Patent Grant
9324255
1. Technical Field
The present invention relates to a technique for performing gray-scale display control by a subfield driving method.
2. Related Art
As a method for expressing gray scales in an electro-optic device using electro-optic elements such as liquid crystal, so-called subfield driving has been known. In the subfield driving, one frame is divided into a plurality of subfields. The subfield driving is a method for performing gray-scale expression using a combination of ON and OFF of the plurality of subfields as a temporal integral value. The number of gray scales capable of being expressed in the subfield driving is determined in principle by the number of subfields. That is, for increasing the number of gray scales, it is necessary to increase the number of subfields per frame. In contrast to this, JP-A-2007-148417 discloses a technique for increasing the number of gray scales capable of being expressed by utilizing the transient response characteristics of liquid crystal without increasing the number of subfields per frame.
In recent years, systems which allow a user to view a three-dimensional (3D) video are under development. One example of methods for allowing a user to view a 3D video is a frame sequential method. The frame sequential method is a method which alternately displays time-divisionally a left-eye image and a right-eye image in a display device to allow a user to view the video via glasses whose shutters for the left eye and the right eye are opened and closed in synchronization with the video. In the case of a two-dimensional (2D) video, all subfields of one frame can be used to perform gray-scale expression. In the case of a 3D video, however, only one-half as many as the subfields of a 2D video can be used at most because a left-eye image and a right-eye image are displayed in one frame. Further in the frame sequential method, since a period during which both of the shutters for the left eye and the right eye are closed is disposed for reducing crosstalk between a left-eye image and a right-eye image, also subfields of this period cannot be used for gray-scale expression. The problem that the number of subfields capable of being used for gray-scale expression is limited occurs not only in a 3D video system, but also in a system or the like in which illumination is turned off in a pulse fashion in synchronization with the video for improving the quality of a moving image. This problem is common to systems in which a video is viewed via a blocking unit which blocks the field of view in a predetermined non-viewing period.
An advantage of some aspects of the invention is to provide a technique for increasing the number of gray scales capable of being expressed in a system in which a video is viewed via a blocking unit which blocks the field of view in a predetermined non-viewing period.
An aspect of the invention provides an electro-optic device including: a plurality of electro-optic elements which are viewed via a blocking unit which blocks the field of view in a predetermined non-viewing period, and each of which is brought into an optical state corresponding to a supplied signal; a converting unit which converts, based on a video signal indicating a video divided into a plurality of frames, a gray-scale value input for each of the frames which is composed of a subfields into a subfield code indicating a combination of ON and OFF of b (2≦b≦a) subfields included in a viewing period other than the non-viewing period and c (1≦c≦b) subfields included in the non-viewing period; and a driving unit which drives the plurality of electro-optic elements by supplying, based on the subfield code converted by the converting unit, the signal for controlling the optical state of each of the plurality of electro-optic elements.
According to this electro-optic device, it is possible to increase the number of gray scales capable of being expressed, compared to the case where gray-scale expression is performed only using subfields of the viewing period.
In a preferred aspect, the converting unit may perform, on a gray-scale value of a current frame as an object to be processed in the plurality of frames, the conversion based on the gray-scale value in the current frame and an optical state of the electro-optic element in an immediately previous frame one frame before the current frame.
According to this electro-optic device, it is possible to control the gray scale also in consideration of the optical state of the immediately previous frame.
In another preferred aspect, the electro-optic device may further include a storage unit which stores a table in which a pair of a gray-scale value and the subfield code are recorded for each of optical states of the immediately previous frame, and the converting unit may perform the conversion with reference to the table stored in the storage unit.
According to this electro-optic device, the conversion to a subfield code can be performed using the table.
In still another preferred aspect, the table may include an identifier indicating an optical state corresponding to the gray-scale value for each of the subfield codes, the storage unit may store the identifier in the immediately previous frame, and the converting unit may perform the conversion based on the identifier and the table stored in the storage unit.
According to this electro-optic device, the identifier included in the table can be used as information indicating the optical state of the immediately previous frame.
In yet another preferred aspect, the response time of the electro-optic element may be longer than the subfield.
According to this electro-optic device, it is possible to increase the number of gray scales capable of being expressed, compared to the case where gray-scale expression is performed only using subfields of the viewing period, in a system using an electro-optic element whose response time is longer than the subfield.
In still yet another preferred aspect, the video signal may indicate a three-dimensional video including a left-eye image and a right-eye image which are alternately switched time-divisionally.
According to this electro-optic device, it is possible to increase the number of gray scales capable of being expressed, compared to the case where gray-scale expression is performed only using subfields of the viewing period, in a system which displays a 3D video.
In further another preferred aspect, the blocking unit may have a light source which is turned on in the viewing period and turned off in the non-viewing period, and the plurality of electro-optic elements may modulate light from the light source according to the optical state.
According to this electro-optic device, it is possible to increase the number of gray scales capable of being expressed, compared to the case where gray-scale expression is performed only using subfields of the viewing period, in a system which performs pseudo-impulse display.
Another aspect of the invention provides an electronic apparatus including the electro-optic device according to any of the aspects described above.
According to this electronic apparatus, it is possible to increase the number of gray scales capable of being expressed, compared to the case where gray-scale expression is performed only using subfields of the viewing period.
The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements.
1-1. Problem Point of Three-Dimensional Display System Using Subfield Driving
Before proceeding to the description of a video display system according to a first embodiment, a problem point of a three-dimensional (3D) video display system using subfield driving will be described. The 3D video display system has a display device and shutter glasses. A 3D video signal indicates a 3D video including a left-eye image and a right-eye image which are alternately switched time-divisionally. The display device alternately displays time-divisionally the left-eye image and the right-eye image according to the 3D video signal. The shutter glasses have a left-eye shutter and a right-eye shutter which are controlled independently of each other. A user views the displayed video via the shutter glasses (3D glasses or stereoscopic vision glasses). The left-eye shutter and the right-eye shutter are shutters which block light entering the left eye and the right eye, respectively. The opening and closing of the left-eye shutter and the right-eye shutter are controlled so as to be synchronized with the left-eye image and the right-eye image.
When a two-dimensional (2D) video is displayed in this display system, 20 subfields are used for displaying one image. That is, the number of subfields capable of being used for gray-scale expression is 20. The number of combinations (to be precise, permutations) of ON and OFF of 20 subfields is 220=1,048,576. That is, when 20 subfields are used, expression ability of up to 1,048,576 gray scales is provided in theory. When a 3D video is displayed with this system, a left-eye frame and a right-eye frame each have 10 subfields. That is, the number of subfields capable of being used for gray-scale expression is 10. The number of combinations of ON and OFF of 10 subfields is 210=1,024. That is, when the time length is reduced to half in this system, expression ability is reduced to about 1/1000 due to that alone.
In a 3D video display system, in addition to the problem that the time length of a frame is reduced to half, there is further a problem of a non-viewing period. In this example, a liquid crystal panel is used as a shutter for shutter glasses. The shutter is an opened state when the liquid crystal panel has a high transmittance ratio (for example, a transmittance ratio of 90% or more), while the shutter is in a closed state when the liquid crystal panel has a low transmittance ratio (for example, a transmittance ratio of 10% or less).
In the example of (A) in
For reducing the crosstalk, it is necessary to provide a period during which the left-eye shutter and the right-eye shutter are both closed. In the example of (B) of
1-2. Outline of Gray-Scale Expression in the Embodiment
In the above description, attention is focused only on the response time of shutter glasses. However, the response time exists also in the display device. When this response time is longer than one subfield, the optical state of a display element in the viewing period is affected by a voltage applied to the display element in the non-viewing period before the viewing period. That is to say, the state of a display element in the non-viewing period affects the optical state of the display element in the viewing period. In the embodiment, this characteristic is utilized to perform gray-scale expression.
Here, a description will be made using an example in which in a display device, the response time of the optical state of a display element to transition from a dark state (luminance is 10% or less) to a bright state (luminance is 90% or more) and the response time to transition from the bright state to the dark state are both 2.0 msec. For simplicity's sake, an example is used in which the transmittance ratio of shutter glasses changes in the form of rectangular wave after 2.5 msec after receiving a signal for causing a transition to the open state or close state. That is, first to third subfields of 10 subfields constitute the non-viewing period, and fourth to tenth subfields constitute the viewing period.
For example, in the case where 256 gray scales (eight bits) with γ=2.2 are expressed, the 111th gray scale can be expressed when “001” is used as the subfield code of the non-viewing period in the above example, while the 83rd gray scale can be expressed when “100” is used.
1-3. Configuration
The light valves 210R, 210G, and 210B are each a device which modulates light, and have liquid crystal panels 100R, 100G, and 100B, respectively. On the liquid crystal panel 100, minified images of the respective colors are formed. The minified images formed respectively by the liquid crystal panels 100R, 100G, and 100B, that is, modulated lights are incident from three directions on the dichroic prism 240. The R light and the B light are reflected at the dichroic prism 240 by 90 degrees, while the G light goes straight. Accordingly, after the respective color images are combined, a color image is projected onto the screen 3000 through the projection lens 250.
Since lights respectively corresponding to R, G, and B are incident on the liquid crystal panels 100R, 100G, and 100B through the dichroic mirrors 2301, it is not necessary to dispose a color filter. Moreover, transmission images of the liquid crystal panels 100R and 100B are projected after being reflected by the dichroic prism 240, whereas a transmission image of the display panel 100G is projected as it is. Accordingly, the horizontal scanning direction of the liquid crystal panels 100R and 100B is opposite to the horizontal scanning direction of the display panel 100G, so that an image whose left and right are inversed is displayed on the liquid crystal panels 100R and 100B.
The liquid crystal panel 100 is a device which displays an image corresponding to a supplied signal. The liquid crystal panel 100 has a display area 101. A plurality of pixels 111 are arranged in the display area 101. In this example, m rows and n columns of pixels 111 are arranged in a matrix. The liquid crystal panel 100 has an element substrate 100a, a counter substrate 100b, and a liquid crystal layer 105. The element substrate 100a and the counter substrate 100b are bonded together with a constant gap therebetween. The liquid crystal layer 105 is interposed between the element substrate 100a and the counter substrate 100b. On the element substrate 100a, m scanning lines 112 and n data lines 114 are disposed. The scanning lines 112 and the data lines 114 are disposed on a surface facing the counter substrate 100b. The scanning line 112 and the data line 114 are electrically insulated from each other. The pixel 111 is disposed corresponding to an intersection of the scanning line 112 and the data line 114. The liquid crystal panel 100 has m×n pixels 111. A pixel electrode 118 and a TFT (Thin Film Transistor) 116 are individually disposed corresponding to each of the pixels 111 on the element substrate 100a. Hereinafter, when the plurality of scanning lines 112 are distinguished from one another, they are referred to as, beginning at the top in
A common electrode 108 is disposed on the counter substrate 100b. The common electrode 108 is disposed on one surface facing the element substrate 100a. The common electrode 108 is common to all of the pixels 111. That is, the common electrode 108 is a so-called solid electrode which is disposed over the substantially entire surface of the counter substrate 100b.
When a signal indicating a voltage at H (High) level is input to the scanning line 112 in the ith row, electrical continuity is established between the source and drain of the TFT 116. When electrical continuity is established between the source and drain of the TFT 116, the pixel electrode 118 has the same potential as that of the data line 114 in the jth column (if an on-resistance between the source and drain of the TFT 116 is ignored). A voltage (hereinafter referred to as “data voltage”, and a signal indicating the data voltage is referred to as “data signal”) corresponding to the gray-scale value of the pixel 111 in the ith row and jth column is applied to the data line 114 in the jth column according to the video signal Vid-in. A common potential LCcom is given to the common electrode 108 by a circuit (not shown). A temporally constant potential Vcom (in this example, Vcom=LCcom) is given to the capacitive line 115 by a circuit (not shown). That is, a voltage corresponding to a difference between the data voltage and the common potential LCcom is applied to the liquid crystal element 120. Hereinafter, a description will be made using an example in which the liquid crystal layer 105 is of VA (Vertical Alignment) type with a normally black mode where the gray scale of the liquid crystal element 120 is in a dark state (black state) when no voltage is applied. Unless otherwise noted, a ground potential which is not shown in the drawing is the standard of voltage (0 V).
Since the liquid crystal panel 100 is driven by subfield driving, the absolute value of a voltage to be applied to the liquid crystal element 120 is one of two values, VH (one example of the first voltage, for example, 5V) and VL (one example of the second voltage, for example, 0 V).
Referring to
The scanning line driving circuit 130 is a circuit which outputs a scanning signal Y according to the control signal Yctr. A scanning signal to be supplied to the scanning line 112 in the ith row is referred to as a scanning signal Yi. In this example, the scanning signal Yi is a signal for sequentially and exclusively selecting one scanning line 112 from the m scanning lines 112. The scanning signal Yi is a signal which serves as a selection voltage (H level) for the scanning line 112 to be selected, while serving as a non-selection voltage (L (Low) level) for the other scanning lines 112. Instead of the driving of sequentially and exclusively selecting one scanning line 112, a so-called MLS (Multiple Line Selection) driving in which the plurality of scanning lines 112 are simultaneously selected may be used.
The data line driving circuit 140 is a circuit which samples the data signal Vx according to the control signal Xctr to output a data signal X. A data signal to be supplied to the data line 114 in the jth column is referred to as a data signal Xj.
The converting section 302 is one example of the converting unit 21. The control section 304, the scanning line driving circuit 130, and the data line driving circuit 140 are one example of the driving unit 22. The memory 301 is one example of the storage unit 23.
1-4. Operation
Referring to
In Step S110, the control section 304 generates a signal corresponding to the subfield code of the object pixel, and outputs this signal as the data signal Vx. More specifically, the control section 304 reads a code of the corresponding subfield from the frame memory 303 at a timing indicated by the start signal DY. For example, when the timing of a first subfield is indicated by the start signal DY, the control section 304 reads, from the frame memory 303, a code “1” of the first subfield in the subfield code “100-1110100” of the object pixel. The control section 304 generates a signal of a voltage (for example, the voltage VH) corresponding to the code “1”, and outputs this signal as the data signal Vx. In another example, when the timing of a second subfield is indicated by the start signal DY, the control section 304 reads, from the frame memory 303, a code “0” of the second subfield in the subfield code “100-1110100” of the object pixel. The control section 304 generates a signal of a voltage (for example, the voltage VL) corresponding to the code “0”, and outputs this signal as the data signal Vx.
The data line driving circuit 140 has a latch circuit (not shown) and holds data corresponding to one row. The control section 304 sequentially outputs the data signal Vx for the pixels 111 in the first to nth columns, and the data line driving circuit 140 holds data of the first to nth columns. At a timing at which the data line driving circuit 140 holds data of kth subfields in the ith row and first to nth columns, the scanning line driving circuit 130 selects the scanning line 112 in the ith row. In this manner, the data of the kth subfields are written to the pixels 111 in the ith row. When writing of data to the mth row is completed, data of (k+1)th subfields are then written sequentially. By repeating the process described above, the liquid crystal element 120 shows a transmittance ratio corresponding to a subfield code.
According to the embodiment, even when the number of subfields of the viewing period is b, expression of gray scales more than b bits (2b gray scales) can be performed by controlling data signals of the c subfields of the non-viewing period.
When observing the subfield codes stored in the LUT 3011 overall of the gray scales, at least one of the c subfields of the non-viewing period of one gray scale is sometime different in state (ON or OFF) from at least one of the c subfields of another gray scale. That is, the state of the c subfields of the non-viewing period is not the same in all of the gray scales, but is sometime different between one gray scale and another gray scale.
The average transmittance ratio of the liquid crystal element 120 in one frame is sometimes affected not only by data signals of the non-viewing period and the viewing period in the frame but also by a transmittance ratio (gray-scale value) in the previous frame (hereinafter referred to as “immediately previous frame”). In a second embodiment, the conversion from a gray-scale value to a subfield code is performed in consideration of the transmittance ratio of the immediately previous frame. That is, in the second embodiment, the converting unit 21 performs, on the gray-scale value of a current frame as an object to be processed in a plurality of frames, the conversion based on the gray-scale value in the current frame and the optical state of an electro-optic element in the immediately previous frame one frame before the current frame. More specifically, the storage unit 23 stores a table in which the pair of a gray-scale value and a subfield code are recorded for each optical state of the immediately previous frame. The converting unit 21 performs the conversion with reference to the table stored in the storage unit 23.
For example, in the case where the gray-scale value of the current frame is the 118th gray scale (eight bits), when the transmittance ratio of the immediately previous frame is 0.75, it is sufficient to use “100-1110100” as a subfield code. Even in the case where the same subfield code “100-1110100” is used, when the transmittance ratio of the immediately previous frame is 1, the transmittance ratio of the current frame is that corresponding to the 120th gray scale. In the case where the gray-scale value of the current frame is the 118th gray scale, when the transmittance ratio of the immediately previous frame is 1, it is sufficient to use “000-1110100” as a subfield code.
The operation of the projector 2000 in the embodiment will be described with reference to
A description will be made with reference again to
In Step S110, the control section 304 generates a signal corresponding to the subfield code of the object pixel, and outputs this signal as the data signal Vx.
According to the embodiment, even when the number of subfields of the viewing period is b, expression of gray scales more than b bits (2b gray scales) can be performed by controlling data signals of the c subfields of the non-viewing period in consideration of the gray-scale value of the immediately previous frame. Moreover, compared to the case where the optical state of the immediately previous frame is not considered, the gray scale can be controlled more precisely.
The invention is not limited to the embodiments described above, but various modifications can be implemented. Hereinafter, some modified examples will be described. Two or more of the modified examples described below may be used in combination.
The blocking unit is not limited to shutter glasses. The invention may be used for, for example, a video display system which displays a 2D video by performing pseudo-impulse display. In this case, the blocking unit has a light source which is turned on in the viewing period and turned off in the non-viewing period. The plurality of electro-optic elements modulate light from this light source according to the optical state. In this video display system, direct-view-type display devices such as liquid crystal televisions are used. In this display device, a backlight (illumination) of a liquid crystal panel is intermittently turned off (that is, the backlight is turned on in a pulse fashion). In this case, the blocking unit is a device which controls the turn-on and turn-off of the backlight. In this display system, a time during which the backlight is turned off is the non-viewing period. When it is intended to perform gray-scale expression using only subfields of the viewing period, the number of subfields capable of being used is reduced, compared to the case where the backlight is not turned off. However, when the gray-scale control technique described in the above embodiments is used, expression of gray scales more than the number of subfields of the viewing period is possible.
In the embodiments, an example has been described in which a plurality of subfields have the same time length. However, the plurality of subfields may not have the same time length. That is, the time length of each of subfields in one frame may be weighted by a given rule, so that they may be different from each other. In this case, the response time of an electro-optic element is longer than a first subfield in one frame (the initial subfield in one frame).
The electronic apparatus according to the invention is not limited to a projector. The invention may be used for televisions, viewfinder-type/monitor direct-view-type video tape recorders, car navigation systems, pagers, electronic notebooks, calculators, word processors, workstations, videophones, POS terminals, digital still cameras, mobile phones, apparatuses equipped with a touch panel, and the like.
The converting unit 21 may convert a gray-scale value into a subfield code without depending on the table stored in the storage unit 23. In this case, the converting unit 21 is programmed so as to convert a gray-scale value into a subfield code without reference to the table.
The configuration of the electro-optic device 2100 is not limited to those illustrated in
The parameters (for example, the number of subfields, the frame rate, the number of pixels, and the like) and the polarity or level of signal described in the embodiments are illustrative only, and the invention is not limited to them.
The entire disclosure of Japanese Patent Application No. 2011-226003, filed Oct. 13, 2011 is expressly incorporated by reference herein.
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| 2011-226003 | Oct 2011 | JP | national |
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