JP2020064103A - Method for driving electro-optical device, electro-optical device, and electronic apparatus - Google Patents

Abstract

To reduce deviation when the gradation changes in sub-field drive.SOLUTION: A frame is one or more blocks; one block is divided into P (P is an integer of 2 or more) sub-fields; the P sub-fields are arranged such that the time length is gradually increased as toward the termination of the blocks. A gradation value designated in video data is converted into a sub-field code designating on or off of pixels for each of the P sub-fields; Q (Q is an integer of 1 or more and smaller than P) codes on the rear end among sub-field codes in a previous frame, and Q codes on the rear end among the sub-field codes in the current frame are compared to each other; part of the sub-field codes on the front end in a predetermined block in the current frame is changed to a code based on a result of comparison.SELECTED DRAWING: Figure 8

Description

  The present invention relates to a driving method of an electro-optical device, an electro-optical device and an electronic device.

  When a liquid crystal element is used as the electro-optical element in the electro-optical device, a sense of blurring occurs due to insufficient optical response when displaying a moving image. There is a so-called overdrive technique to reduce the blurring feeling (see Patent Document 1). In this overdrive, when a change in gradation occurs, the target gradation is eventually changed after the change by applying a voltage to the liquid crystal element in excess of the analog voltage corresponding to the desired gradation. It is a technology that can be obtained.

  On the other hand, in the electro-optical device, the electro-optical element is driven either on or off for each of a plurality of subfields obtained by dividing one frame, and the ratio of the time of driving on or off is changed so that the intermediate So-called subfield driving, which expresses gradation, has also been proposed (see Patent Document 2).

JP, 6-189232, A JP, 2001-337643, A

  However, in the sub-field driving, the voltage applied to the electro-optical element is limited to only two types, an on-voltage for turning on and an off-voltage for turning off. Therefore, in order to improve the optical response of the electro-optical element, if it is assumed that overdrive and sub-field driving are combined, an analog voltage that is more than the voltage corresponding to the target gray scale of the electro-optical element is applied. Face the contradiction of applying.

  In order to solve one of the above problems, a driving method of an electro-optical device according to one embodiment of the present invention is such that a pixel has brightness in a predetermined frame according to a grayscale value specified by video data. In the driving method of the electro-optical device, the frame is one or more blocks, and one block is divided into P (P is an integer of 2 or more) subfields and The P subfields are arranged so that the time length increases toward the end of the block, and the gradation value specified by the video data is turned on for each of the P subfields. Or, it is converted into a sub-field code designating OFF, and among the sub-field codes, Q (Q is an integer of 1 or more smaller than P) sub-field codes on the rear end side are converted into the pixel whose gradation value is the pixel. of In the driving method, the OFF is designated in order from the subfield on the end side of the block, and in the driving method, Q codes on the rear end side among the subfield codes of the previous frame, and Among the subfield codes of the current frame, the Q codes on the rear end side are compared with each other, and among the subfield codes of the current frame, a part of the (PQ) subfield codes on the front end side of a predetermined block Is changed to a subfield code based on the result of the comparison.

3 is a perspective view showing the configuration of the electro-optical device according to the embodiment. FIG. FIG. 3 is a cross-sectional view showing a structure of a liquid crystal panel in the electro-optical device. It is a block diagram which shows the electric constitution of an electro-optical device. It is a figure which shows the equivalent circuit of the pixel in a liquid crystal panel. It is a figure which shows the kind of voltage applied to the pixel electrode in a liquid crystal panel. FIG. 6 is a diagram showing an arrangement of subfields in the electro-optical device. It is a figure for demonstrating operation | movement of an electro-optical device. It is a figure which shows the structure of the processing circuit in an electro-optical device. It is a figure which shows an example of the conversion content of the Sfc conversion table in a processing circuit. It is a figure which shows the optical response in a liquid crystal panel. It is a figure which shows the relationship between the last 4 bits of a subfield code, and a terminal transmittance. 7 is a flowchart showing a subfield code changing operation. It is a figure which shows the number of differences of the trailing 4 codes of a subfield code. It is a figure which shows the optical response when a subfield code is not changed. It is a figure which shows the optical response when a subfield code is changed. It is a figure which shows the structure of the liquid crystal projector to which the electro-optical device which concerns on embodiment etc. is applied.

  Embodiments for carrying out the present invention will be described below with reference to the drawings.

  FIG. 1 is a perspective view showing a configuration of an electro-optical device 1 according to the embodiment. The electro-optical device 1 includes a transmissive liquid crystal panel 10 used as a light valve of a liquid crystal projector, for example. In the electro-optical device 1, the liquid crystal panel 10 is housed in a frame-shaped case 72 that opens in a rectangular display region. One end of an FPC board 74 is connected to the liquid crystal panel 10. Note that FPC is an abbreviation for Flexible Printed Circuits. A plurality of terminals 76 are provided at the other end of the FPC board 74 and are connected to a host device (not shown). The control circuit 3 of the semiconductor chip is mounted on the FPC board 74, and video data is supplied from the host device via a plurality of terminals 76 in synchronization with the synchronization signal.

The video data defines the gradation value of the pixel of the image to be displayed for each RGB, which is 7 bits in this embodiment. The sync signal includes a vertical sync signal for instructing the start of screen scanning in the display area, a horizontal sync signal for instructing the start of horizontal scanning of one row (line) of the screen, and one pixel of video data. A dot clock signal indicating the timing of the minutes is included.
Details of the control circuit 3 will be described later. In FIG. 1, the longitudinal direction of the display area is the X direction, the lateral direction is the Y direction, and the incident direction of light is the Z direction.

FIG. 2 is a cross-sectional view showing the structure when the liquid crystal panel 10 is broken along the YZ plane in FIG. As shown in FIG. 2, the liquid crystal panel 10 includes an element substrate 100 in which various elements and pixel electrodes 118 and the like are formed on a base material 101, and a counter substrate 200 in which a counter electrode 208 and the like are formed on a base material 201. The sealing material 9 including the spacers 7 is bonded so that a constant gap is maintained and the electrode forming surfaces are opposed to each other, and the liquid crystal 5 is sealed in the gap.
In addition, in the present embodiment, substrates having optical transparency are used for the base materials 101 and 201.

In the element substrate 100, a plurality of terminals 107 are formed in a region facing the counter substrate 200 and located on one outer side of the sealing material 9, and an FPC substrate 74 (not shown in FIG. 2) is connected thereto. It
In addition, the counter electrode 208 provided on the counter substrate 200 is electrically connected to any of the plurality of terminals 107 formed on the element substrate 100 by a conductive material (not shown) such as silver paste, and temporally. A substantially constant voltage is applied to.

FIG. 3 is a block diagram showing an electrical configuration of the electro-optical device 1, and FIG. 4 is a diagram showing an equivalent circuit of the pixel P in the liquid crystal panel 10 of the electro-optical device 1.
As shown in FIG. 3, the electro-optical device 1 includes a control circuit 3 and a liquid crystal panel 10. Of these, the control circuit 3 includes a processing circuit 30 and a scanning control circuit 35. Further, the liquid crystal panel 10 includes m scanning lines 112 extending in the X direction, n data lines 114 extending in the Y direction, m scanning lines 112, and The pixel P formed corresponding to each intersection with the n data lines 114, the scanning line driving circuit 130, and the data line driving circuit 140 are included.
In addition, both m and n are integers of 2 or more.

  Video data Vid-in and a sync signal Sync are supplied to the control circuit 3 from a host device. Of these, the control circuit 3 includes a processing circuit 30 and a scanning control circuit 35. The processing circuit 30 converts the gradation value of the video data Vid-in into a data signal Vsf for driving the liquid crystal element included in the pixel P either on or off, and supplies the data signal Vsf to the data line driving circuit 140. The scanning control circuit 35 controls the scanning line driving circuit 130 and the data line driving circuit 140 on the basis of the synchronization signal Sync, and causes the processing circuit 30 to indicate a subfield Sfn indicating a subfield being scanned among subfields to be described later. Alternatively, the write polarity in the subfield is designated.

  The scanning line driving circuit 130 sequentially outputs the scanning signals Y1, Y2, Y3, ..., Ym for each subfield in the first, second, and third rows in accordance with the control signal Yctr supplied from the scanning control circuit 35. It is supplied to the scanning line 112 of the mth line ,. Specifically, the scanning line driving circuit 130 sequentially and exclusively selects the scanning lines 112 from the first row to the m-th row for each subfield, and supplies the H-level scanning signal to the selected scanning lines 112. Then, an L level scanning signal is supplied to the unselected scanning lines 112.

The data line driving circuit 140 is supplied with the data signal Vsf supplied from the processing circuit 30 from the scanning control circuit 35 while the scanning line driving circuit 130 selects the i-th row scanning line 112. , Xn are supplied to the data lines 114 of the first, second, third, ..., Nth columns according to the control signal Xctr.
Note that i is a code used to generalize and explain one scanning line 112 among the m scanning lines 112, and is an arbitrary integer of 1 or more and m or less.

As shown in FIG. 4, in the pixel P provided corresponding to the intersection of the scanning line 112 and the data line 114, for example, the gate region of the N-channel type thin film transistor 116 is connected to the scanning line 112 and the source region thereof. Is connected to the data line 114, and its drain region is connected to the pixel electrode 118.
In the present embodiment, as described above, the pixel electrode 118 faces the counter electrode 208 to which the voltage Vcom is applied, and the liquid crystal 5 is located between both electrodes. Therefore, one end of the liquid crystal element is the pixel electrode 118. , And the other end is used as a counter electrode 208 to sandwich the liquid crystal 5.

In such a configuration, in the pixel P corresponding to the intersection of a certain one scanning line 112 and a certain one data line 114, when the scanning signal supplied to the scanning line 112 becomes the H level, the pixel concerned. At P, the thin film transistor 116 is turned on, and a voltage corresponding to the difference between the voltage of the signal supplied to the data line 114 and the voltage Vcom of the counter electrode 208 is written in the liquid crystal element.
When the scan signal supplied to the scan line 112 becomes L level, the thin film transistor 116 is turned off, but the liquid crystal element holds the voltage written when the thin film transistor 116 is on due to its capacitance.
In the present description, the voltage is based on 0 volt as the L level of the non-selection voltage applied to the scanning line 112, unless otherwise specified as in the liquid crystal element.

Now, on each of the facing surfaces of the element substrate 100 and the counter substrate 200, an alignment film for orienting the molecules of the liquid crystal 5 in a predetermined direction is provided, and on the back side thereof, the absorption axis corresponds to the alignment direction. Polarizers are provided so as to be oriented.
Therefore, the liquid crystal panel 10 has a minimum transmittance when the effective value of the voltage applied to the liquid crystal element is zero, while the transmittance gradually increases as the effective value of the voltage increases, and finally the transmittance is finally increased. It is in normally black mode where the rate is maximum.
Alignment films, polarizers, etc. are not directly related to the present case, and are not shown in the drawing.
In addition, since the liquid crystal panel is a transmissive type here, the transmissivity is used. However, if the liquid crystal panel is a reflective type, the transmissivity can be read as a reflective rate. If a self-luminous type such as OLED is used as the electro-optical element, it can be read as a ratio indicating brightness.

In the normally black mode, when the transmittance in the darkest state is set to 0% and the transmittance in the brightest state is normalized to 100%, the voltage at which the relative transmittance becomes 10% of the voltage applied to the liquid crystal element. Is called the optical threshold voltage, and the voltage at which the relative transmittance is 90% is called the optical saturation voltage.
In the voltage modulation method, when the pixel P has an intermediate gradation, the liquid crystal element is designed so that a voltage that is equal to or higher than the optical threshold value and equal to or lower than the optical saturation voltage is applied. When designed in this manner, the transmittance of the pixel P has a value that reflects the voltage applied to the liquid crystal element.

  In contrast to the voltage modulation method, in the present embodiment, since subfield driving is performed, the liquid crystal element is driven by either ON when the voltage applied is equal to or higher than the saturation voltage or OFF when the voltage is equal to or lower than the threshold voltage. Has become. In this structure, in order to express an intermediate gradation in the pixel P, the liquid crystal element is driven on or off in units of a subfield obtained by dividing one frame into a plurality of units, and is turned on (or off) over one frame. It is configured to control the distribution of the driving period.

Here, when the liquid crystal element is driven to be turned on, an ON signal whose absolute value is equal to or higher than the saturation voltage with respect to the voltage Vcom of the counter electrode 208 is applied to the pixel electrode 118 of the liquid crystal element. There are two types of the voltage of the ON signal: a positive ON voltage higher than the voltage Vcom and a negative ON voltage lower than the voltage Vcom.
On the other hand, when the liquid crystal element is driven off, a voltage of an off signal is applied to the pixel electrode 118 of the liquid crystal element so that the absolute value of the voltage Vcom is equal to or lower than the optical threshold voltage. In the present embodiment, the voltage Vcom is used as the voltage of the off signal in order to have both the positive polarity and the negative polarity.

  FIG. 5 is a diagram showing the voltages of the ON signal and the OFF signal applied to the pixel electrode 118 in relation to the voltage Vcom applied to the counter electrode 208. For example, when the voltage Vcom is 7 volts and the voltage of the positive ON signal is 12 volts, the voltage of the negative ON signal is 2 volts in order to provide symmetry. The off signal of positive polarity and the off signal of negative polarity have the same voltage Vcom of 7 volts.

FIG. 6 is a diagram showing an arrangement of subfields. As shown in this figure, one frame is divided into subfields Sf1 to Sf20 in a temporal order.
Here, “one frame” refers to a unit period required to repeatedly scan all the pixels P for each subfield so that each of the pixels P has a gradation specified by a gradation value.
Therefore, the period of the vertical synchronizing signal included in the synchronizing signal Sync, that is, the period in which the video data Vid-in for one screen to be displayed, which is supplied from the host device, and the one screen in the electro-optical device 1 are supplied. Although it may be different from the period required for the scan for creating, in the present embodiment, it is the same for simplification of the description.

  In the present embodiment, one frame is roughly divided into two blocks, that is, the first half subfields Sf1 to Sf10 and the second half subfields Sf11 to Sf20. Among these, in the first half subfields Sf1 to Sf10, the period length gradually increases toward the rear in time. Regarding the write polarity in the subfields Sf1 to Sf10, the positive polarity is designated in the subfield Sf1, the negative polarity is designated in the subfield Sf2, and the positive polarity or the negative polarity alternates from the following subfields Sf3 to Sf10. Specified in.

  The period lengths of the subfields Sf11 to Sf20 in the latter half of one frame period are the same as the period lengths of the subfields Sf1 to Sf10, respectively. The write polarities of the subfields Sf11 to Sf20 are in a relationship in which the write polarities of the subfields Sf1 to Sf20 are inverted.

FIG. 7 is a diagram for explaining scanning in the liquid crystal panel 10.
As shown in the upper column of FIG. 7, the start pulse Spx is supplied from the scan control circuit 35 to the scan line drive circuit at the start timing of the subfields Sf1 to Sf20 in the pixels on the first row.
The scanning line drive circuit 130 outputs the scanning signals Y1 to Ym by sequentially transferring the start pulse Spx by a clock signal (not shown). As a result, the scanning signals Y1 to Ym sequentially become H level, and the scanning lines 112 are exclusively selected row by row.
The start pulse Spx and the clock signal are included in the control signal Yctr described above.

In FIG. 7, the lower column shows the scanning lines selected by the scanning signals Y1 to Ym when the vertical axis represents the first row to the m-th row, which is the number of rows of the scanning lines 112, and the horizontal axis represents the elapsed time. It is a figure which shows a time transition.
If the selection of scanning lines is indicated by black dots for each scanning line, the scanning lines 112 are exclusively selected one by one, so that the selected scanning lines are sequentially arranged from the 1st row to the mth row with the lapse of time. Transition. Therefore, a black dot indicating the selected scanning line is shown as a downward-sloping continuous point with the lapse of time, and is shown by a downward-sloping solid line in the figure for simplification.

  When the scanning line 112 of the i-th row is selected in a certain subfield, the data line 114 of the j-th column specifies ON or OFF of the liquid crystal element of the i-th row and the j-th column in the subfield, and A data signal having a voltage corresponding to the write polarity is supplied. Therefore, in the sub-field, the liquid crystal element in the i-th row and the j-th column is driven at a designated ON or OFF.

  FIG. 8 is a block diagram showing the electrical configuration of the processing circuit 30 in the electro-optical device 1 according to this embodiment. As shown in FIG. 8, the processing circuit 30 includes an Sfc conversion table 302, a frame memory 304, a change direction detection unit 306, an Sfc conversion table 308, a comparison unit 310, an Sfc changing unit 320, an Sfc memory 322, and a polarity adding unit 324. including.

The Sfc conversion table 302 is an example of a subfield code conversion unit, and converts the gradation value specified by the video data Vid-in supplied from the higher-level device into a subfield code for each pixel.
Here, in order to express a certain gradation value in the pixel P, the subfield code means whether the liquid crystal element included in the pixel P is turned on or off in each of the subfields Sf1 to Sf20. Is a code that specifies.
The conversion contents of the Sfc conversion table 302 will be described later.

  In addition, in the present embodiment, one frame is composed of 20 subfields, so that the subfield code corresponding to one pixel P is 20 in total. In the embodiment, since the video data Vid-in is 7 bits, the gradation value for the pixel P is designated in 128 steps from "0" having the lowest transmittance to "127" having the highest transmittance. .

In the frame memory 304, the video data Vid-in supplied from the host device is temporarily stored, and the video data is read out after one frame has elapsed. For this reason, the video data read from the frame memory 304 has a relationship of being delayed by one frame for each pixel P with respect to the video data Vid-in supplied from the host device.
In order to distinguish the video signals, the frame specified by the video data Vid-in supplied from the higher-level device is called a “current frame” and specified by the video data read out from the frame memory 304 with a delay. Although the frame is referred to as a “previous frame”, the current frame may have a relatively delayed relationship with the previous frame.

  The change direction detection unit 306 compares the video data of the current frame with the video data of the previous frame in units of pixels P, and the gradation value of the pixel indicated by the video data of the current frame is indicated by the video data of the previous frame. It is detected whether or not the gradation value is smaller than the gradation value of the pixel to be displayed, that is, whether or not the change in the gradation value is in the darkening direction.

  The Sfc conversion table 308 converts the gradation value specified by the video data of the previous frame into a subfield code for each pixel. However, unlike the Sfc conversion table 302, the Sfc conversion table 308 outputs the trailing end 4 codes corresponding to the subfields Sf17 to Sf20 among the 20 codes.

FIG. 9 is a diagram showing an example of conversion contents of the Sfc conversion table 302 in the processing circuit 30.
As shown in this figure, in the Sfc conversion table 302, for each gradation value from “0” to “127”, in the subfields Sf1 to Sf20, the liquid crystal element included in the pixel P is turned on or turned off. A code that specifies whether or not is associated.
Here, a white square frame corresponding to one subfield means that the liquid crystal element is designated to be ON in the subfield, and a black square frame corresponding to any one subfield indicates the liquid crystal element. It means to specify off in the subfield.
When the code designating ON is “1” and the code designating OFF is “0”, for example, the video data Vid-in having the gradation value “80” is displayed in the subfield by the Sfc conversion table 302. In Sf1 to Sf20, the subfield codes of “11111100000011110000” are sequentially converted.

According to the conversion content shown in FIG. 9, the subfield code corresponding to the gradation value is designated so that the transmittance of the liquid crystal element when viewed in one frame sequentially changes according to the increase or decrease of the gradation value. .
The characteristic points of the conversion content shown in FIG. 9 are as follows.
Specifically, first, the conversion contents of the first half subfields Sf1 to Sf10 and the conversion contents of the second half subfields Sf11 to Sf20 are similar.
Secondly, when the gradation value decreases from the highest "127", the sub-fields with longer periods are converted to the off code "0" in order. Specifically, in the example of FIG. 9, when the gradation value is “127” and the gradation value is “122”, OFF is designated in the subfield Sf20, and the gradation value further decreases. When it becomes "117", OFF is designated in the subfield Sf19, and when the gradation value decreases to "107", OFF is designated in the subfield Sf18 and when the gradation value decreases to "83", Off is designated in subfield Sf17.

  Therefore, the dominant factor that determines the transmittance is the sum of ON or OFF period lengths of the subfields Sf17 to Sf20 (Sf7 to Sf10) having a relatively long period. On the other hand, the factor that finely determines the transmittance is the sum of the on / off period lengths of the subfields Sf11 to Sf16 (Sf1 to Sf6) having a relatively short period.

In FIG. 9, for convenience, the range of gradation values in which the subfields Sf17 to Sf20 are all “1” is set as a group A, and among the subfields Sf17 to Sf20, the subfields Sf17 to Sf19 are “1”, The gradation value range in which the field Sf20 is "0" is group B, the gradation value range in which the subfields Sf17 and Sf18 are "1", and the subfields Sf19 and Sf20 is "0" is group C, The range of gradation values in which the subfield Sf17 is "1" and the subfields Sf18 to Sf20 are "0" is group D, and the range of gradation values in which the subfields Sf17 to Sf20 are all "0" is group E. And
For gradation values that belong to the same group, the subfield code is determined so that the transmittance is different by adjusting the sum of the ON or OFF period lengths of the subfields Sf11 to Sf16 (Sf1 to Sf6). Has been done.

FIG. 10 is a diagram showing the transmittance of each group over time. Specifically, in FIG. 10, in one frame, typical gradation values in groups A to E are converted into subfield codes, and the liquid crystal element is driven on or off according to the subfield codes. Shows the temporal change of the transmittance of the liquid crystal element when the liquid crystal element is driven almost completely off in order to obtain the lowest gradation value "0" in the next one frame.
The on / off patterns of the subfields Sf11 to Sf16, which have a relatively long period and are located on the end side of one frame period, are the same for each of the groups A to E. Therefore, the transmittances of the liquid crystal elements at the end timing of one frame (hereinafter, referred to as “terminal transmittances”) are substantially equal in each of the groups A to E.

FIG. 11 is a diagram showing the range of the terminal transmittance in each group.
As shown in FIG.
In the group A, the terminal transmittance is in the range of 0.9 to 1.0,
In the group B, the terminal transmittance is in the range of 0.7 to 0.9,
In the group C, the terminal transmittance is in the range of 0.3 to 0.7,
In the group D, the terminal transmittance is set in the range of 0.1 to 0.3,
In the group E, the terminal transmittance is in the range of 0.0 to 0.1,
Each is set.

  Returning to FIG. 8 again, the comparison unit 310 converts the rear end 4 code and the previous frame video data of a total of 20 subfield codes obtained by converting the video data of the current frame for a certain pixel P. The trailing end 4 codes are compared to obtain the exclusive OR. Therefore, the number of “1” s in the exclusive OR is the number of different codes (hereinafter, “the number of differences” when comparing the rear end 4 codes of the current frame and the rear end 4 codes of the previous frame). Will be shown).

  The Sfc changing unit 320 is an example of a subfield code changing unit, and for a certain pixel P, a part or all of the front 4 codes among the subfield codes obtained by converting the video data of the current frame is changed to the changing direction detecting unit 306. The change is made based on the gradation value change direction detected by and the number of differences obtained by the comparison unit 310. The Sfc changing unit 320 may not change the subfield code.

The Sfc memory 322 temporarily stores the subfield code output from the Sfc changing unit 320, and is a pixel that is a scan target of the liquid crystal panel 100 by the scan control circuit 35 and is specified by the subfield number Sfn. The subfield, that is, the code corresponding to the subfield at the time of scanning is read.
The polarity assigning unit 324 converts the code read from the Sfc memory into a data signal of the write polarity Pol designated by the scan control circuit 35. Specifically, as shown in FIG. 5, if the read code is “1” and positive polarity writing is designated, the polarity imparting unit 324 outputs a positive voltage ON signal of 12 volts. , If the negative polarity writing is designated, the negative polarity ON signal of 2V is output respectively, and if the read code is "0", the 7V is turned off regardless of the write polarity. Output a signal.

FIG. 12 is a flowchart for explaining the changing operation of the subfield code of the specific pixel P, focusing on a specific pixel P among the operations of the processing circuit 30.
The gradation value specified for the pixel P by the 7-bit video data Vid-in, that is, the gradation value specified for the pixel P in the current frame is converted into a subfield code of 20 codes by the Sfc conversion table 302. It is converted (step S10).
The 7-bit video data Vid-in that specifies the gradation value of the pixel P is temporarily stored in the frame memory 304, while the video data that specifies the gradation value of the pixel P in the previous frame is stored in the frame memory 304. Is read from (step S12).

The video data read from the frame memory 304 and designating the gradation value of the pixel P in the previous frame is converted by the Sfc conversion table 308 into the rear end 4 codes corresponding to the subfields Sf17 to Sf20 (step S14). .
Whether the gradation value specified for the pixel P in the current frame is compared with the gradation value specified for the pixel P in the previous frame to determine whether the change in the gradation value is small, that is, The change direction detection unit 306 determines whether or not the direction is dark (step S16).

The comparing unit 310 compares the trailing end 4 code of the subfield codes in the current frame with the trailing end 4 code of the subfield codes in the previous frame for the pixel P, regardless of the changing direction of the gradation value. Then, the code difference number is obtained (steps S18 and S38).
Next, the Sfc changing unit 320 determines whether the difference number by the comparing unit 310 is “0” (steps S20 and S40).
If the number of differences is “0” (if the determination results of steps S20 and S40 are “Yes”), the subfield code in the current frame is output as it is without being changed by the Sfc changing unit 320. The changing process for the pixel P ends, the target of the process shifts to the next pixel, and the similar changing operation is executed.

If the number of differences is not "0" (if the determination results of steps S20 and S40 are "No"), the Sfc changing unit 320 determines whether the number of differences is "1" (step S22, S42).
When the difference number is "1" (when the determination result of steps S22 and S42 is "Yes"), the change in the gradation value is in the direction of decreasing (the determination result of step S16 is " If “Yes”), the Sfc changing unit 320 changes the front end 1 code of the subfield codes in the current frame to “0” (step S24). On the other hand, in this case, if the change in gradation value is not in the direction of decreasing (if the determination result of step S16 is “No”), the Sfc changing unit 320 causes the subfield code of the current frame to be changed. Among them, the front end 1 code is changed to "1" (step S44).

If the difference number is not "1" (if the determination results of steps S22 and S42 are "No"), the Sfc changing unit 320 determines whether the difference number is "2" (step S26, S46).
When the difference number is "2" (when the determination results of steps S26 and S46 are "Yes"), the change in the gradation value is in the direction of decreasing (the determination result of step S16 is " If “Yes”), the Sfc changing unit 320 changes the front end 2 code of the subfield codes in the current frame to “00” (step S28). On the other hand, in this case, if the change in gradation value is not in the direction of decreasing (if the determination result of step S16 is “No”), the Sfc changing unit 320 causes the subfield code of the current frame to be changed. Among them, the front end 2 code is changed to "11" (step S48).

If the difference number is not "2" (if the determination results of steps S26 and S46 are "No"), the Sfc changing unit 320 determines whether the difference number is "3" (step S30, S50).
When the difference number is "3" (when the determination results of steps S30 and S50 are "Yes"), the change in the gradation value is in the direction of decreasing (the determination result of step S16 is " If "Yes"), the Sfc changing unit 320 changes the leading 3 codes of the subfield codes in the current frame to "000" (step S32). On the other hand, in this case, if the change in gradation value is not in the direction of decreasing (if the determination result of step S16 is “No”), the Sfc changing unit 320 causes the subfield code of the current frame to be changed. Among them, the front end 3 code is changed to "111" (step S52).

If the difference number is not “3” (if the determination results of steps S30 and S50 are “No”), it means that the difference number is “4”.
In this case, if the change in the gradation value is in the direction of decreasing (if the determination result in step S16 is “Yes”), the Sfc changing unit 320 causes the subfield code in the current frame to be The front end 4 code is changed to "0000" (step S36). On the other hand, in this case, if the change in gradation value is not in the direction of decreasing (if the determination result of step S16 is “No”), the Sfc changing unit 320 causes the subfield code of the current frame to be changed. Among them, the front end 4 code is changed to "1111" (step S56).

  After the change in step S24, S28, S32, S36, S44, S48, S52, or S56, the change process for the pixel P ends, the process target shifts to the next pixel, and the same change operation is executed. To be done.

  As described above, in the present embodiment, one frame is divided into the subfields Sf1 to Sf20 shown in FIG. 6, and the gradation value designated by the video data Vid-in is turned on for each subfield Sf1 to Sf20. Alternatively, assuming that the sub-field code that specifies off driving is converted, if the gradation value change from the previous frame to the current frame is in the dark direction in the off direction, the front-end code of the sub-field code in the current frame is displayed. Are sequentially changed to “0” which is an off code according to the number of different codes, and if the change of the gradation value is in the bright direction of the on direction, the front end side code of the subfield code in the current frame is sequentially changed. The code is changed to "1" which is the ON code according to the number of differences.

The reason for having such a configuration will be described.
When the range of gradation values from “0” to “127” is classified into groups A to E, if the groups having the same gradation value change, the end transmittances are almost the same, the current frame is the same. The desired optical response (change in transmittance) can be obtained without changing the subfield code of.
However, when the gradation value of the previous frame belongs to a certain group, the difference in the terminal transmittance increases as the gradation value of the current frame moves away from the group to which the gradation value belongs.

In this embodiment, the group to which the gradation value belongs is specified by the last four codes of the subfield code, as shown in FIG. 11, and the conversion contents from the gradation value to the subfield code are in the direction of darkening. The code on the rear end side sequentially becomes “0” as it goes to. Therefore, as shown in FIG. 13, the greater the difference number between the trailing 4 codes of the sub-field code of the previous frame and the trailing 4 codes of the sub-field code of the current frame, the greater the difference in the terminal transmittance. Become.
Note that the upper right half of FIG. 13 shows the case where the gradation value changing direction is small, that is, the case where it changes in the dark direction, and the lower left half shows the case where the gradation value changing direction is large, that is, the direction where it becomes brighter. Shows the case of changing.

  In this embodiment, one frame is divided into the first half subfields Sf1 to Sf10 and the second half subfields Sf11 to Sf20. As shown in FIG. 9, the conversion contents from the gradation value to the subfield code are not completely the same, but are substantially the same in the subfield having a relatively long period.

Among the groups A to E, if the gradation values change in the same group, the reason why the terminal transmittances are almost the same is that the on / off patterns are aligned in the subfields having a long period length. Is. From this reason, it is considered that the transmittances at the end of not only the latter half blocks but also the first half blocks are almost the same if the gradation values change in the same group.
In each group, the transmittance at the end of the first half block is almost the same, that is, the transmittance at the start of the second half block is almost the same. Therefore, if it is limited to the latter half block, it can be said that the optical response of the liquid crystal element in the block is hardly affected by the gradation value of the previous frame.

  Therefore, it is considered that the reason why the optical characteristic corresponding to the subfield code is not obtained in a certain frame is the influence of the terminal transmittance in the previous frame. Specifically, it is considered that if the difference in the terminal transmittance is large, even if the liquid crystal element is driven according to the subfield code, the optical characteristics corresponding to the subfield code deviate on the front end side of one frame. To be

Therefore, in the present embodiment, the subfield code on the front end side of one frame is changed to a code for changing the gradation value in accordance with the difference in the terminal transmittance, that is, in accordance with the number of differences. The content is to correct at the front end of the frame.
This point will be described with reference to the drawings.

FIG. 14 is a diagram showing an optical response in the case where the subfield code is not changed in order to explain the superiority of the present embodiment, and FIG. 15 is changed in the subfield code as in the present embodiment. It is a figure which shows the optical response in the case.
14 and 15 show an example in which the gradation value is changed to "80" in the current frame in the state where the gradation value "127" is specified in the previous frame.
Here, the subfield code when the gradation value is “127” is all “1”, and the subfield code when the gradation value is “80” is “111111000000111110000” as described above.
In addition, in the examples of FIGS. 14 and 15, the solid line shows the actual chemical response of the liquid crystal element, and the broken line shows the ideal or designed chemistry of the liquid crystal element when the gradation value is “80”. Shows the response.

As shown in FIG. 14, when the sub-field code is not changed, the sub-field code when the gradation value is “80” is continuous with “1” on the front end side, and therefore the end transmission in the previous frame is performed. The state of the rate of “1.0” continues, and in the first half block, there is a deviation from the ideal state where the transmittance gradually increases when the gradation value is “80”. Note that, in the latter half block, as described above, the influence of the previous frame is less likely to occur, so the actual optical response substantially matches the ideal characteristics.
Therefore, when the gradation value changes from "127" to "80" in the current frame, the optical characteristic corresponding to the gradation value "80" is not obtained due to the deviation in the first half block of the current frame. There is.

On the other hand, in the present embodiment, when the gradation value changes from “127” to “80” in the current frame, the gradation value change tends to decrease, and the group A to which the gradation value “127” belongs To the group E to which the gradation value “80” belongs, the code difference number is “4”.
Therefore, the subfield code having the gradation value of “80” is changed to “0000” corresponding to the front direction four codes of the “11111100000011110000” in the OFF direction and the difference number “4”.
Therefore, in the present embodiment, as shown in FIG. 15, since the current frame is changed to the off direction which is the change direction from the start, the degree of the deviation can be suppressed to be small.

  As described above, according to the present embodiment, it is possible to improve the optical response of an electro-optical element such as a liquid crystal element even in subfield driving, and therefore, a gradation deviation suppression effect such as overdrive can be obtained. Obtainable.

In subfield driving, a configuration may be considered in which the grayscale value of the previous frame and the grayscale value of the current frame are simply used as two arguments to determine the subfield code of the current frame.
However, in such a configuration, for example, if the gradation value is 7 bits, there are 16384 (= 2 ⁷ × 2 ⁷ ) combinations of the gradation value of the previous frame and the gradation value of the current frame. It is necessary to actually measure the optical response to determine what subfield code is to be used for one combination. If the time required for measurement for one combination is 1 second, it will take about 4.6 hours to measure all the above combinations. Further, it is known that an electro-optical element such as a liquid crystal element changes its optical response characteristics when the surrounding environment (typically temperature) of the liquid crystal panel changes. Therefore, it is necessary to prepare a plurality of Sfc conversion tables in advance according to the temperature and the like and select one of the Sfc conversion tables according to the temperature. Therefore, the time required for measurement is equal to the number of prepared tables. It will be doubled further.
Therefore, the above-mentioned configuration has a drawback that not only the time required for measurement takes a long time, but also the storage capacity of the table becomes large.

On the other hand, in the present embodiment, the dominant factor that determines the transmittance is the sum of the ON or OFF period lengths in the subfields that have a relatively long period, while the factor that finely determines the transmittance is It is the sum of the on / off period lengths of subfields with a short target period.
Therefore, in the present embodiment, even if the surrounding environment of the liquid crystal panel changes, the on / off pattern in the subfield having a relatively long period is not changed, but only the on / off pattern in the subfield having a relatively short period is changed. Therefore, it is possible to reduce the influence of the drawbacks such as the above-mentioned configuration.

  Although the liquid crystal element has been described as a normally black mode in the embodiment, it may be a normally white mode. In the normally white mode, the direction in which the liquid crystal element is turned on is the direction in which the gradation value is reduced (direction to darken), and the direction in which the liquid crystal element is turned off is the direction in which the gradation value is increased (direction to be brightened).

  Further, although the number of blocks is “2” in the embodiment, it may be an even number of “4” or more. The reason for setting the number of blocks to an even number is to ensure the symmetry of the periods in the positive polarity writing and the negative polarity writing for AC driving the liquid crystal element. When the electro-optical element is other than the liquid crystal element, that is, when it is not necessary to perform AC driving, for example, when an OLED is used for the electro-optical element, the number of blocks is “1” or more. OLED is an abbreviation for Organic Light Emitting Diode.

  When the number of blocks is “2” or more, the terminal transmittances of the blocks are almost the same. Therefore, it is preferable to change the front end code of the first block of the first frame in the sense that it is easily affected by the terminal transmittance of the previous frame. However, if the number of blocks is large, it may be affected by the second and subsequent blocks, so the front end code of the second and subsequent blocks may be changed.

  In the embodiment, the trailing end 4 code of the subfield code in the previous frame and the trailing end 4 code of the subfield code in the current frame are compared. However, depending on the conversion contents from the gradation value to the subfield code, the trailing end On the side, other than 4 codes may be used.

Further, in the embodiment, the front end 4 code of the current frame is changed according to the number of differences, but may be other than “4” as long as the rear end code to be compared is not changed. Further, the number of differences and the number of codes to be changed do not have to match.
For example, in the configuration in which the rear end 4 codes are compared with each other, among the front end 5 codes of the current frame, the front end 1 code is selected if the difference number is "1", and the front end 2 code is selected if the difference number is "2". If the number of differences is “3”, the leading 4 codes may be changed, and if the number of differences is “4”, the leading 5 codes may be changed. That is, when the change in gradation value is large, the number of codes to be changed may be increased.

<Electronic equipment>
Next, electronic equipment to which the electro-optical device 1 according to the embodiment and the like is applied will be described.

  FIG. 15 is a diagram showing a configuration of a three-plate type liquid crystal projector using the liquid crystal panel 10 of the electro-optical device 1 described above as a light valve. As shown in FIG. 15, the liquid crystal projector 2100 includes liquid crystal panels 10R, 10G and 10B. The liquid crystal panels 10R, 10G, and 10B are the same as the liquid crystal panel 10 in the embodiment and the like, and transmit images based on video data corresponding to respective colors of R, G, and B supplied from a higher-level device (not shown), respectively. To generate.

Inside the liquid crystal projector 2100, a lamp unit 2102 including a white light source such as a halogen lamp is provided. The projection light emitted from the lamp unit 2102 is separated into three primary colors of red, green and blue by the three mirrors 2106 and the two dichroic mirrors 2108 arranged inside. Of these, red light is incident on the liquid crystal panel 10R, green light is incident on the liquid crystal panel 10G, and blue light is incident on the liquid crystal panel 10B.
Note that the optical path of blue light is longer than other red and green light paths. Therefore, the blue light is guided to the liquid crystal panel 10B via the relay lens system 2121 including the entrance lens 2122, the relay lens 2123, and the exit lens 2124 in order to prevent loss in the optical path.

The liquid crystal panel 10R writes the data signal of the red component for each pixel P by the scanning line driving circuit 130 and the data line driving circuit 140. In the liquid crystal panel 10R, when the data signal is written in the pixel P, the transmittance becomes according to the data signal. Therefore, in the liquid crystal panel 10R, the transmittance of the incident red light is controlled for each pixel, so that a transmission image of the red component of the image to be displayed is generated.
Similarly, in the liquid crystal panels 10G and 10B, the data signal of the green component and the data signal of the blue component are written by the drive circuit for each pixel, and the transmission images of the green and blue components of the images to be displayed are displayed. Is generated.

The transmission images of the respective colors generated by the liquid crystal panels 10R, 10G and 10B enter the dichroic prism 2112 from three directions. Then, in the dichroic prism 2112, the R and B lights are refracted by 90 degrees, while the G light goes straight. Therefore, after the images of the respective colors are combined, a color image is projected on the screen 2120 by the projection lens 2114.
The transmission images of the liquid crystal panels 10R and 10B are projected after being reflected by the dichroic prism 2112, whereas the transmission images of the liquid crystal panel 10G are projected straight. For this reason, the transmission images of the liquid crystal panels 10R and 10B are in a laterally inverted relationship with respect to the transmission image of the liquid crystal panel 10G.

  DESCRIPTION OF SYMBOLS 1 ... Electro-optical device, 3 ... Control circuit, 5 ... Liquid crystal, 10 ... Liquid crystal panel, 100 ... Element substrate, 118 ... Pixel electrode, 130 ... Scan line drive circuit, 140 ... Data line drive circuit, 200 ... Counter substrate, 208 ... counter electrode, 302, 308 ... Sfc conversion table, 306 ... change direction detection section, 310 ... comparison section, 320 ... Sfc change section.

Claims (8)

A driving method of an electro-optical device, comprising: controlling a pixel to have a brightness according to a gradation value specified by video data in a predetermined frame,
The frame is one or more blocks,
In one block,
It is divided into P (P is an integer of 2 or more) subfields, and the P subfields are arranged so that the time length increases toward the end of the block.
Converting the gradation value designated by the video data into a subfield code designating ON or OFF of the pixel for each of the P subfields,
Of the subfield codes, Q subfield codes on the rear end side (where Q is an integer of 1 or more smaller than P) are used for the block when the gradation value is toward the off brightness of the pixel. Specify the off from the subfield on the end side in order,
In the driving method,
Of the subfield codes of the previous frame, the Q codes on the rear end side and the subfield codes of the current frame on the rear end side are compared with each other.
A method of driving an electro-optical device, wherein a part of (PQ) subfield codes on the front end side of a predetermined block among subfield codes of the current frame is changed to a subfield code based on the result of the comparison. As a result of the comparison, as the number of different codes increases,
The driving method of the electro-optical device according to claim 1, wherein the code is changed in order from the code on the front end side. In the comparison,
Calculating the number of differences between the Q code on the rear end side in the subfield code of the previous frame and the Q code on the rear end side in the subfield code of the current frame,
The driving method of the electro-optical device according to claim 2, wherein among the codes on the front end side in the subfield code of the current frame, a code corresponding to the calculated difference number is changed. In the comparison,
Compare the gradation value of the previous frame and the gradation value of the current frame, specify the direction of change of the gradation value,
The code on the front end side in the subfield code of the current frame is
When the changing direction of the gradation value is toward the off brightness of the pixel, it is changed to an off code,
The method of driving an electro-optical device according to claim 1, wherein when the changing direction of the gradation value is toward the ON brightness of the pixel, the ON code is changed. The electro-optical device according to claim 1, wherein the predetermined block is a first block in the frame. If the frame has more than one block,
The electro-optical device according to claim 1, wherein the predetermined block is a block other than the first block in the frame. Pixels are controlled in a predetermined frame so that the brightness becomes according to the gradation value specified by the video data,
The frame is one or more blocks,
In one block,
An electro-optical device which is divided into P (P is an integer of 2 or more) subfields and in which the P subfields are arranged so that the time length increases toward the end of the block. ,
The gradation value designated by the video data is converted into a subfield code designating ON or OFF of the pixel for each of the P subfields, and a Q ( Q is an integer greater than or equal to 1 smaller than P), and when the gradation value is toward the off brightness of the pixel, the off field is sequentially designated from the subfield on the end side of the block. A subfield code conversion unit,
Of the sub-field codes of the previous frame, the rear end side Q codes, and among the sub-frame codes of the current frame, the rear end side Q codes are compared with each other.
A subfield code changing unit that changes a part of the (PQ) subfield codes on the front end side of the predetermined block among the subfield codes of the current frame to a subfield code based on the result of the comparison.
An electro-optical device having:   An electronic apparatus including the electro-optical device according to claim 7.

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