFT. When a voltage is applied to an object liquid crystal cell (e.g., when writing a video signal), gate line L2 becomes “high” and the TFT becomes conductive.
After writing the video signal, gate line L2 becomes “Low,” following which the written video signal is held. When gate line L2 switches from “High” to “Low,” the liquid crystal cell voltage VLC drops due to the differential effect of the parasitic capacitance CGD. The amount of drop in voltage is uniform without any dependence on the polarity of the video signal. In addition, due to the drop in voltage that results from the combined capacitance of the liquid crystal cell capacitance CLC and the added capacitance CS, common voltage Vcom attains the optimum adjustment value at a voltage that is lower than reference voltage Vref, which is the central value of the image data.
Nematic liquid crystal used in a liquid crystal display device typically has a rod shape and further has a dielectric anisotropy in which the dielectric constant in the direction of the major axis is greater than the dielectric constant in the direction of the minor axis. The liquid crystal molecules are arranged in a substantially horizontal state when in a state in which voltage is not applied, but assume an increasingly vertical state according to the degree of voltage applied. In this way, the dielectric constant varies according to the applied voltage.
When the dielectric constant changes, the electrostatic capacitance also changes. The potential of a liquid crystal cell comes under the influence of the stray capacitance between the gate and drain of the TFT and the combined capacitance of the liquid crystal capacitance and the added capacitance, and the common voltage is adjusted by a voltage that, to the degree of the amount of drop in voltage of the combined capacitance, is lower than the liquid crystal cell potential. The voltage drop of the combined capacitance is changed by the voltage applied to the liquid crystal cell (e.g., image data).
V-T/gamma correction circuit 100 is composed of: V-T correction circuit for correcting the voltage (V)—transmittance (T) characteristic of the liquid crystal panel, which is represented by an S-shaped curve, to a linear characteristic (in which V changes proportionally with respect to T); and a gamma correction circuit for correcting the input/output characteristic to a non-linear form.
Typically, this V-T correction circuit and gamma correction circuit can both be made up from look-up tables (hereinbelow abbreviated as “LUT”). The V-T correction circuit and gamma correction circuit may be realized as two LUTs for separately storing correction data used in the two correction circuits, or may be realized as a single LUT for storing correction data capable of both corrections. Japanese Patent number 3033912 (hereinbelow referred to as “Patent Document 2”) discloses a case in which V-T correction circuit and gamma correction circuit are realized as a single LUT.
A video signal that has undergone V-T/gamma correction by V-T/gamma correction circuit 100 is supplied to liquid crystal drive circuit 101. In liquid crystal drive circuit 101, the video signal from V-T/gamma correction circuit 100 is subjected to line reversal and frame reversal processing. The video signal that has undergone line reversal-frame reversal in liquid crystal drive circuit 101 is then supplied to liquid crystal panel 103. In liquid crystal panel 103, common voltage is supplied from common voltage generation circuit 102.
In the liquid crystal panel driver described in the foregoing explanation, the combined use of line-reversal drive and frame-reversal drive in which line data of positive polarity and line data of negative polarity are switched with each frame can eliminate noticeable flicker even when the adjustment of common voltage Vcom diverges somewhat from the optimum value.
In addition, a “double speed drive system” is also known in which, in frame-reversal drive, a liquid crystal panel is driven at twice the drive frequency of the input video signal (e.g., see JP-A-2006-099034 (hereinbelow referred to as “Patent Document 3”)).
In frame-reversal drive, differences in luminance occur in frame units and the frequency at which flicker occurs (with respect to frame frequency) is therefore low. Flicker for which the frequency of occurrence is low is problematic because it is readily apparent to the human eye.
In contrast, using the frame-reversal double-speed drive system to drive a liquid crystal panel at twice the drive frequency of the input video signal increases the frequency of generation of flicker compared to normal frame reversal drive and can therefore be assumed to render flicker difficult to discern according to the capabilities of the human eye. In the latest high-luminance projectors, however, flicker becomes more readily noticeable with the increase in brightness, even when the frame-reversal double-speed drive system is adopted. The reasons for this problem are described below.
Assuming the frequency of occurrence of flicker is fixed, according to the characteristics of human vision, flicker becomes more easily discernible in proportion to the increase in luminance of the screen. Increasing the angle of field also makes flicker more noticeable. For example, when viewing an image projected onto a screen from the latest high-luminance projector, flicker is more easily noticeable when viewed from a position close to the screen than when viewed from a position far from the screen. Flicker may therefore be noticed when viewing a projected image from a position close to the screen even with a high-luminance projector that adopts the frame-reversal double-speed drive system.
In addition, when the common voltage is not appropriate, giving rise to a difference between the potential of liquid crystal cells and the standard voltage (e.g., reference voltage) that is the standard for the reversal of image data of positive polarity and negative polarity, the difference between the brightness of display images that are based on image data of positive polarity and the brightness of display images that are based on image data of negative polarity increases, and flicker becomes more easily noticed. Normally, in a projector in which the AC drive system is adopted, the projected image is rated by visual inspection or by measuring equipment and the common voltage then adjusted to minimize the amount of flicker.
That is, the common voltage is adjusted and balance established for the voltage applied to the liquid crystal cells by means of the time interval of the application of image data of positive polarity and the time interval of the application of image data of negative polarity, whereby the difference in brightness between the image data of positive polarity and the image of negative polarity is minimized. However, the combined capacitance of the liquid crystal cell capacitance and the added capacitance differ due to the amplitude and polarity of the image data, and as a result, when the common voltage is uniform, a minute difference will inevitably occur between the brightness of displayed images that are based on image data of positive polarity and the brightness of displayed images that are based on image data of negative polarity. This minute difference in brightness is one cause of noticeable flicker.
It is further believed that flicker that is produced by the difference between the common voltage and standard voltage (e.g., reference voltage) can be limited by rapidly modulating the common voltage at, for example, the frame frequency. However, the high-speed modulation of the common voltage is problematic, first because confronting electrodes to which the common voltage is supplied are common to all of the liquid crystal cells, resulting in high electrostatic capacity, and further, because of the low carrier mobility of the silicon material of the wiring portion of the TFT substrate.
In the invention described in Patent Document 1, uniformly adjusting flicker over the entire surface enables a solution to the problem that the optimum value of the common voltage for minimizing flicker differs for the central portion and peripheral portions of the liquid crystal panel. However, Patent Document 1 makes no disclosure regarding the configuration for limiting the occurrence of flicker during frame-reversal double-speed drive. Suppression of the occurrence of flicker during frame-reversal double-speed drive is therefore a problem in the invention disclosed in Patent Document 1.
In the invention disclosed in Patent Document 2, V-T correction and gamma correction can be realized by a simple configuration. However, the invention described in Patent Document 2 does not have a configuration for suppressing the occurrence of flicker during frame-reversal double-speed drive.
Patent Document 3 discloses a method for adjusting the common voltage so as to minimize flicker during frame-reversal double-speed drive, but the danger remains that merely suppressing the occurrence of flicker by adjusting the common voltage leaves open the potential for noticeable flicker and a drop in the quality of displayed images.
In view of the foregoing and other problems, disadvantages, and drawbacks of the related art, it is a purpose of the present invention to provide a liquid crystal display device that may solve the above-described problems and may enable a suppression of the occurrence of flicker during AC drive.
To achieve the above-described purpose, an exemplary aspect of the present invention is directed to a liquid crystal display device which may include a liquid crystal panel including a plurality of liquid crystal cells, a common voltage generation circuit that may supply a fixed common voltage to common electrodes to which the plurality of liquid crystal cells are connected in common, and a liquid crystal drive circuit that may implement control both for supplying the plurality of liquid crystal cells with voltage that accords with a video signal supplied as input from the outside to display images on the liquid crystal panel, and for reversing the polarity of the voltage supplied to the plurality of liquid crystal cells at a prescribed period.
In this exemplary aspect, the liquid crystal display device may also include an adjustment module that may include a memory module for storing characteristics data for correcting fluctuations in brightness that originate from the polarity and amplitude of voltage supplied to the liquid crystal cells, and, based on the characteristics data, may adjust the brightness of display images that are based on image data of positive polarity for which the polarity of the voltage is taken as positive and the brightness of display images that are based on image data of negative polarity for which the polarity of the voltage is taken as negative.
The above-described liquid crystal display device of the present invention may include a configuration that renders flicker difficult to discern through the adjustment by means of the adjustment module of the balance of the brightness of display images that are based on image data of positive polarity and the brightness of display images that are based on image data of negative polarity based on characteristics data that have been obtained in advance.
The optimum value of the common voltage may change according to the polarity and amplitude of image data. Thus, in a configuration in which a fixed DC voltage is supplied as the common voltage, fluctuation may occur in the brightness when there is a great amount of shift in the potential applied to liquid crystal cells at times when the image data is of positive polarity and at times when the image data is of negative polarity, and as a result, flicker becomes easily discernible. According to an exemplary aspect of the present invention, the adjustment module may implement adjustment such that little difference occurs in brightness between display images that are based on image data of positive polarity and display images that are based on image data of negative polarity, whereby flicker becomes difficult to discern.
The present invention as described hereinabove may suppress the occurrence of flicker in frame-reversal double-speed drive, and thus may provide a high-quality image in which flicker is difficult to discern in a high-luminance liquid crystal display device that employs the frame-reversal double-speed drive system.
The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of exemplary embodiments of the invention with reference to the drawings, in which:
Referring to
V-T/gamma correction circuit 10 may be of the same configuration as V-T/gamma correction circuit 100 shown in
Based on a timing signal Vd from control signal generation module 14, switch circuit 11 switches the output destination of the video signal received as input from V-T/gamma correction circuit 10 to either of a first and a second output. The first output is supplied to LUT 12a, and the second output is supplied to LUT 12b.
Data for adjusting the brightness of display images that are based on image data of positive polarity may be stored in advance in LUT 12a, and data for adjusting the brightness of display images that are based on image data of negative polarity are stored in advance in LUT 12b. Adjusting the balance of the brightness of display images that are based on image data of positive polarity and display images that are based on image data of negative polarity by means of these LUTs 12a and 12b may enable the suppression of flicker that is produced by the difference between the value of the standard voltage that is the standard for reversal of image data of positive polarity and image data of negative polarity and the value of the common voltage that is supplied as output from common voltage generation circuit 15.
Control signal generation module 14 may generate timing signal Vd that is an integer multiple (a positive integer multiple, normally “2”) of the vertical synchronizing signal Vsync that indicates the frame period of the video signal from the outside. The timing signal Vd generated by control signal generation module 14 may be supplied to switch circuit 11. The switching of positive polarity LUT 12a and negative polarity LUT 12b may be carried out based on this timing signal Vd.
Common voltage generation circuit 15 generates common voltage Vcom that may be applied to the common electrodes of each liquid crystal cell of liquid crystal panel 16. The value of common voltage Vcom may be adjusted in advance to minimize flicker that is generated by the reversal of the image data. As one method that can be considered for adjusting common voltage Vcom, synchronization of measuring equipment may be first obtained by means of timing signal Vd, each of the brightness of display images relating to image data (e.g., frames) of positive polarity and the brightness of display images relating to image data (e.g., frames) of negative polarity are measured by the measuring equipment, and common voltage Vcom then adjusted to minimize the difference between the two brightness values. Alternatively, a signal in which positive polarity is all white and negative polarity is all black and a signal in which positive polarity is all black and negative polarity is all white may be prepared, following which common voltage Vcom may be adjusted to minimize the difference in brightness between the images that are displayed by the two signals.
Liquid crystal drive circuit 13 may implement control both for supplying to each liquid crystal cell of liquid crystal panel 16 voltage that accords with image data that have been supplied by way of LUTs 12a and 12b to display an image on liquid crystal panel 16, and for reversing the polarity of voltage supplied to each liquid crystal cell at a frequency that is an integer multiple of the frame frequency of the video signal.
Although not shown in
The following explanation discusses the details of the data that may be stored in LUTs 12a and 12b.
Normally, it is known at the time of shipment of a product that a minute difference will occur between the brightness of image data of positive polarity and image data of negative polarity due to the liquid crystal characteristics or the inadequacy of the detection accuracy of flicker in the adjustment of the common voltage. Accordingly, characteristics data that differ according to this minute difference may be stored in LUTs 12a and 12b for adjusting the balance of the brightness of the display images that are based on the image data of positive polarity and the display images that are based on the image data of negative polarity.
Storing the characteristics data shown in
In the example shown in
Adjustment may also be carried out in both LUTs 12a and 12b to reduce the difference in brightness between the display images that are based on image data of positive polarity and the display images that are based on image data of negative polarity. In this case as well, the occurrence of flicker may be suppressed.
Further, in contrast to the example shown in
Still further, the inclination of the characteristics data stored in LUTs 12a and 12b need not be fixed and may be any data that may enable adjustment of the brightness of display images.
In the interest of clarifying the explanation of the relation between data stored in LUTs 12a and 12b, the example shown in
A video signal may be supplied to V-T/gamma correction circuit 10 from the outside, and vertical synchronizing signal Vsyn that is sync-separated from this video signal may be supplied to control signal generation module 14. In V-T/gamma correction circuit 10, the video signal that has been received as input may be subjected to a V-T/gamma correction process. The input video signal contains data (picture element data) for each liquid crystal cell, and may be applied as input to V-T/gamma correction circuit 10 in a state of already having undergone gamma correction. For example, image data received from a broadcast station may have undergone gamma correction that takes into consideration the characteristics of a Braun tube. In V-T/gamma correction circuit 10, the input video signal that has undergone this gamma correction may be subjected to correction (VT correction and gamma correction) that accords with the characteristics of liquid crystal panel 16 for each pixel unit (cell unit).
When the vertical synchronizing signal Vsync is supplied to control signal generation module 14, control signal generation module 14 may generate timing signal Vd of double the input vertical synchronizing signal Vsync and may supply this timing signal Vd to switch circuit 11. Based on timing signal Vd, switch circuit 11 may supply the image data from V-T/gamma correction circuit 10 that have undergone V-T/gamma correction to one of LUTs 12a and 12b.
In accordance with the characteristics data that have been stored in advance, LUT 12a may correct (e.g., adjust the brightness of) the image data that have been supplied by way of switch circuit 11 in pixel units (cell units). Similarly, in accordance with the characteristics data that have been stored in advance, LUT 12b may correct (e.g., adjust the brightness of) image data that have been supplied by way of switch circuit 11 in pixel units (cell units). It is here assumed that the characteristics data shown in
Of the image data from V-T/gamma correction circuit 10 that have undergone V-T/gamma correction, data that are taken to be image data of positive polarity may be supplied to liquid crystal drive circuit 13 by way of LUT 12a, and data that are taken to be image data of negative polarity may be supplied to liquid crystal drive circuit 13 by way of LUT 12b. Liquid crystal drive circuit 13 may drive liquid crystal panel 16 based on the image data that have been supplied by way of LUTs 12a and 12b. In liquid crystal panel 16, the common voltage from common voltage generation circuit 15 may be supplied to a shared common electrode in each liquid crystal cell, and image display may be realized based on the image data from liquid crystal drive circuit 13.
According to the liquid crystal display device of the present embodiment described in the foregoing explanation, two types of LUT may be used while switching according to the polarity of the image data, whereby changes in brightness that are apparent to the eye may be minimized and the occurrence of flicker suppressed. As a result, a configuration can be provided in which flicker is difficult to discern in a liquid crystal projector of the high-luminance frame-reversal double-speed drive system. In addition, the use of an LUT configuration may enable a minimization of the influence exerted by the amplitude of the image data.
In addition, the adoption of a configuration in which switching is implemented according to polarity between two LUTs, one for positive polarity and one for negative polarity, can facilitate application not only to liquid crystal panels in which characteristics that arise from polarity have already been ascertained, but also to liquid crystal panels that will be developed in the future and that have as yet unknown characteristics.
The configuration that employs LUTs may further enable driving the liquid crystal panels of a normally white liquid crystal panel and a normally black liquid crystal panel by the same circuit. Switching between normally white and normally black can be easily handled by preparing data in the descending or ascending order of LUT addresses.
In the present embodiment, the independent formation of an LUT for gamma correction and an LUT for flicker suppression may increase the degree of freedom of adjustment of the two LUTs. In this case, moreover, the present embodiment may allow easy adjustment of gamma correction to realize characteristics according to the preferences of the projector operator.
The liquid crystal display device of the present embodiment as described above is only an example of the present invention and its configuration and operation are open to modification as appropriate within the scope that does not diverge from the gist of the present invention. For example, the switch circuit and LUTs may also be arranged in a section that follows the liquid crystal drive circuit.
In addition, when three liquid crystal panels are provided corresponding to the three primary colors R, G, and B, V-T/gamma correction circuit 10, switch circuit 11, LUTs 12a and 12b, and liquid crystal drive circuit 13 may be provided for each liquid crystal panel. In this case, control signal generation module 14 may generate timing signals for each liquid crystal panel to supply these timing signals to each switch circuit 11. Each liquid crystal panel may also be supplied with a common voltage from the common voltage generation circuit.
Characteristics data for adjusting the brightness of display images that relate to image data of positive polarity or negative polarity may be stored in LUT 20. In offset table 21, characteristics data may be stored for returning (offsetting) the image data in which brightness has undergone adjustment by the characteristics data of LUT 20 to the image data of brightness that preceded adjustment.
Image data from V-T/gamma correction circuit 10 may be supplied to liquid crystal drive circuit 13 by way of LUT 20. Timing signal Vd that has been generated in control signal generation module 14 and the characteristics data of offset table 21 may each be supplied to liquid crystal drive circuit 13. Based on timing signal Vd that has been generated in control signal generation module 14, liquid crystal drive circuit 13 may return those data from among the image data that have been received as input by way of LUT 20 that are taken to be image data of positive polarity or negative polarity to image data of the brightness that preceded adjustment by means of offset table 21. For example, when characteristics data of the LUT for positive polarity shown in
Although liquid crystal drive circuit 13 may carry out an offset process for making the image data of negative polarity the original brightness in the above-described operation, liquid crystal drive circuit 13 may conversely also carry out an offset process for making the image data of positive polarity the original brightness. In this case, characteristics data for adjusting the brightness of display images that relate to image data of negative polarity may be stored in LUT 20.
The liquid crystal display device of the present embodiment may exhibit the same effects as for the above-described first embodiment.
In addition, adopting a configuration having just one LUT may enable a reduction in circuit scale. The present embodiment may be suitable for an already existing liquid crystal panel drive method in which the liquid crystal characteristics have been ascertained.
V-T correction circuit 30 may be basically identical to the V-T correction circuit that makes up V-T/gamma correction circuit 10 and is made up from an LUT.
Gamma correction circuit 31a may be composed of one LUT for storing characteristics data for carrying out both gamma correction and brightness adjustment relating to image data of positive polarity (characteristics data in which are combined: the characteristics data of the LUT relating to the gamma correction circuit that makes up V-T/gamma correction circuit 10, and characteristics data relating to LUT 12a shown in
Gamma correction circuit 31b may be also composed of one LUT for storing characteristics data for carrying out both gamma correction and adjustment of brightness relating to image data of negative polarity (characteristic data in which are combined: the characteristics data of the LUT relating to the gamma correction circuit that makes up V-T/gamma correction circuit 10, and characteristics data relating to LUT 12b shown in
In the liquid crystal display device of the present embodiment, video signals supplied from the outside may first undergo V-T correction in V-T correction circuit 30 and may then be supplied to switch circuit 11. Based on timing signal Vd from control signal generation module 14, switch circuit 11 may supply, of the image data from V-T correction circuit 30, those data that are taken to be image data of positive polarity to gamma correction circuit 31a and may supply data that are taken to be image data of negative polarity to gamma correction circuit 31b.
In gamma correction circuit 31a, image data from V-T correction circuit 30 (e.g., corresponding to image data of positive polarity) may be subjected to gamma correction and display image brightness adjustment. Similarly, in gamma correction circuit 31b, image data from V-T correction circuit 30 (e.g., corresponding to image data of negative polarity) may be subjected to gamma correction and display image brightness adjustment. In this way, adjustment may be realized by means of gamma correction circuits 31a and 31b that may reduce the difference in brightness between the display images that are based on image data of positive polarity and the display images that are based on image data of negative polarity, thereby enabling suppression of the flicker that is generated when a minute difference occurs between the brightness of image data of positive polarity and image data of negative polarity.
The liquid crystal display device of the present embodiment may also exhibit the same effects as the previously described first embodiment.
In addition, the processes for gamma correction and flicker suppression can be carried out by a single LUT, whereby circuit scale may be reduced from the configuration of the first embodiment. However, the degree of freedom of gamma correction and brightness adjustment that use the LUT may also be reduced.
In addition, V-T correction circuit can be integrated with gamma correction circuits 31a and 31b to enable a further reduction of circuit scale. However, this configuration may result in a further reduction of the degree of freedom of the V-T correction, gamma correction, and brightness adjustment that use the LUT.
Liquid crystal drive circuit 40 may implement control both for supplying liquid crystal cells with voltage that accords with image data that have been received as input from V-T/gamma correction circuit 10 to display images on liquid crystal panel 16, and for reversing the polarity of the voltage supplied to each liquid crystal cell at a frequency that is an integer multiple of the frame frequency of the video signals. Liquid crystal drive circuit 40 may further, based on characteristics data that have been obtained in advance from the minute difference in brightness between image data of positive polarity and image data of negative polarity, adjust the brightness of the display images that are based on image data of positive polarity and the brightness of the display images that are based on image data of negative polarity.
This adjustment circuit may include, for example, an amplitude amplification circuit for increasing and decreasing the amplitude of image data that have been received as input; a black level adjustment circuit for adjusting the black level; and/or a control module for controlling the amplitude by means of the amplitude amplification circuit and the adjustment of level by means of the black level adjustment circuit in accordance with the polarity of the image data.
The present embodiment may enable suppression of flicker by means of a configuration having circuit scale equivalent to the prior art. The present embodiment may be particularly effective for liquid crystal panels in which the amount of divergence from the optimum value of common voltage that results from the amplitude of the image data is comparatively small while the influence due to polarity is dominant. However, control of the amplitude or black level of image data of positive polarity or negative polarity may in some cases result in a reduction of the dynamic range or the occurrence of white-out (or black-out). These problems may be in a trade-off relationship with the perception of flicker.
A normally white/normally black liquid crystal panel can be easily handled by inverting each bit of image data.
In each of the above-described first to fourth embodiments, image data may be adjusted by means of, for example, an LUT to suppress flicker, but an equivalent effect can also be obtained by means of modulation, according to the polarity of image data, of the standard voltage (e.g., reference voltage Vref) that is the standard of the reversal of image data of positive polarity and image data of negative polarity.
The present invention as explained hereinabove can be generally applied to liquid crystal display devices that use AC-drive liquid crystal panels in which the polarity of the voltage supplied to a plurality of liquid crystal cells is reversed at a prescribed period, and in particular, when applied to a liquid crystal display device that employs frame-reversal double-speed drive, exhibits the special effect of suppressing flicker.
While exemplary embodiments of the present invention have been described using specific terms, such description is for illustrates purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.
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