# Patent

## Title
Display device, picture signal processing method, and program

## Abstract

Provided is a display device including a display unit having luminescence elements that individually becomes luminous depending on a current amount. The luminescence elements are arranged in a matrix pattern. The display device comprises a luminescence amount regulator for setting a reference duty for regulating a luminescence amount per unit time for each of the luminescence elements, according to picture information of an input picture signal, and also comprises an adjuster for adjusting, based on the reference duty, an effective duty regulating a luminous time for which the luminescence elements become luminous within a unit time, so that the effective duty is within a predetermined range, and for adjusting a gain of the picture signal, so that a luminescence amount regulated with the effective duty and with the gain of the picture signal equals to the luminescence amount regulated with the reference duty.

## Background
TECHNICAL FIELD
The present invention relates to a display device, a method of processing a picture signal, and a program.
BACKGROUND ART
In recent years, various display devices, such as organic EL displays (organic ElectroLuminescence displays, also called as OLED displays (Organic Light Emitting Diode displays)), FEDs (Field Emission Displays), PDPs (Plasma Display Panels), and the like, have been developed as devices to replace CTR displays (Cathode Ray Tube displays).
Amongst the various display devices mentioned above, the organic EL displays are self-luminescence type display devices that use an electroluminescence phenomenon. They have drawn particular attention of people as devices for the next generation, because they are superior to display devices in their moving image characteristics, viewing angle characteristics, colour reproducibility, etc. The electroluminescence phenomenon is a phenomenon in which the state of an electron of a material (an organic EL element) changes from the ground state to the excited state so as to return from the excited state, which is unstable, to the ground state, which is stable, whereby the difference of energy is emitted in the form of light.
In such circumstances, various techniques related to the self-luminescence type display devices have been developed. An example of the techniques related to luminous time control for a unit time on a self-luminescence type display device can be found in the following Patent Literature 1.

    Patent Literature 1: JP 2006-038967 (A)


DISCLOSURE OF INVENTION
Object to be Achieved by Invention
However, the typical techniques related to luminous time control for a unit time merely shortens the luminous time per unit time and lower the signal level of a picture signal in response to higher average luminance of the picture signal. Thus, when a picture signal at extremely high luminance is input into a self-luminescence type display device, the luminescence amount of a picture displayed (signal level of picture signal×luminous time) becomes much too large, which could result in the current overflowing into the luminescence elements.
Moreover, self-luminescence type display devices using the typical techniques related to luminous time control for a unit time cause the luminance to be lowered because the luminescence amount for a picture displayed (signal level of picture signal×luminous time) is smaller than the luminescence amount indicated by an input picture signal.
The present invention is made in view of the above-mentioned issue, and aims to provide a display device, a method of processing a picture signal, and a program, which are novel and improved, and which are capable of controlling the luminous time per unit time based on an input picture signal to prevent the current from overflowing into the luminescence elements and also of controlling the gains of picture signals as well to achieve higher display quality.
Solution for Achieving Object
According to the first aspect of the present invention in order to achieving the above-mentioned object, there is provided a display device including a display unit having luminescence elements that individually becomes luminous depending on a current amount. The luminescence elements are arranged in a matrix pattern. The display device includes a luminescence amount regulator for setting a reference duty for regulating a luminescence amount per unit time for each of the luminescence elements according to picture information of an input picture signal, and also includes an adjuster for adjusting, based on the reference duty, an effective duty regulating a luminous time for which the luminescence elements become luminous within a unit time, so that the effective duty is within a predetermined range, and for adjusting a gain of the picture signal, so that a luminescence amount regulated with the effective duty and with the gain of the picture signal equals to the luminescence amount regulated with the reference duty.
The display device may include a luminescence amount regulator and an adjuster. The luminescence amount regulator may set a reference duty for regulating a luminescence amount per unit time for each of the luminescence elements, according to picture information of an input picture signal. Now, the unit time may be a unit time that passes one after another cyclically, for example. And, for example, the luminescence amount regulator may use an average of the luminance of the picture signal, the histogram of the picture signal, or the like for the picture information of the picture signal. The adjuster may adjust, based on the reference duty, an effective duty regulating a luminous time for which the luminescence elements become luminous within a unit time, so that the effective duty is within a predetermined range, where the predetermined range may be set by use of a lower and/or upper limit value of the effective duty. The lower limit value of the effective duty is set so that the occurrence of flickers will not be obviously noticed. The upper limit value of the effective duty is set so that blurred movements will not be obviously noticed, which movements lower the quality of moving pictures. Also, the adjuster may adjust a gain of the picture signal, so that a luminescence amount regulated with the effective duty and with the gain of the picture signal equals to the luminescence amount regulated with the reference duty. According to such a configuration, the current can be prevented from overflowing into the luminescence elements by controlling the luminous time per unit time, and further, higher display quality can be achieved by controlling the gain of the picture signal as well.
Also, the adjuster may include a luminous time adjuster for outputting, as the effective duty, the reference duty adjusted to a predetermined lower or upper limit value if the reference duty set by the luminescence amount regulator is out of the predetermined range, and also may include a gain adjuster for adjusting the gain of the picture signal based on the reference duty set by the luminescence amount regulator and on the effective duty output from the luminous time adjuster.
According to such a configuration, higher display quality can be achieved by controlling both the luminous time per unit time and the gain of the picture signal.
Also, the gain adjuster may damp the gain of the picture signal, depending upon an increasing ratio of the effective duty to the reference duty, if the luminous time adjuster has output the effective duty adjusted to the lower limit value.
According to such a configuration, each of the luminous time and the gain of the picture signal can be adjusted with the luminescence amount kept constant.
Also, the gain adjuster may amplify the gain of the picture signal, depending upon a decreasing ratio of the effective duty to the reference duty, if the luminous time adjuster has output the effective duty adjusted to the upper limit value.
According to such a configuration, each of the luminous time and the gain of the picture signal can be adjusted with the luminescence amount kept constant.
Also, the gain adjuster may include a primary gain adjuster for multiply the input picture signal by the reference duty, and may also include a secondary gain adjuster for dividing the adjusted picture signal output from the primary gain adjuster by the effective duty output from the luminous time adjuster.
According to such a configuration, each of the luminous time and the gain of the picture signal can be adjusted with the luminescence amount kept constant.
Also, an average luminance calculator may be further included for calculating average luminance of the input picture signal for a predetermined period. And, the luminescence amount regulator may set the reference duty, depending upon the average luminance calculated by the average luminance calculator.
According to such a configuration, the current can be prevented from overflowing into the luminescence elements by controlling the luminous time per unit time.
Also, the luminescence amount regulator may memorise a look-up table, in which luminance of the picture signal and the reference duty are correlated to each other, and may uniquely set the reference duty depending upon the average luminance calculated by the average luminance calculator.
According to such a configuration, the luminescence amount per unit time can be regulated.
Also, the predetermined period for the average luminance calculator to calculate the average luminance may be one frame.
According to such a configuration, the luminous time within each frame period can be controlled more precisely.
Also, the average luminance calculator may include a current ratio adjuster for multiplying primary colour signals of the picture signal by respective adjustment values for each of the respective primary colour signals based on a voltage-current characteristic, and may also include an average value calculator for calculating the average luminance of the picture signal output from the current ratio adjuster for the predetermined period.
According to such a configuration, a picture and an image can be displayed accurately according to the input picture signal.
Also, a linear converter may be further included for adjusting the input picture signal to a linear picture signal by gamma adjustment. And, a picture signal input into the luminescence amount regulator may be the adjusted picture signal.
According to such a configuration, the current can be prevented from overflowing into the luminescence elements by controlling the luminous time per unit time.
Also, a gamma converter may be further included for performing, on the picture signal, gamma adjustment according to a gamma characteristic of the display unit.
According to such a configuration, a picture and an image can be displayed accurately according to the input picture signal.
Also, according to the second aspect of the present invention in order to solve the above-mentioned object, there is provided a picture signal processing method of a display device including a display unit having luminescence elements that individually becomes luminous depending on a current amount. The luminescence elements are arranged in a matrix pattern. The picture signal processing method includes the step of setting a reference duty for regulating a luminescence amount per unit time for each of the luminescence elements, according to picture information of an input picture signal, and also includes the step of adjusting, based on the reference duty, an effective duty regulating a luminous time for which the luminescence elements become luminous within a unit time, so that the effective duty is within a predetermined range, and adjusting a gain of the picture signal, so that a luminescence amount regulated with the effective duty and with the gain of the picture signal equals to the luminescence amount regulated with the reference duty.
By use of such a method, the current can be prevented from overflowing into the luminescence elements by controlling the luminous time per unit time, and further, higher display quality can be achieved by controlling the gain of the picture signal as well.
Also, according to the third aspect of the present invention in order to solve the above-mentioned object, there is provided a program for use in a display device including a display unit having luminescence elements that individually becomes luminous depending on a current amount. The luminescence elements are arranged in a matrix pattern. The program is configured to cause a computer to function as the step of setting a reference duty for regulating a luminescence amount per unit time for each of the luminescence elements, according to picture information of an input picture signal, and also to function as the step of adjusting, based on the reference duty, an effective duty regulating a luminous time for which the luminescence elements become luminous within a unit time, so that the effective duty is within a predetermined range, and adjusting a gain of the picture signal, so that a luminescence amount regulated with the effective duty and with the gain of the picture signal equals to the luminescence amount regulated with the reference duty.
According to such a program, the current can be prevented from overflowing into the luminescence elements by controlling the luminous time per unit time, and further, higher display quality can be achieved by controlling the gain of the picture signal as well.
Also, according to the forth aspect of the present invention in order to achieving the above-mentioned object, there is provided a display device including a display unit having pixels, each of which includes a luminescence element that individually becomes luminous depending on a current amount and a pixel circuit for controlling a current applied to the luminescence element according to a voltage signal, scan lines which supply a selection signal for selecting pixels to be luminous to the pixels in a predetermined scanning cycle, and data lines which supply to the pixels the voltage signal according to an input picture signal, where the pixels, the scan lines, and the data lines are arranged in a matrix pattern. The display device includes an average luminance calculator for calculating average luminance of the input picture signal for a predetermined period. The display device also includes a luminescence amount regulator for setting a reference duty for regulating a luminescence amount per unit time for each of the luminescence elements depending upon the average luminance calculated by the average luminance calculator. The display device also includes an adjuster for adjusting, based on the reference duty, an effective duty regulating a luminous time for which the luminescence elements become luminous within a unit time, so that the effective duty is within a predetermined range, and for adjusting a gain of the picture signal, so that a luminescence amount regulated with the effective duty and with the gain of the picture signal equals to the luminescence amount regulated with the reference duty.
According to such a configuration, the current can be prevented from overflowing into the luminescence elements by controlling the luminous time per unit time, and further, higher display quality can be achieved by controlling the gain of the picture signal as well.
Advantage of Invention
According to the present invention, the current can be prevented from overflowing into luminescence elements by controlling a luminous time per unit time, and further, higher display quality can be achieved by controlling the gain of a picture signal as well.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is an illustration that shows one example of the configuration of a display device according to an embodiment of the present invention.
FIG. 2A is an illustration that schematically shows changes in signal characteristics in respect to a display device according to an embodiment of the present invention.
FIG. 2B is an illustration that schematically shows changes in signal characteristics in respect to a display device according to an embodiment of the present invention.
FIG. 2C is an illustration that schematically shows changes in signal characteristics in respect to a display device according to an embodiment of the present invention.
FIG. 2D is an illustration that schematically shows changes in signal characteristics in respect to a display device according to an embodiment of the present invention.
FIG. 2E is an illustration that schematically shows changes in signal characteristics in respect to a display device according to an embodiment of the present invention.
FIG. 2F is an illustration that schematically shows changes in signal characteristics in respect to a display device according to an embodiment of the present invention.
FIG. 3 is a cross-sectional diagram that shows an example of the cross-sectional structure of a pixel circuit provided for a panel of a display device according to an embodiment of the present invention.
FIG. 4 is an illustration that shows an equivalent circuit for a 5Tr/1C driving circuit according to an embodiment of the present invention.
FIG. 5 is a timing chart for driving of the 5Tr/1C driving circuit according to an embodiment of the present invention.
FIG. 6A is an illustration that typically shows ON/OFF state of each of the transistors included in the 5Tr/1C driving circuit according to an embodiment of the present invention, etc.
FIG. 6B is an illustration that typically shows ON/OFF state of each of the transistors included in the 5Tr/1C driving circuit according to an embodiment of the present invention, etc.
FIG. 6C is an illustration that typically shows ON/OFF state of each of the transistors included in the 5Tr/1C driving circuit according to an embodiment of the present invention, etc.
FIG. 6D is an illustration that typically shows ON/OFF state of each of the transistors included in the 5Tr/1C driving circuit according to an embodiment of the present invention, etc.
FIG. 6E is an illustration that typically shows ON/OFF state of each of the transistors included in the 5Tr/1C driving circuit according to an embodiment of the present invention, etc.
FIG. 6F is an illustration that typically shows ON/OFF state of each of the transistors included in the 5Tr/1C driving circuit according to an embodiment of the present invention, etc.
FIG. 6G is an illustration that typically shows ON/OFF state of each of the transistors included in the 5Tr/1C driving circuit according to an embodiment of the present invention, etc.
FIG. 6H is an illustration that typically shows ON/OFF state of each of the transistors included in the 5Tr/1C driving circuit according to an embodiment of the present invention, etc.
FIG. 6I is an illustration that typically shows ON/OFF state of each of the transistors included in the 5Tr/1C driving circuit according to an embodiment of the present invention, etc.
FIG. 7 is an illustration that shows an equivalent circuit for a 2Tr/1C driving circuit according to an embodiment of the present invention.
FIG. 8 is a timing chart for driving of the 2Tr/1C driving circuit according to an embodiment of the present invention.
FIG. 9A is an illustration that typically shows ON/OFF state of each of the transistors included in the 2Tr/1C driving circuit according to an embodiment of the present invention, etc.
FIG. 9B is an illustration that typically shows ON/OFF state of each of the transistors included in the 2Tr/1C driving circuit according to an embodiment of the present invention, etc.
FIG. 9C is an illustration that typically shows ON/OFF state of each of the transistors included in the 2Tr/1C driving circuit according to an embodiment of the present invention, etc.
FIG. 9D is an illustration that typically shows ON/OFF state of each of the transistors included in the 2Tr/1C driving circuit according to an embodiment of the present invention, etc.
FIG. 9E is an illustration that typically shows ON/OFF state of each of the transistors included in the 2Tr/1C driving circuit according to an embodiment of the present invention, etc.
FIG. 9F is an illustration that typically shows ON/OFF state of each of the transistors included in the 2Tr/1C driving circuit according to an embodiment of the present invention, etc.
FIG. 10 is an illustration that shows an equivalent circuit for a 4Tr/1C driving circuit according to the embodiment of the present invention.
FIG. 11 is an illustration that shows an equivalent circuit for a 3Tr/1C driving circuit according to the embodiment of the present invention.
FIG. 12 is a block diagram that shows an example of a luminous time controller according to the embodiment of the present invention.
FIG. 13 is a block diagram that shows an average luminance calculator according to the embodiment of the present invention.
FIG. 14 is an illustration that shows an example of each V-I ratio of a luminescence element for each colour included in a pixel according to the embodiment of the present invention.
FIG. 15 is an illustration that illustrates the way of deriving a value held in a look-up table according to the embodiment of the present invention.
FIG. 16 is an illustration for illustrating the first way of adjusting an effective duty by the luminous time adjuster according to the embodiment of the present invention.
FIG. 17 is an illustration for illustrating the second way of adjusting an effective duty by the luminous time adjuster according to the embodiment of the present invention.
FIG. 18 is an illustration for illustrating the third way of adjusting an effective duty by the luminous time adjuster according to the embodiment of the present invention.
FIG. 19 is a flow diagram that shows an example of the method of processing a picture signal according to the embodiment of the present invention.



REFERENCE SIGNS LIST


    100 display device
    110 picture signal processor
    116 linear converter
    126 luminous time controller
    132 gamma converter
    160 adjustment signal generator
    200 average luminance calculator
    202 luminescence amount regulator
    204 adjuster
    206 luminous time adjuster
    208 gain adjuster
    210 primary gain adjuster
    212 secondary gain adjuster
    250 current ratio adjuster
    252 average value calculator


DESCRIPTION OF EMBODIMENTS
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the appended drawings. Note that, in this specification and the drawings, elements that have substantially the same function and structure are denoted with the same reference numerals, and repeated explanation is omitted.
Example of Display Device According to Embodiment of Invention
First, an example of the configuration of a display device according to an embodiment of the present invention will be described. FIG. 1 is an illustration that shows an example of the configuration of the display device 100 according to an embodiment of the present invention. Besides, in the following, an organic EL display, which is a self-luminescence display device, will be described as an example of the display devices according to an embodiment of the present invention. Also, in the following, the explanation will be provided with assumption that a picture signal input into the display device 100 is a digital signal used in digital broadcasting, for example, though it is not limited as such; for example, such a picture signal may be an analogue signal used in analogue broadcasting, for example.
With reference to FIG. 1, the display device 100 includes a controller 104, a recorder 106, a picture signal processor 110, a memory 150, a data driver 152, a gamma circuit 154, an overflowing-current detector 156, and a panel 158. Also, the display device 100 may include one or more ROMs (Read Only Memories) in which data for control and signal processing software are recorded, an operating unit (not shown) operable for users, etc. Now, examples of the operating unit (not shown) include, but are not limited to, buttons, directional keys, a rotary selector, such as a Jog-dial, and any combinations thereof.
The controller 104 includes an MPU (Micro Processing Unit), for example, and controls the entire display device 100.
The control that is executed by the controller 104 includes executing a signal process on a signal transmitted from the picture signal processor 110, and passing a processing result to the picture signal processor 110. Now, the above signal process by the controller 104 includes, for example, calculating a gain for use in adjustment on the luminance of an image to be displayed on the panel 158, but is not limited thereto.
The recorder 106 is one means for storing included in the display device 100, and able to hold information for controlling the picture signal processor 110 by the controller 104. The information held in the recorder 106 includes, for example, a table in which parameters are preset for executing by the controller 104 a signal process on a signal transmitted from the picture signal processor 110. And, examples of the recorder 106 include, but are not limited to, magnetic recording media like Hard Disks, and non volatile memories like EEPROMs (Electrically Erasable and Programmable Read Only Memories), flash memories, MRAMs (Magnetoresistive Random Access Memories), FeRAMs (Ferroelectric Random Access Memories), and PRAMs (Phase change Random Access Memories).
The signal processor 110 may perform a signal process on a picture signal input. Now, the signal processor 110 may perform a signal process by hardware (e.g., signal processing circuits) or software (signal processing software). In the following, an example of the configuration of the picture signal processor 110 will be explained.
[One Example of Configuration of Picture Signal Processor 110]
The signal processor 110 includes an edge blurrer 112, an I/F 114, a linear converter 116, a pattern generator 118, a colour temperature adjuster 120, a still image detector 122, a long-term colour temperature adjuster 124, a luminous time controller 126, a signal level adjuster 128, an unevenness adjuster 130, a gamma converter 132, a dither processor 134, a signal output 136, a long-term colour temperature adjusting detector 138, a gate pulse output 140, and a gamma circuit controller 142.
The edge blurrer 112 executes on an input picture signal a signal process for blurring the edge. Specifically, the edge blurrer 112 prevents a sticking phenomenon of an image onto the panel 158 (which will be described later) by intentionally shifting an image that is indicated by the picture signal and blurring its edge. Now, the sticking phenomenon is a deterioration phenomenon of luminescence characteristics that occurs in the case where the frequency for a particular pixel of the panel 158 to become luminous is higher than those of the other pixels. The luminance of a pixel that has deteriorated of the sticking phenomenon of an image is lower than the luminance of the other pixels that have not deteriorated. Therefore, difference in luminance between a pixel which has been and the surrounding pixels which have not deteriorated becomes larger. Due to such difference in luminance, users of the display device 100 who see pictures and images displayed by the display device 100 would find the screen as if letters are sticking on it.
For example, the I/F 114 is an interface for transmitting/receiving a signal to/from elements outside the picture signal processor 110, such as the controller 104.
The linear converter 116 executes gamma adjustment on an input picture signal to adjust it to a linear picture signal. For example, if the gamma value of an input signal is “2.2,” the linear converter 116 adjusts the picture signal so that its gamma value becomes “1.0.”
The pattern generator 118 generates test patterns for use in image processes inside the display device 100. The test patterns for used in image processes inside the display device 100 include, for example, a test pattern which is used for display check on the panel 158, but are not limited thereto.
The colour temperature adjuster 120 adjusts the colour temperature of an image indicated by a picture signal, and adjusts colours to be displayed on the panel 158 of the display device 100. Besides, the display device 100 may include colour temperature adjusting means (not shown) by which a user who uses the display device 100 can adjust colour temperature. By the display device 100 including the colour temperature adjusting means (not shown), users can adjust the colour temperature of an image displayed on the screen. Now, examples of the colour temperature adjusting means (not shown) which the can be included in the display device include, but are not limited to, buttons, directional keys, a rotary selector, such as a Jog-dial, and any combinations thereof. Moreover, the colour temperature adjusting means (not shown) may be an integrated unit combined with the operating unit (not shown).
The still image detector 122 detects a chronological difference between input picture signals. And it determines that the input picture signals indicate a still image if a predetermined time difference is not detected. The detection result from the still image detector 122 may used for preventing a sticking phenomenon on the panel 158 and inhibiting deterioration of luminescence elements, for example.
The long-term colour temperature adjuster 124 adjusts aging-related changes of red (designated “R” below), green (designated “G” below), and blue (designated “B” below) sub-pixels included in each pixel of the panel 158. Now, respective luminescence elements (organic EL elements) for respective colours included in a sub-pixel of a pixel vary in L-T characteristics (luminance-time characteristics). Hence, with aging-related deterioration of luminescence elements, the colour balance will be lost when an image indicated by a picture signal is displayed on the panel 158. Therefore, the long-term colour temperature adjuster 124 compensates a luminescence element (organic EL element) for each colour included in a sub-pixel for its aging-related deterioration.
The luminous time controller 126 controls the luminous time per unit time for each pixel of the panel 158. More specifically, the luminous time controller 126 controls the ratio of the luminous time of a luminescence element to a unit time (or rather, the ratio of luminousness to dead screen for a unit time, which will be called a “duty” below). The display device 100 can display the image indicated by a picture signal for a predetermined time period by applying a current selectively to the pixels of the panel 158. And, a “unit time” according to the embodiment of the present invention may be assumed as a “unit time that passes one after another cyclically.” Besides, in the following context, the explanation will be provided with assumption that the “unit time” is “one frame period,” but “unit times” according to the embodiment of the present invention is not limited to such “one frame period,” of course.
Also, the luminous time controller 126 may control the luminous time (duty) so as to prevent the current from overflowing into each of the pixels (strictly, the luminescence elements of each of the pixels) of the panel 158. Now an overflowing current to be prevented by the luminous time controller 126 mainly represents the fact (an overload) that a larger current amount larger than tolerance of the pixels of the panel 158 flows the pixels.
Moreover, the luminous time controller 126 may control a luminous time (duty) and also control the gain of a picture signal. By the luminous time controller 126 controlling a luminous time (duty) and the gain of a picture signal, an overflowing current may be prevented, and higher display quality may also be achieved by controlling the occurrence of the phenomena that lower the display quality, such as flickers and blurred movements, for example.
The details will be described later of the configuration of the luminous time controller 126 according to the embodiment of the present invention and control over a luminous time and the gain of a picture signal in respect to the display device 100 according to the embodiment of the present invention.
The signal level adjuster 128 determines a risk degree for developing an image sticking phenomenon in order to prevent the image sticking phenomenon. And, the signal level adjuster 128 adjusts luminance of a picture to be displayed on the panel 158 by adjusting the signal level of a picture signal in order to prevent an image sticking phenomenon when the risk degree is equal to or over a predetermined value.
The long-term colour temperature adjusting detector 138 detects information for use by the long-term colour temperature adjuster 124 in compensating a luminescence element with its aging-related deterioration. The information detected by the long-term colour temperature adjusting detector 138 may be sent to the controller 104 through the I/F 114 to be recorded onto the recorder 106 via the controller 104.
The unevenness adjuster 130 adjusts the unevenness, such as horizontal stripes, vertical stripes, and spots in the whole screen, which might occur when an image or a picture indicated by a picture signal is displayed on the panel 158. For example, the unevenness adjuster 130 may perform an adjustment with reference to the level of an input signal and a coordinate position.
The gamma converter 132 executes a gamma adjustment on the picture signal into which a picture signal has been converted to have a linear characteristic by the linear converter 116 (more strictly, a picture signal output from the unevenness adjuster 130) so as to perform adjustment so that the picture signal have a predetermined gamma value. Now, such a predetermined gamma value is a value by which the V-I characteristic of a pixel circuit (to be described later) included in the panel 158 of the display device 100 (voltage-current characteristics; more strictly, the V-I characteristic of a transistor included in the picture circuit) can be cancelled. By the gamma converter 132 executing the gamma adjustment on a picture signal to give it a predetermined gamma value as described above, the relation between light amount of an object indicated by the picture signal and a current to be applied to luminescence elements can be handled linearly.
The dither processor 134 performs a dithering process on the picture signal which has been executed a gamma adjustment by the gamma converter 132. Now, the dithering is to display with displayable colours combined in order to represent medium colours in an environment where the number of available colours is small. Colours which can not be normally displayed on the panel can be seemingly represented, produced by performing dithering by the dither processor 134.
The signal output 136 outputs to the outside of the picture signal processor 110 the picture signal on which a dithering process is performed by the dither processor 134. Now, the picture signal output from the signal output 136 may be provided as a signal separately given for each colour of R, G, and B.
The gate pulse output 140 outputs a selection signal for controlling the luminousness and the luminous time of each pixel of the panel 158. Now, the selection signal is based on a duty output by the luminous time controller 126; thus, for example, luminescence elements of a pixel may be luminous when a selection signal is at a high level, and luminescence elements of a pixel may be not luminous when a selection signal is at a low level.
The gamma circuit controller 142 outputs a predetermined setting value to the gamma circuit 154 (to be described later). Now, such a predetermined setting value output from the gamma circuit controller 142 by the gamma circuit controller 142 can be a reference voltage to be given to a ladder resistance of a D/A converter (Digital-Analogue Converter) included in the data driver 152 (to be described later).
The picture signal processor 110 may execute various signal processes on an input picture signal by the configurations described above.
The memory 150 is alternative means for storing included in the display device 100. The information held in the memory 150 includes, for example, information necessary in the case where the signal level adjuster 128 adjusts luminance; the information has information on a pixel or a group of pixels which are luminous at the luminance over a predetermined luminance and corresponding information on the exceeding quantity. However, the information held in the memory 150 is not limited thereto. And, examples of the memory 150 include, but are not limited to, volatile memories, such as SDRAMs (Synchronous Dynamic Random Access Memory) and SRAMs (Static Random Access Memory). For example, the memory 150 may be a magnetic recording medium, such as a hard disk, or a non volatile memory, such as a flash memory.
The data driver 152 converts the signal output from the signal output 136 into a voltage signal to be applied to each pixel of the panel 158, and outputs the voltage signal to the panel 158. Now, the data driver 152 may include a D/A converter for converting a picture signal as a digital signal into a voltage signal as an analogue signal.
The gamma circuit 154 outputs a reference voltage to be given to a ladder resistance of the D/A converter included in the data driver 152. The reference voltage output to the data driver 152 by the gamma circuit 154 may be controlled by the gamma circuit controller 142.
When an overflowing current is generated due to, for example, a short circuit on a substrate (not shown), the overflowing current detector 156 detects the overflowing current, and informs the gate pulse output 140 of the generation of the overflowing current. For example, the gate pulse output 140 informed of the overflowing current generation by the overflowing current detector 156 may refrain from applying a selection signal to each pixel of the panel 158, so that the overflowing current is prevented from being applied to the panel 158.
The panel 158 is a display included in the display device 100. The panel 158 has a plurality of pixels arranged in a matrix pattern. Also, the panel 158 has data lines, to which a voltage signal depending on a picture signal in correspondence to each pixel is applied, and scan lines, to which a selection signal is applied. For example, the panel 158 which displays a picture at definition of SD (Standard Definition) has at least 640×480=307200 (Data Lines×Scan Lines) pixels, and if these pixels are formed out of R, G, and B sub-pixels for provide coloured display, then it has 640×480×3=921600 (Data Lines×Scan Lines×Number of Sub-Pixels) sub-pixels. Similarly, the panel 158 which displays a picture at definition of HD (High Definition) has 1920×1080 pixels, and for coloured display, it has 1920×1080×3 sub-pixels.
[Application Example of Sub-Pixels: with Organic EL Elements Included]
If the luminescence elements included in a sub-pixel of each pixel are organic EL elements, the I-L characteristics will be linear. As described above, the display device 100 can get the relation between the light amount of an object indicated by a picture signal and the current amount to be applied to the luminescence elements to be linear by the gamma adjustment by the gamma converter 132. Thus, the display device 100 can get the relation between the light amount of an object indicated by a picture signal and a luminescence amount to be linear, so that a picture and an image can be displayed accurately in accordance to the picture signal.
Also, the panel 158 includes in each pixel a pixel circuit for controlling a current amount to be applied. A pixel circuit includes a switching element and a driving element for controlling a current amount by an applied scan signal and an applied voltage signal, and also a capacitor for holding a voltage signal, for example. The switching element and the driving element are formed out of TFTs (Thin Film Transistors), for example. Now, because the transistors included in pixel circuits are different from each other in V-I characteristic, the V-I characteristic of the panel 158 as a whole is different from the V-I characteristics of the panels included in the other display devices that are configured similarly to the display device 100. Therefore, the display device 100 gets the relation between the light amount of an object indicated by a picture signal and the current amount to be applied to luminescence elements to be linear by performing a gamma adjustment in correspondence to the panel 158 by the above-described gamma converter 132 so as to cancel the V-I characteristic of the panel 158. Besides, there will be described later examples of the configuration of a pixel circuit included in the panel 158 according to an embodiment of the present invention.
The display device 100 according to an embodiment of the present invention can display a picture and an image according to an input picture signal, configured as shown in FIG. 1. Besides, although the picture signal processor 110 is shown in FIG. 1 with the linear converter 116 followed by the pattern generator 118, it is not limited to such a configuration, and a picture signal processor may have the pattern generator 118 followed by the linear converter 116.
(Outline of Changes in Signal Characteristics for Display Device 100)
Next, there will be described the outline of changes in signal characteristics in respect to the above-described display device 100 according to an embodiment of the present invention will be described. Each of FIG. 2A-FIG. 2F is an illustration that schematically shows changes in signal characteristics in respect to the display device 100 according to an embodiment of the present invention.
Now, each graph in FIG. 2A-FIG. 2F shows chronologically a process in the display device 100, and the left diagrams in FIG. 2B-FIG. 2E show signal characteristics as results of the respective preceding processes; for example, “the signal characteristic as a result of the process in FIG. 2A corresponds to the left diagram in FIG. 2B.” The right diagrams in FIG. 2A-FIG. 2E show signal characteristics for use as coefficients in the processes.
[First Signal Characteristic Change: Change Due to Process by Linear Converter 116]
As shown in the left diagram of FIG. 2A, for example, a picture signal transmitted from a broadcasting station or the like (a picture signal input into the picture signal processor 110) has a predetermined gamma value (e.g., “2.2”). The linear converter 116 of the picture signal processor 110 adjusts it into a picture signal with a characteristic that gives a linear relation between the light amount of an object indicated by a picture signal and an output B, by multiplying the gamma curve (linear gamma: the right diagram of FIG. 2A) that is inverse to the gamma curve (the left diagram of the FIG. 2A) indicated by the picture signal input into the picture signal processor 110, so that the gamma value of the picture signal input into the picture signal processor 110 is cancelled.
[Second Signal Characteristic Change: Change Due to Process by Gamma Converter 132]
The gamma converter 132 of the picture signal processor 110 multiplies the gamma curve (panel gamma: the right diagram of the FIG. 2B) inverse to the gamma curve unique to the panel 158 in advance in order to cancel the V-I characteristic (the right diagram of the FIG. 2D) of a transistor included in the panel 158.
[Third Signal Characteristic Change: Change Due to D/A Conversion by Data Driver 152]
FIG. 2C shows the case where the picture signal is D/A-converted by the data driver 152. As shown in FIG. 2C, the picture signal is D/A-converted by the data driver 152, so that the relation for the picture signal between the light amount of an object indicated by the picture signal and the voltage signal into which the picture signal is D/A-converted will be as the left diagram of the FIG. 2D.
[Forth Signal Characteristic Change: Change at Pixel Circuit of Panel 158]
FIG. 2D shows the case where the voltage signal is applied to a pixel circuit included in the panel 158 by the data driver 152. As shown in FIG. 2B, the gamma converter 132 of the picture signal processor 110 has multiplied a panel gamma in correspondence to the V-I characteristic of a transistor included in the panel 158 in advance. Therefore, if the voltage signal is applied to the pixel circuit included in the panel 158, the relation for the picture signal between the light amount of an object indicated by the picture signal and the current to be applied to the pixel circuit will be linear as shown in the left diagram of FIG. 2E.
[Fifth Signal Characteristic Change: Change at Luminescence Element (Organic EL Element) of Panel 158]
As shown in the right diagram of FIG. 2E, the I-L characteristic of an organic EL element (OLED). Therefore, at a luminescence element of the panel 158, since both of the multiplied factors have linear signal characteristics as shown in FIG. 2E, the relation for the picture signal between the light amount of an object indicated by the picture signal and the luminescence amount of the luminescence element is a linear relation (FIG. 2F).
As shown in FIG. 2A-FIG. 2F, the display device 100 may have a linear relation between the light amount of an object indicated by an input picture signal and the luminescence amount of a luminescence element. Therefore, the display device 100 can display a picture and an image accurately according to the picture signal.
(Example of Configuration of Pixel Circuit Included in Panel 158 of Display Device 100)
Next, there will be described an example of the configuration of a pixel circuit included in the panel 158 of the display device 100 according to an embodiment of the present invention. And, in the following, the explanation will be provided with assumption that the luminescence element is an organic EL element, for example.
[1] Structure of Pixel Circuit
First, the structure of a pixel circuit included in the panel 158 will be described. FIG. 3 is a cross-sectional diagram that shows an example of the cross-sectional structure of a pixel circuit provided for the panel 158 of the display device 100 according to the present invention.
With reference to FIG. 3, the pixel circuit provided for the panel 158 is configured to have a dielectric film 1202, a dielectric planarising film 1203, and a window dielectric film 1204, each of which is formed in this order on a glass substrate 1201 where a driving transistor 1022 and the like are formed, and to have organic EL elements 1021 provided for recessed parts 1204A in this window dielectric film 1204. Besides, in FIG. 3, only the driving transistor 1022 of each element of the driving circuit is depicted, and depictions for the other elements are omitted.
An organic EL element 1021 includes an anode electrode 1205 made of metals and the like formed at the bottom part of a recessed part 1204A in the above-mentioned window dielectric film 1204, and an organic layer (electron transport layer, luminescence layer, and hole transmit layer/hole inject layer) 1206 formed on this anode electrode 1205, a cathode electrode 1207 made of a transparent conductive film and the like formed on this organic layer commonly for all of the elements.
In the organic EL element 1021, the organic layer is formed by sequentially depositing a hole transmit layer/hole inject layer 2061, and a luminescence layer 2062, an electrode transport layer 2063, and an electrode inject layer (not shown) on the anode electrode 1205. Now, with a current flowing from the driving transistor 1022 to the organic layer 1206 through the anode electrode 1205, the organic EL element 1021 becomes luminous when an electron and a hole recombine at the luminescence layer 2062.
The driving transistor 1022 includes a gate electrode 1221, a source/drain area 1223 provided on one side of a semiconductor layer 1222, a drain/source area 1224 provided on the other side of the semiconductor layer 1222, a channel forming area 1225 which is a part opposite to the gate electrode 1221 of the semiconductor layer 1222. And, the source/drain area 1223 is electrically connected to the anode electrode 1205 of the organic EL element 1021 via a contact hole.
After the organic EL element 1021 has been formed on a pixel basis on the glass substrate 1201 on which the driving circuit is formed, a sealing substrate 1209 is bonded via a passivation film 1208 by adhesive 1210, and then the organic EL element 1021 is sealed by this sealing substrate 1209, thus the panel 158 is formed.
[2] Driving Circuit
Next, an example of the configuration of a driving circuit provided for the panel 158 will be described.
The driving circuit included in a pixel circuit of the panel 158 including organic EL elements could vary depending on the number of transistors and the number of capacitors, where the transistors and the capacitors are included in the driving circuit. Examples of the driving circuit includes a driving circuit including 5 transistors/1 capacitor (which may be designated below as a “5Tr/1C driving circuit”), a driving circuit including 4 transistors/1 capacitor (which may be designated below as a “4Tr/1C driving circuit”), a driving circuit including 3 transistors/1 capacitor (which may be designated below as a “3Tr/1C driving circuit”), and a driving circuit including 2 transistors/1 capacitor (which may be designated below as a “2Tr/1C driving circuit”). Then, first of all, the common matters amongst the above driving circuits will be described.
In the following, for reasons of simplicity, each transistor included in a driving circuit will be described with the assumption that it includes an n-channel type TFT. Besides, a driving circuit according to an embodiment of the present invention can, of course, include p-channel type TFTs. And, a driving circuit according to an embodiment of the present invention can be configured to have transistors formed on a semiconductor substrate or the like. In other words, the structure of a transistor included in a driving circuit according to an embodiment of the present invention is not particularly limited. And, in the following, a transistor included in a driving circuit according to an embodiment of the present invention will be described with the assumption that it is enhancement type, though it is not limited thereto; a depression type transistor may be also used. Furthermore, a transistor included in a driving circuit according to an embodiment of the present invention may be single gate type or dual gate type.
And, in the following explanation, it is assumed that the panel 158 includes (N/3)×M pixels arranged in a 2-dimension matrix pattern (M is a natural number larger than 1; N/3 is a natural number larger than 1), and that each pixel include three sub-pixels (an R luminescence sub-pixel that generates red light, a G luminescence sub-pixel that generates green light, and a B luminescence sub-pixel that emits blue light). And, luminescence elements included in each pixel are assumed to be line sequentially driven, and the display frame rate is represented by FR (frames/sec.). Now, luminescence elements included in each of (N/3) pixels arranged in the m-th row (m=1, 2, 3, . . . , M), or more specifically N sub-pixels, will be driven simultaneously. In other words, the timing for emitting light or not of each luminescence element included in one row is controlled on the basis of the row to which they belong. Now, the process for writing a picture signal onto each pixel included in one row may be a process of writing a picture signal simultaneously onto all of the pixels (which may be designated as the “simultaneous writing process”), or a process of writing a picture signal sequentially onto each pixel (which may be designated as the “sequential writing process”). Either of the writing processes is optionally chosen depending on the configuration of a driving circuit.
And, in the following, driving and operating related to the luminescence element located on the m-th row and the n-th column (n=1, 2, 3, . . . , N) will be described, where such a luminescence element is designated as the (n, m) luminescence element or the (n, m) sub-pixel.
Until a horizontal scanning period (m-th horizontal scanning period) for each luminescence element arranged in m-th row expires, various processes (the threshold voltage cancelling process, the writing process, and the mobility adjusting process, each of which will be described below) are performed in the driving circuit. Now, the writing process and the mobility adjusting process are necessarily performed during the m-th horizontal scanning period, for example. And, with some types of driving circuit, the threshold voltage cancelling process and the corresponding pre-process can be performed prior to the m-th horizontal scanning period.
Then, after all of the above-mentioned various processes are done, a luminescence part included in each luminescence element arranged in the m-th row is made luminous by the driving circuit. Now, the driving circuit may make the luminescence parts luminous immediately when all of the above-mentioned various processes are done, or after a predetermined period (e.g., a horizontal scanning period for the predetermined number of rows) expires. And, such periods can be optionally set, depending on the specification of a display device and the configuration of a driving circuit and the like. Besides, in the following explanation, for reasons of simplicity, luminescence parts are assumed to be made luminous immediately when various processes are done.
The luminosity of a luminescence part included in each luminescence element arranged in the m-th row is maintained, for example, until just before beginning of the horizontal scanning period of each luminescence element arranged in (m+m′)-th row, where “m′” is determined according to the design specification of a display device. In other words, the luminosity of a luminescence part included in each luminescence element arranged in the m-th row in a given display frame is maintained until the (m+m′−1)-th horizontal scanning period. And, for example, from the beginning of the (m+m′)-th horizontal scanning period until the writing process or the mobility adjusting process are done within the m-th horizontal scanning period in the next display frame, a luminescence part included in each luminescence element arranged in the m-th row maintains non luminous state. And, the time length of a horizontal scanning period is a time length shorter than (1/FR)×(1/M) seconds, for example. Now, if the value of (m+m′) is above M, the horizontal scanning period for the extra is managed in the next display frame, for example.
By provide the above-mentioned period of non luminous state (which may be simply designated as non luminous period in the following), afterimage blur involved in active matrix driving is reduced for the display device 100, and quality of moving image can be more excellent. Besides, the luminous state/non luminous state of each sub-pixel (more strictly a luminescence element included in a sub-pixel) according to an embodiment of the present invention is not limited as such.
And, in the following, for two source/drain areas of one transistor, the term “one source/drain area” may be used in the meaning of the source/drain area on the side connected to a power source. And, the case where a transistor is in ON state means a situation that a channel is formed between source/drain areas. It does not matter here whether a current flows from one source/drain area of this transistor to another. And, the case where a transistor is in OFF state means a situation that no channel is formed between source/drain areas. And, the case where a source/drain area of a given transistor is connected to source/drain area of another transistor embraces a mode where the source/drain area of the given transistor and the source/drain area of the other transistor possess the same area. Furthermore, a source/drain area can be formed not only from conductive materials, such as polysilicon, amorphous silicon and the like, but also from metals, alloys, conductive particles, layered structure thereof, and a layer made of organic materials (conductive polymers), for example.
Furthermore, in the following, timing charts would be shown for explaining driving circuits according to an embodiment of the present invention, where lengths (time lengths) along the transverse axis indicating respective periods are typical, and they do not indicate any rate of time lengths of various periods.
[2-2] Driving Method of Driving Circuit
Next, a method of driving a driving circuit according to an embodiment of the present invention will be described. FIG. 4 is an illustration that shows an equivalent circuit for a 5Tr/1C driving circuit according to an embodiment of the present invention. Besides, in the following, the method of driving a driving circuit according to an embodiment of the present invention will be described with an exemplary 5Tr/1C driving circuit with reference to FIG. 4, whilst a similar driving method is basically used for the other driving circuits.
A driving circuit according to an embodiment of the present invention is driven by (a) the pre-process, (b) the threshold voltage cancelling process, (c) the writing process, and (d) the luminescence process shown below, for example.
(a) Pre-Process
In the pre-process, a first-node initialising voltage is applied to the first node ND1, and a second-node initialising voltage is applied to the second node ND2. Now, the first-node initialising voltage and the second-node initialising voltage are applied, so that the potential difference between the first node ND1 and the second node ND2 is above the threshold voltage of the driving transistor TRD and the potential difference between the second node ND2 and the cathode electrode included in the luminescence part ELP is not above the threshold voltage of the luminescence part ELP.
(b) Threshold Voltage Cancelling Process
In the threshold voltage cancelling process, the voltage of the second node ND2 is changed towards a voltage obtained by subtracting the threshold voltage of the driving transistor TRD from the voltage of the first node ND1, with the voltage of the first node ND1 maintained.
More specifically speaking, in order to change the voltage of the first node ND1 towards the voltage obtained by subtracting the threshold voltage of the driving transistor TRD from the voltage of the first node ND1, a voltage which is above a voltage obtained by adding the threshold voltage of the driving transistor TRD to the voltage of the second node ND2 in the process of (a) is applied to one source/drain area of the driving transistor TRD. Now, in the threshold voltage cancelling process, how close the potential difference between the first node ND1 and the second node ND2 (i.e., the potential difference the gate electrode and the source area of the driving transistor TRD) approaches to the threshold voltage of the driving transistor TRD depends qualitatively on time for the threshold voltage cancelling process. Therefore, as in a mode where enough long time is secured for the threshold voltage cancelling process, the voltage of the second node ND2 reaches at the voltage obtained by subtracting the threshold voltage of the driving transistor TRD from the voltage of the first node ND1, and the driving transistor TRD gets in OFF state. On the other hand, as in a mode where there is no choice but to set the time for the threshold voltage cancelling process short, the potential difference between the first node ND1 and the second node ND2 may be larger than the threshold voltage of the driving transistor TRD, and the driving transistor TRD may be not get in OFF state. Hence, in the threshold voltage cancelling process, the driving transistor TRD does not necessarily get in OFF state as a result of the threshold voltage cancelling process,
(c) Writing Process
In the writing process, a picture signal is applied to the first node ND1 from the data line DTL via the writing transistor TRW that is made to be in ON state by a signal from the scan line SCL.
(d) Luminescence Process
In the Luminescence Process, the luminescence part ELP become luminous (is driven) by making the writing transistor TRW to be in OFF state by a signal from the scan line SCL to make the first node ND1 to be in floating state and running a current depending on the value of the potential difference between the first node ND1 and the second node ND2 from the power source unit 2100 to the luminescence part ELP via the driving transistor TRD.
A driving circuit according to an embodiment of the present invention is driven by the above processes of (a)-(d), for example.
[2-3] Examples of Configuration of Driving Circuit and Specific Examples of Driving Method
Next, for each driving circuit, examples of the configurations of the driving circuits and a method of driving such driving circuits will be described specifically below. Besides, in the following, a 5Tr/1C driving circuit and a 2Tr/1C driving circuit out of various driving circuits will be described.
[2-3-1] 5Tr/1C Driving Circuit
First, a 5Tr/1C driving circuit will be described with reference to FIG. 4-FIG. 6I. FIG. 5 is a timing chart for driving of the 5Tr/1C driving circuit according to an embodiment of the present invention. FIG. 6A-FIG. 6I are illustrations that typically show respective ON/OFF states of the transistors included in the 5Tr/1C driving circuit according to an embodiment of the present invention shown in FIG. 4, etc.
With reference to FIG. 4, the 5Tr/1C driving circuit includes a writing transistor TRW, a driving transistor TRD, a first transistor TR1, a second transistor TR2, a third transistor TR3, and a capacitor C1; namely, the 5Tr/1C driving circuit includes five transistors and one capacitor. Besides, in the example shown in FIG. 4, the writing transistor TRW, the first transistor TR1, the second transistor TR2, and the third transistor TR3 are formed out of n-channel type TFTs, though they are not limited thereto; they may also be formed out of p-channel type TFTs. And, the capacitor C1 may be formed out of a capacitor with a predetermined capacitance.
<First Transistor TR1>
One source/drain area of the first transistor TR1 is connected to a power source unit 2100 (voltage Vcc), and the other source/drain area of the first transistor TR1 is connected to one source/drain area of the driving transistor TRD. And, the ON/OFF operation of the first transistor TR1 is controlled by a first-transistor control line CL1, which is extended from a first-transistor control circuit 2111 to connect to the gate electrode of the first transistor TR1. Now, the power source unit 2100 is provided for supply a current to a luminescence part ELP to make the luminescence part ELP luminous.
<Driving Transistor TRD>
One source/drain area of the driving transistor TRD is connected to the other source/drain area of the first transistor TR1. And, the other source/drain area of the driving transistor TRD is connected to the anode electrode of the luminescence part ELP, the other source/drain area of the second transistor TR2, and one source/drain area of the capacitor C1, and forms a second node ND2. And, the gate electrode of the driving transistor TRD is connected to the other source/drain area of the writing transistor TRW, the other source/drain area of the third transistor TR3, and the other electrode of the capacitor C1, and forms a first node ND1.
Now, in the case of the luminous state of a luminescence element, the driving transistor TRD is driven to flow a drain current Ids according to Equation 1 below, for example, where “μ” shown in Equation 1 denotes a “effective mobility,” and “L” denotes a “channel length.” And similarly, “W” shown in Equation 1 denotes a “channel width,” “Vgs” denotes the “potential difference between the gate electrode and the source area, “Vth” denotes a “threshold voltage,” “Cox” denotes “(Relative Permittivity of Gate Dielectric Layer)×(Permittivity of Vacuum)/(Thickness of Gate Dielectric Layer),” and “k” denotes “k=(1⁄2)·(W/L)·Cox,” respectively.

Ids=k·μ·(Vgs−Vth)2  Equation 1

And, in the case of the luminous state of a luminescence element, one source/drain area of the driving transistor TRD works as a drain area, and the other source/drain area works as a source area. Besides, in the following, for the reason of simplicity of explanation, in the following explanation, one source/drain area of the driving transistor TRD may be simply designated as the “drain area”, and the other source/drain area may be simply designated as the “source area”.
The luminescence part ELP becomes luminous due to the drain current Ids shown in Equation 1 flowing thereto, for example. Now, the luminescence state (luminance) of the luminescence part ELP is controlled depending on the magnitude of the value of the drain current Ids.
<Writing Transistor TRW>
The other source/drain area of the writing transistor TRW is connected to the gate electrode of the driving transistor TRD. And, one source/drain area of the writing transistor TRD is connected a data line DTL, which is extended from a signal output circuit 2102. Then, a picture signal VSig for controlling the luminance of the luminescence part ELP is supplied to the one source/drain area via the data line DTL. Besides, various signals and voltages (signals for pre-charge driving, various reference voltages, etc.) except for the picture signal VSig may be supplied to the one source/drain area via the data line DTL. And, the ON/OFF operation of the writing transistor TRW is controlled by a scan line SCL, which is extended from a scanning circuit 2101 to connect to the gate electrode of the writing transistor TRW.
<Second Transistor TR2>
The other source/drain area of the second transistor TR2 is connected to the source area of the driving transistor TRD. And, a voltage VSS for initialising the potential of the second node ND2 (i.e., the potential of the source area of the driving transistor TRD) is supplied to one source/drain area of the second transistor TR2. And, the ON/OFF operation of the second transistor TR2 is controlled by a second-transistor control line AZ2, which is extended from a second-transistor control circuit 2112 to connect to the gate electrode of the second transistor TR2.
<Third Transistor TR3>
The other source/drain area of the third transistor TR3 is connected to the gate electrode of the driving transistor TRD. And, a voltage VOfs for initialising the potential of the first node ND1 (i.e., the potential of the gate electrode of the driving transistor TRD) is supplied to one source/drain area of the third transistor TR3. And, the ON/OFF operation of the third transistor TR3 is controlled by a third-transistor control line AZ3, which is extended from a third-transistor control circuit 2113 to connect to the gate electrode of the third transistor TR3.
<Luminescence Part ELP>
The anode electrode of the luminescence part ELP is connected to the source area of the driving transistor TRD. And, a voltage VCat is applied to the cathode electrode of the luminescence part ELP. In FIG. 4, the capacitance of the luminescence part ELP is represented by a symbol: CEL. And, a threshold voltage which is necessary for the luminescence part ELP to be luminous is represented by Vth-EL. Then, when voltage equal to or more than Vth-EL is applied between the anode and cathode electrodes of the luminescence part ELP, the luminescence part ELP becomes luminous.
Besides, in the following, “VSig” represents a picture signal for controlling luminance of the luminescence part ELP, “VCC” represents the voltage of the power source unit 2100, and “VOfs” represents the voltage for initialising the potential of the gate electrode of the driving transistor TRD (the potential of the first node ND1). And, in the following, “VSS” represents the voltage for initialising the potential of the source area of the driving transistor TRD (the potential of the second node ND2), “Vth” represents a threshold voltage of the driving transistor TRD, “VCat” represents the voltage applied to the cathode electrode of the luminescence part ELP, and “Vth-EL” represents a threshold voltage of the luminescence part ELP. Furthermore, in the following, the respective values of voltages or potentials are explained, given as follows for example, though respective values of voltages or potentials according to an embodiment of the present invention are not limited as follows, of course.
VSig: 0 [volt]-10 [volt]
VCC: 20 [volt]
VOfs: 0 [volt]
VSS: −10 [volt]
Vth: 3 [volt]
VCat: 0 [volt]
Vth-EL: 3 [volt]
In the following, with reference to FIG. 5 and FIG. 6A-FIG. 6I, the operation of a 5Tr/1C driving transistor will be described. Besides, in the following, the explanation will be provided with the assumption that luminous state starts immediately after all of the above-described various processes (the threshold voltage cancelling process, the writing process, the mobility adjusting process) are done in the 5Tr/1C driving transistor, though it is not limited thereto. The explanations of 4Tr/1C driving circuit, 3Tr/1C driving circuit, and 2Tr/1C driving circuit are similarly provided below.
<A-1> [Period—TP(5)−1] (See FIG. 5 and FIG. 6A)
[Period—TP(5)−1] indicates, for example, an operation in the previous display frame, and is a period for which the (n, m) luminescence element is in luminous state after the last various processes are done. Thus, a drain current I′ based on the equation (5) below flows into a luminescence part ELP of a luminescence element included in the (n, m) sub-pixel, and the luminance of the luminescence element included in the (n, m) sub-pixel is a value depending on this drain current I′. Here, the writing transistor TRW, the second transistor TR2, and the third transistor TR3 are in OFF state, and the first transistor TR1 and the driving transistor TRD are in ON state. The luminous state of the (n, m) luminescence element is maintained until just before the beginning of the horizontal scanning period for a luminescence element arranged in the (m+m′)-th row.
[Period—TP(5)0]-[Period—TP(5)4] are operation periods laid after the luminous state after completion of the last various processes ends, and just before the next writing process is executed. In other words, these [Period—TP(5)0]-[Period—TP(5)4] corresponds to the period of a particular time length from the beginning of the (m+m′)-th horizontal scanning period in the previous display frame to the end of the (m−1)-th horizontal scanning period in the current display frame. Besides, [Period—TP(5)0]-[Period—TP(5)4] may be configured to be included within the m-th horizontal scanning period in the current display frame.
And, for [Period—TP(5)0]-[Period—TP(5)4], the (n, m) luminescence element is basically in non luminous state. In other words, for [Period—TP(5)0]-[Period—TP(5)1] and [Period—TP(5)3]-[Period—TP(5)4], the luminescence element does not emit light since the first transistor TR1 is in OFF state. Now, for [Period—TP(5)2], the first transistor TR1 is in ON state. However, the threshold voltage cancelling process to be described below is executed for [Period—TP(5)2]. Therefore, given that Equation 2 below is satisfied, the luminescence element will not be luminous.
In the following, each period of [Period—TP(5)0]-[Period—TP(5)4] will be described. Besides, the beginning of [Period—TP(5)1], and the length of each period of [Period—TP(5)0]-[Period—TP(5)4] are optionally set according the settings of the display device 100.
<A-2> [Period—TP(5)0]
As described above, for [Period—TP(5)0], the (n, m) luminescence element is in non luminous state. And, the writing transistor TRW, the second transistor TR2, and the third transistor TR3 are in OFF state. Now, because the first transistor TR1 gets into OFF state at the time point for transition from [Period—TP(5)1] to [Period—TP(5)0], the potential of the second node ND2 (the source area of the driving transistor TRD or the anode electrode of the luminescence part ELP) is lowered to (Vth-EL+VCat), and the luminescence part ELP gets into non luminous state. And, as the potential of the second node ND2 gets lower, the potential of the first node ND1 in floating state (the gate electrode of the driving transistor TRD) is also lowered.
<A-3> [Period—TP(5)1] (See FIG. 5, FIG. 6B and FIG. 6C)
For [Period—TP(5)1], there is executed a pre-process for executing the threshold voltage cancelling process. More specifically, at the beginning of [Period—TP(5)1], the second transistor TR2 and the third transistor TR3 are got into ON state by getting the second-transistor control line AZ2 and the third-transistor control line AZ3 to be at high level. As a result, the potential of the first node ND1 becomes VOfs (e.g., 0 [volt]), and the potential of the second node ND2 becomes VSS (e.g., −10 [volt]). Then, before the expiration of [Period—TP(5)1], the second transistor TR2 is got into OFF state by getting the second-transistor control line AZ2 to be at low level. Now, the second transistor TR2 and the third transistor TR3 may be synchronously got into ON state, though they are not limited as such; for example, the second transistor TR2 may be first got into ON state, or the third transistor TR3 may be first got into ON state.
By the process above, the potential between the gate electrode and source area of the driving transistor TRD becomes above Vth. Now, the driving transistor TRD is in ON state.
<A-4> [Period—TP(5)2] (See FIG. 5 and FIG. 6D)
For [Period—TP(5)2], the threshold voltage cancelling process is executed. More specifically, the first transistor TR1 is got into ON state by getting the first-transistor control line CL1 to be at high level with the third transistor TR3 maintained in ON state. As a result, the potential of the first node ND1 does not change (VOfs=0 [volt] maintained), whilst the potential of the second node ND2 changes towards the potential obtained by subtracting the threshold voltage Vth of the driving transistor TRD from the potential of the first node ND1. In other words, the potential of the second node ND2 in floating state increases. Then, when the potential difference between the gate electrode and source area of the driving transistor TRD reaches to Vth, the driving transistor TRD gets into OFF state. Specifically, the potential of the second node ND2 in floating state approaches to (VOfs−Vth=−3 [volt]>VSS) to be (VOfs−Vth) in the end. Now, if Equation 2 below is assured, in other words, if the potentials are selected and determined to satisfy Equation 2, the luminescence part ELP will not be luminous.

(VOfs−Vth)<(Vth-EL+VCat)  Equation 2

For [Period—TP(5)5], the potential of the second node ND2 will be (VOfs−Vth) eventually. Now, the potential of the second node ND2 is determined, depending on the threshold voltage Vth of the driving transistor TRD, and on the potential VOfs for initialising the gate electrode of the driving transistor TRD; namely the potential of the second node ND2 does not depend on the threshold voltage Vth-EL of the luminescence part ELP.
<A-5> [Period—TP(5)3] (See FIG. 5 and FIG. 6E)
For [Period—TP(5)3], the first transistor TR1 is got into OFF state by getting the first-transistor control line CL1 to be at low level with the third transistor TR3 maintained in ON state. As a result, the potential of the first node ND1 does not change (VOfs=0 [volt] maintained), nor the potential of the second node ND2 does not change. Therefore, the potential of the second node ND2 is maintained (VOfs−Vth=−3 [volt]).
<A-6> [Period—TP(5)4] (See FIG. 5 and FIG. 6F)
For [Period—TP(5)4], the third transistor TR3 is got into OFF state by getting the third-transistor control line AZ3 to be at low level. Now, the potentials of the first node ND1 and the second node ND2 do not change substantially. Besides, in practice, potential changes might occur by electrostatic bonding of parasitic capacitances or the like; however, these can be normally neglected.
For [Period—TP(5)0]-[Period—TP(5)4], a 5Tr/1C driving transistor operates as described above. Next, each period of [Period—TP(5)5]-[Period—TP(5)7] will be described. Now, the writing process is executed for [Period—TP(5)5], and the mobility adjusting process is executed for [Period—TP(5)6]. The above-mentioned processes are necessarily executed within the m-th horizontal scanning period, for example. In the following, for the reason of simplicity of the explanation, the explanation will be provided with the assumption that the beginning of [Period—TP(5)5] and the end of [Period—TP(5)6] match the beginning and end of the m-th horizontal scanning period, respectively.
<A-7> [Period—TP(5)5] (See FIG. 5 and FIG. 6G)
For [Period—TP(5)5], the writing process for the driving transistor TRD is executed. Specifically, the data line DTL is made to be VSig for controlling the luminance of the luminescence part ELP with the first transistor TR1, the second transistor TR2, and the third transistor TR3 maintained in OFF state; next, the writing transistor TRW is got into ON state by getting the scan line SCL to be at high level. As a result, the potential of the first node ND1 increases to VSig.
Now, the value of the capacitance of the capacitor C1 is represented by c1, the value of the capacitance of the capacitance CEL of the luminescence part ELP is represented by cEL, and the value of the parasitic capacitance between the gate electrode and source area of the driving transistor TRD is represented by cgs. When the potential of the gate electrode of the driving transistor TRD changes from VOfs to VSig (>VOfs), the potentials of both sides of the capacitor C1 (the potentials of the first node ND1 and the second node ND2) basically change. In other words, potentials based on the change (VSig−VOfs) of the potential of the gate electrode of the driving transistor TRD (=the potential of the first node ND1) are allotted to the capacitor C1, the capacitance CEL of the luminescence part ELP, and the parasitic capacitance between the gate electrode and source area of the driving transistor TRD. Thus, if the value cEL is enough larger than the value c1 and the value cgs, the change of the potential of the source area of the driving transistor TRD (the second node ND2) based on the change (VSig−VOfs) of the potential of the driving transistor TRD is small. Now, in general, the capacitance value cEL of the capacitance CEL of the luminescence part ELP is larger than the capacitance value c1 of the capacitor C1 and the value cgs of the parasitic capacitance of the driving transistor TRD. Thus, in the following, for the reason of simplicity of the explanation, the explanation will be provided, except for the cases in particular necessities, without any regard to potential changes of the second node ND2 which occur by potential changes of the first node ND1. It is the same as described above for the other driving circuits shown below. And, FIG. 5 is shown without any regard to potential changes of the second node ND2 which occur by potential changes of the first node ND1.
And, the value of Vg is as “Vg=VSig” and the value of Vs is as “Vs≈VOfs−Vth,” where Vg is the potential of the gate electrode of the driving transistor TRD (the first node ND1) and Vs is the potential of the source area of the driving transistor TRD (the second node ND2). Therefore, the potential difference between the first node ND1 and the second node ND2, namely the potential difference Vgs between the gate electrode and source area of the driving transistor TRD can be expressed by Equation 3 below.

Vgs≈VSig−(VOfs−Vth)  Equation 3

As shown in Equation 3, Vgs obtained in the writing process for the driving transistor TRD depends on only the picture signal VSig for controlling the luminance of the luminescence part ELP, the threshold voltage Vth of the driving transistor TRD, and the voltage VOfs for initialising the gate electrode of the driving transistor TRD. And it can be seen from Equation 3 that Vgs obtained in the writing process for the driving transistor TRD does not depend on the threshold voltage Vth-EL of the luminescence part ELP.
<A-8> [Period—TP(5)6] (See FIG. 5-FIG. 6H)
For [Period—TP(5)6], an adjustment (mobility adjustment process) on the potential of the source area of the driving transistor TRD (the second node ND2) based on the magnitude of the mobility g of the driving transistor TRD is executed.
In general, if the driving transistor TRD is made of a polysilicon film transistor or the like, it is hard to avoid that the mobility μ varies amongst transistors. Therefore, even if picture signals VSigs of the same value are applied to gate electrodes of a plurality of driving transistors TRDs of different mobility μs, there might be found a difference between a drain current Ids flowing a driving transistor TRD with large mobility μ and a drain Ids flowing a driving transistor TRD with small mobility μ. Then, if such a difference occurs, the uniformity of the screen of the display device 100 will be lost.
Then, for [Period—TP(5)6], the mobility adjusting process is executed in order to prevent the issues described above from occurring. Specifically, the first transistor TR1 is got into ON state by getting the first transistor control line CL1 to be at high level with the writing transistor TRW maintained in ON state: next, by getting the first transistor control line CL1 to be at high level after a predetermined time (t0) has passed, the first transistor TR1 is got into ON state, and next, by getting the scan line SCL to be at low level after a predetermined time (t0) has passed, the writing transistor TRW is got into OFF state, and the first node ND1 (the gate electrode of the driving transistor TRD) is got into floating state. As a result, if the value of the mobility μ of the driving transistor TRD is large, then the increased amount ΔV (potential adjustment value) of the potential of the source area of the driving transistor TRD is large, and if the value of the mobility μ of the driving transistor TRD is small, then the increased amount ΔV (potential adjustment value) of the potential of the source area of the driving transistor TRD is small. Now, the potential difference Vgs between the gate electrode and source area of the driving transistor TRD is transformed, for example, as Equation 4 below, based on Equation 3.

Vgs≈VSig−(VOfs−Vth)−ΔV  Equation 4

Besides, the predetermined time for executing the mobility adjusting process (the total time t0 of [Period—TP(5)6]) can be determined in advance as a configuration value during the configuration of the display device 100. And, the total time t0 of [Period—TP(5)6] can be determined so that the potential of the source area of the driving transistor TRD in this case (VOfs−Vth+ΔV) satisfy Equation 5 below. In such a case, the luminescence part ELP will not be luminous for [Period—TP(5)6]. Moreover, an adjustment on the variation of the coefficient k(≡1⁄2)·(W/L)·Cox) is also executed simultaneously by this mobility adjusting process.

VOfs−Vth+ΔV<(Vth-EL+VCat)  Equation 5

<A-9> [Period—TP(5)7] (See FIG. 6I)

By the above-described operations, the threshold voltage cancelling process, the writing process, and the mobility adjusting process are done. Now, for [Period—TP(5)7], low level of the scan line SCL results in OFF state of the writing transistor TRW, and floating state of the first node ND1, namely the gate electrode of the driving transistor TRD. On the other hand, the first transistor TR1 maintains ON state, the drain area of the driving transistor TRD is in connection with the power source 2100 (voltage Vcc, e.g., 20 [volt]). Thus, for [Period—TP(5)7], the potential of the second transistor TR2 increases.
Now, the gate electrode of the driving transistor TRD is in floating state, and because of the existence of the capacitor C1, the same phenomenon as in so-called bootstrap circuit occurs in the gate electrode of the driving transistor TRD, and also the potential of the first node ND1 increases. As a result, the potential difference Vgs between the gate electrode and source area of the driving transistor TRD maintains the value of Equation 4.
And, for [Period—TP(5)7], the luminescence part ELP starts to be luminous because the potential of the second node ND2 increases to be above (Vth-EL+VCat). At this point, the current flowing to the luminescence part ELP can be expressed by Equation 1 above because it is the drain current Ids flowing from the drain area of the driving transistor TRD to the source area of the driving transistor TRD; where, from Equation 1 above and Equation 4 above, Equation 1 above can be transformed into Equation 6 below, for example.

Ids=k·μ·(VSig−VOfs−ΔV)2  Equation 6

Therefore, for example, if VOfs is set to 0 [volt], the current Ids flowing to the luminescence part ELP is proportional to the square of the value obtained by subtracting the value of the picture signal VSig for controlling the luminance of the luminescence part ELP from the value of the potential adjustment value ΔV of the second node ND2 (the source area of the driving transistor TRD) resulted from the mobility μ of the driving transistor TRD. In other words, the current Ids flowing to the luminescence part ELP does not depend on the threshold voltage Vth-EL of the luminescence part ELP and the threshold voltage Vth of the driving transistor TRD; namely, the luminescence amount (luminance) of the luminescence part ELP is not affected by the threshold voltage Vth-EL of the luminescence part ELP and the threshold voltage Vth of the driving transistor TRD. Then, the luminance of the (n, m) luminescence element is a value corresponding to this current Ids.
And, larger mobility μ of the driving transistor TRD results in a larger potential adjustment value ΔV, then the value of Vgs on the left side of Equation 4 above becomes smaller. Therefore, even if the value of the mobility μ is large in Equation 6, the value of (VSig−VOfs−ΔV)2 becomes small, and as a result, the drain current Ids can be adjusted. Thus, also if values of picture signal VSigs are the same amongst driving transistors TRDs with different mobility μ, the drain currents Idss will be almost the same, and as a result, the currents Idss flowing to the luminescence part ELP for controlling the luminance of the luminescence part ELP is uniformed. Thus, a 5Tr/1C driving circuit can adjust the variation of the luminance of the luminescence parts resulted from the variation of the mobility μ (and further, the variation of k).
And, luminous state of the luminescence part ELP is maintained until the (m+m′−1)-th horizontal scanning period. This time point corresponds to the end of [Period—TP(5)−1].
A 5Tr/1C driving circuit makes a luminescence element luminous by operating as described above.
[2-3-2] 2Tr/1C Driving Circuit
Next, a 2Tr/1C driving circuit will be described. FIG. 7 is an illustration that shows an equivalent circuit for the 2Tr/1C driving circuit according to an embodiment of the present invention. FIG. 8 is a timing chart for driving of the 2Tr/1C driving circuit according to an embodiment of the present invention. FIG. 9A-FIG. 9F are illustrations that typically show ON/OFF state of each of the transistors included in the 2Tr/1C driving circuit according to an embodiment of the present invention, etc.
With reference to FIG. 7, the 2Tr/1C driving circuit omits three transistors, which are the first transistor TR1, the second transistor TR2, and the third transistor TR3, are omitted from the 5Tr/1C driving circuit shown in FIG. 4 described above. In other words, the 2Tr/1C driving circuit includes a writing transistor TRW, a driving transistor TRW, and a capacitor C1.
<Driving Transistor TRD>
The detailed explanation of the configuration the driving transistor TRD is omitted since it is the same as the configuration of the driving transistor TRD described with regard to the 5Tr/1C driving circuit shown in FIG. 4. Besides, the drain area of the driving transistor TRD is connected to the power source unit 2100. And, from the power source unit 2100, the voltage VCC-H for getting the luminescence part ELP luminous and the voltage VCC-L for controlling the potential of the source area of the driving transistor TRD are supplied. Now, the values of the voltages VCC-H and VCC-L could be as “VCC-H=20 [volt]” and “VCC-H=−10 [volt],” for example, though they are not limited thereto, of course.
<Writing Transistor TRW>
The configuration of the writing transistor TRW is the same as the configuration of the writing transistor TRW described with regard to the 5Tr/1C driving circuit shown in FIG. 4. Therefore, the detailed explanation of the configuration the writing transistor TRW is omitted.
<Luminescence Part ELP>
The configuration of the luminescence part ELP is the same as the configuration of the luminescence part ELP described with regard to the 5Tr/1C driving circuit shown in FIG. 4. Therefore, the detailed explanation of the configuration the luminescence part ELP is omitted.
In the following, the operation of the 2Tr/1C driving circuit will be described with reference to FIG. 8 and FIG. 9A-FIG. 9F, respectively.
<B-1> [Period—TP(2)−1] (See FIG. 8 and FIG. 9A)
[Period—TP(2)−1] indicates, for example, an operation for a previous display frame, and it is substantially the same operation as that of [Period—TP(5)−1] shown in FIG. 5 described with regard to the 5Tr/1C driving circuit.
[Period—TP(2)0]-[Period—TP(2)2] shown in FIG. 8 are periods corresponding to [Period—TP(5)0]-[Period—TP(5)4] shown in FIG. 5, and operation periods until just before the next writing process is executed. And, for [Period—TP(2)0]-[Period—TP(2)2], similarly to the 5Tr/1C driving circuit described above, the (n, m) luminescence element is basically in non luminous state. Now, the operation of the 2Tr/1C driving circuit is different from the operation of the 5Tr/1C driving circuit in that [Period—TP(2)1]-[Period—TP(2)2] are included in the m-th horizontal scanning period in addition to [Period—TP(2)3], as shown in FIG. 8. Besides, in the following, for the reason of simplicity of the explanation, the explanation will be provided with the assumption that the beginning of [Period—TP(2)1] and the end of [Period—TP(2)3] match the beginning and end of the m-th horizontal scanning period, respectively.
In the following, each period of [Period—TP(2)0]-[Period—TP(2)2] will be described. Besides, the length of each period of [Period—TP(2)1]-[Period—TP(2)2] can be optionally set according to the settings of the display device 100, similarly to the 5Tr/1C driving circuit described above.
<B-2> [Period—TP(2)0] (See FIG. 8 and FIG. 9B)
[Period—TP(2)0] indicates, for example, an operation from the previous display frame to the current display frame. More specifically, [Period—TP(2)0] is a period from the (m+m′)-th horizontal scanning period in the previous display frame to the (m−1)-th horizontal scanning period in the current display frame. And for this [Period—TP(2)0], the (n, m) luminescence element is in non luminous state. Now, at the time point for transition from [Period—TP(2)−1] to [Period—TP(2)0], the voltage supplied from the power source unit 2100 is switched from VCC-H to voltage VCC-L. As a result, the potential of the second node ND2 is lowered to VCC-L, and the luminescence part ELP gets into non luminous state. And, as the potential of the second node ND2 gets lower, the potential of the first node ND1 in floating state (the gate electrode of the driving transistor TRD) is also lowered.
<B-3> [Period—TP(2)1] (See FIG. 8 and FIG. 9C)
The horizontal scanning period for the m-th row begins at [Period—TP(2)1]. Now, for this [Period—TP(2)1], a pre-process for executing the threshold voltage cancelling process is executed. At the beginning of [Period—TP(2)1], the writing transistor TRW is got into ON state, by getting the potential of the scan line SCL to be at high level. As a result, the potential of the first node ND1 becomes VOfs (e.g., 0 [volt]). And, the potential of the second node ND2 is maintained at VCC-L (e.g., −10 [volt]).
Thus, for [Period—TP(2)1], the potential between the gate electrode and source area of the driving transistor TRD becomes above Vth, and the driving transistor TRD gets into ON state.
<B-4> [Period—TP(2)2] (See FIG. 8 and FIG. 9D)
The threshold voltage cancelling process is executed for [Period—TP(2)2]. Specifically, for [Period—TP(2)2], the voltage supplied from the power source unit 2100 is switched from VCC-L to the voltage VCC-H, with the writing transistor TRW maintained in ON state. As a result, for [Period—TP(2)2], the potential of the first node ND1 does not change (VOfs=0 [volt] maintained), whilst the potential of the second node ND2 changes towards the potential obtained by subtracting the threshold voltage Vth of the driving transistor TRD from the potential of the first node ND1. Hence, the potential of the second node ND2 in floating state increases. Then, when the potential difference between the gate electrode and source area of the driving transistor TRD reaches to Vth, the driving transistor TRD gets into OFF state. More specifically, the potential of the second node ND2 in floating state approaches to (VOfs−Vth=−3 [volt]) to be (VOfs−Vth) in the end. Now, if Equation 2 above is assured, in other words, if the potentials are selected and determined to satisfy Equation 2 above, the luminescence part ELP will not be luminous.
For [Period—TP(2)3], the potential of the second node ND2 will be (VOfs−Vth) eventually. Therefore, the potential of the second node ND2 is determined, depending on the threshold voltage Vth of the driving transistor TRD, and on the potential VOfs for initialising the gate electrode of the driving transistor TRD. In other words, the potential of the second node ND2 does not depend on the threshold voltage Vth-EL of the luminescence part ELP.
<B-5> [Period—TP(2)3] (See FIG. 8 and FIG. 9E)
For [Period—TP(2)3], the writing process for the driving transistor TRD, and an adjustment (mobility adjustment process) on the potential of the source area of the driving transistor TRD (the second node ND2) based on the magnitude of the mobility μ of the driving transistor TRD are executed. Specifically, for [Period—TP(2)3], the data line DTL is made to be VSig for controlling the luminance of the luminescence part ELP with the writing transistor TRW maintained in OFF state. As a result, the potential of the first node ND1 increases to VSig, and the driving transistor TRD gets into ON state. Besides, the way of bringing the driving transistor TRD into ON state is not limited thereto; for example, the driving transistor TRD gets into ON state by bringing the writing transistor TRW into ON state. Hence, for example, the 2Tr/1C driving circuit can bring the driving transistor TRD into ON state by getting the writing transistor TRW into OFF state temporally, changing the potential of the data line DTL into a picture signal VSig for controlling the luminance of the luminescence part ELP, getting the scan line SCL to be at high level, and then bringing the writing transistor TRW into ON state.
Now, for [Period—TP(2)3], unlike the case of the 5Tr/1C described above, the potential of the source area of the driving transistor TRD increases since the voltage VCC-H is applied to the drain area of the driving transistor TRD by power source unit 2100. And for [Period—TP(2)3], by getting the scan line SCL to be at low level after a predetermined time (t0) has passed, the writing transistor TRW is brought into OFF state, and the first node ND1 (the gate electrode of the driving transistor TRD) gets into floating state. Now, the total time t0 of [Period—TP(2)3] may be determined in advance as a configuration value during the configuration of the display device 100 so that the potential of the second node ND2 is (VOfs−Vth+ΔV).
For [Period—TP(2)3], by the processes described above, if the value of the mobility μ of the driving transistor TRD is large, then the increased amount ΔV of the potential of the source area of the driving transistor TRD is large, and if the value of the mobility μ of the driving transistor TRD is small, then the increased amount ΔV of the potential of the source area of the driving transistor TRD is small. Thus, adjustment on mobility is executed for [Period—TP(2)3].
<B-6> [Period—TP(2)4] (See FIG. 8 and FIG. 9E)
By the operations described above, the threshold voltage cancelling process, the writing process, and the mobility adjusting process are done in the 2Tr/1C driving circuit. For [Period—TP(2)4], the same process as that of [Period—TP(5)7] described with regard to the 5Tr/1C driving circuit is executed; namely, for [Period—TP(2)4], the potential of the second node ND2 increases to be above (Vth-EL+VCat), so that the luminescence part ELP starts to be luminous. And at this point, the current flowing to the luminescence part ELP can be specified by Equation 6 above, therefore, the current Ids flowing to the luminescence part ELP does not depend on the threshold voltage Vth-EL of the luminescence part ELP and the threshold voltage Vth of the driving transistor TRD; namely, the luminescence amount (luminance) of the luminescence part ELP is not affected by the threshold voltage Vth-EL of the luminescence part ELP and the threshold voltage Vth of the driving transistor TRD. Furthermore, the 2Tr/1C driving circuit may prevent the occurrence of the variation of the drain current Ids resulted from the variation of the mobility μ of the driving transistor TRD.
Then, Luminous state of the luminescence part ELP is maintained until the (m+m′−1)-th horizontal scanning period. This time point corresponds to the end of [Period—TP(5)−1].
Thus, the luminescence operation of the luminescence element 10 included in the (n, m) sub-pixel is done.
In the above, the 5Tr/1C driving circuit and the 2Tr/1C driving circuit have been described as driving circuits according to an embodiment of the present invention, though driving circuits according to an embodiment of the present invention are not limited thereto. For example, a driving circuit according to an embodiment of the present invention may be formed out of a 4Tr/1C driving circuit shown in FIG. 10 or a 3Tr/1C driving circuit shown in FIG. 11.
Also in the above, it is illustrated that the writing process and the mobility adjustment are executed individually, though the operation of a 5Tr/1C driving circuit according an embodiment of the present invention is not limited thereto. For example, similarly to the 2Tr/1C driving circuit described above, a 5Tr/1C driving circuit may be configured to execute the writing process along with the mobility adjusting process. Specifically, a 5Tr/1C may configured to apply a picture signal VSig—m to the first node from a data line DTL via a writing transistor TSig for [Period—TP(5)5] in FIG. 5, for example, with a luminescence control transistor TEL—C in ON state.
The panel 158 of the display device 100 according to an embodiment of the present invention may be configured to include pixel circuits and driving circuits as described above. Besides, the panel 158 according to an embodiment of the present invention is not, of course, limited to the configuration in which pixel circuits and driving circuits as described above are included.
(Control Over Luminous Time within 1 Frame Period and Gain of Picture Signal)
Next, there will be described control over a luminous time within one frame period (duty) and the gain of a picture signal according to the embodiment of the present invention. The control over a luminous time within one frame period and the gain of a picture signal according to the embodiment of the present invention may be executed by the luminous time controller 126 of the picture signal processor 110.
FIG. 12 is a block diagram that shows an example of the luminous time controller 126 according to an embodiment of the present invention. In the following, the explanation will be provided with assumption that a picture signal input into the luminous time controller 126 is a signal which corresponds to an image for each one frame period (unit time) and which is provided separately for each colour of R, G, and B.
With reference to FIG. 12, the luminous time controller 126 includes an average luminance calculator 200, a luminescence amount regulator 202, and a adjuster 204.
The average luminance calculator 200 calculates an average value of luminance for a predetermined period. Now, such a predetermined period could be one frame period, for example, though it is not limited thereto; it could be two frame periods, for example.
Also, the average luminance calculator 200 may calculate an average value of luminance for each predetermined period, for example (i.e., calculate an average value of luminance in a certain cycle), however it is not limited as such; for example, the predetermined period may be a variable period.
In the following explanation, the predetermined period is set to one frame period, and the average luminance calculator 200 is configured to calculate an average value of luminance for each one frame period.
[Configuration of Average Luminance Calculator 200]
FIG. 13 is a block diagram that shows the average luminance calculator 200 according to the embodiment of the present invention. With reference to FIG. 13, the average luminance calculator 200 includes a current ratio adjuster 250 and an average value calculator 252.
The current ratio adjuster 250 adjusts the current ratio for input picture signals for R, G, and B by respectively multiplying the input picture signals for R, G, and B by adjustment coefficients, which are respectively predetermined for the colours. Now, for example, the above-mentioned predetermined adjustment coefficients are values that correspond to respective V-I ratios (voltage-current ratios) of an R luminescence element, a G luminescence element, and a B luminescence element which are included in a pixel of the panel 158 so as to differ from each other in respect to their corresponding colours.
FIG. 14 is an illustration that shows an example of each V-I ratio of a luminescence element for each colour included in a pixel according to an embodiment of the present invention. As shown in FIG. 14, the V-I ratio of a luminescence element for a colour included in a pixel is different from the ratios of those for the other colours, as “B luminescence element>R luminescence element>G luminescence element.” Now, as shown in FIG. 2A-FIG. 2F, the display device 100 can execute a process in a linear region with the gamma value unique to the panel 158 cancelled by multiplying a gamma curve inverse to the gamma curve that is unique to the panel 158 by the gamma converter 132. Thus, for example, respective V-I ratios of an R luminescence element, a G luminescence element, and a B luminescence element can be obtained in advance by fixing the duty to a predetermined value (e.g., “0.25”) and deriving in advance the V-I relations as shown in FIG. 14.
Besides, the current ratio adjuster 250 may include memory means, and the above-mentioned adjustment coefficients used by the current ratio adjuster 250 may be stored in the memory means. Now examples of such memory means included in the current ratio adjuster 250 include non volatile memories, such as EEPROMs and flash memories, but are not limited thereto. And, the above-mentioned adjustment coefficients used by the current ratio adjuster 250 may be held in memory means included in the display device 100, such as the recorder 106 or the memory 150, and read out by the current ratio adjuster 250 at any appropriate occasions.
The average value calculator 252 calculates average luminance (APL: Average Picture Level) for one frame period from R, G, and B picture signals adjusted by the current ratio adjuster 250. Now, examples of the way of calculating average luminance for one frame period by the average value calculator 252 include using the arithmetic mean, but are not limited thereto; for example, the calculation may be carried out by use of the geometric mean and a weighted mean.
The average luminance calculator 200 calculates average luminance for one frame period as described above, and outputs it.
With reference to FIG. 12 again, the luminescence amount regulator 202 set a reference duty depending on average luminance for one frame period calculated by the average luminance calculator 200, where the reference duty is a duty as a reference for regulating per unit time (i.e., one frame period) a luminescence amount for which the pixels (luminescence elements) are luminous.
A luminescence amount for one frame period can be expressed by Equation 7 below, where “Lum” shown in Equation 7 denotes a “luminescence amount,” “Sig” shown in Equation 7 denotes a “signal level,” and “Duty” shown in Equation 7 denotes a “luminous time.”

Lum=(Sig)×(Duty)  (Equation 7)

As shown in Equation 7, by setting a reference duty, a luminescence amount will depend only on the signal level of a picture signal, namely the gain of a picture signal.
And, a reference duty can be set by the luminescence amount regulator 202 by use of a Look Up Table, in which average luminance for one frame period and reference duties are correlated, for example. Now, the luminescence amount regulator 202 may store the Look Up Table in memory means, such as non volatile memories like EEPROMs and flash memories, or as magnetic recording media like Hard Disks, for example.
[Way of Deriving Value Held in Look Up Table According to Embodiment of Present Invention]
Now, the way of deriving a value held in the Look Up Table according to an embodiment of the present invention will be described. FIG. 15 is an illustration that illustrates the way of deriving a value held in the Look Up Table according to an embodiment of the present invention, where the relation between average luminance (APL) for one frame period and a reference duty is shown. Besides, there is shown in FIG. 15 for example the case where the average luminance for one frame period is represented by digital data of 10 bits, whilst average luminance for one frame period is not, of course, limited to digital data of 10 bits.
And, the Look Up Table according to an embodiment of the present invention is derived with reference to the luminescence amount for the case where the luminance is at its maximum for a predetermined duty, for example (and in this case, an image in “white” is displayed on the panel 158).
The area S shown in FIG. 15 represents the luminescence amount for the case where the reference duty is set to 25% so that the luminance is at its maximum. Besides, the predetermined duty according to an embodiment of the present invention is not limited to 25%. It may be set according to the properties (e.g., the properties of the luminescence elements) of the panel 158 included in the display device 100 or to MTBF (Mean Time Between Failure) of the display device 100.
The curve a shown in FIG. 15 is a curve passing through values of average luminance (APL) for one frame period and the reference duty that have their products equal to the area S in the case where the reference duty is larger than 25%.
The straight line b shown in FIG. 15 is a straight line that regulates the upper limit value L of the reference duty for the curve a. As shown in FIG. 15, in the Look Up Table according to an embodiment of the present invention, an upper limit value may be provided for the reference duty. For example, an upper limit value may be provided for the reference duty in the embodiment of the present invention for purpose of solving an issue due to the relation of trade off between “luminance” related to the duty and “blurred movement” given when a moving image is displayed. The issue due to the relation of trade off between “luminance” according to the duty and “blurred movement” here is as follows.
<For Large Duty>
Luminance: higher
Blurred Movement: heavier
<For Small Duty>
Luminance: lower
Blurred Movement: lighter
Thus, in the Look Up Table according to an embodiment of the present invention, the upper limit value L of a reference duty is set and a certain balance between “luminance” and “blurred movement” is achieved for purpose of solving the issue due to the relation of trade off between luminance and blurred movement. Now, the upper limit value L of the reference duty may be set, for example, according to the characteristic of the panel 158 included in the display device 100 (e.g., characteristics of luminescence elements).
For example, by use of the Look Up Table in which average luminance for one frame period and reference duties are held in respective correlation so as to take values on the curve a and the straight line b shown in FIG. 15, the luminescence amount regulator 202 may set a reference duty according to the average luminance for one frame period calculated by the average luminance calculator 200. Besides, the example has been shown in the above explanation where an upper limit value L of the reference duty is set by the luminescence regulator 202, as shown in FIG. 15, for example, though the embodiment of the present invention is not limited thereto. For example, a luminous time adjuster 206 (to be described later) of the adjuster 204 may provide a predetermined upper limit value for a duty.
FIG. 12 is now referred again for explaining the luminous time controller 126. The adjuster 204 includes a luminous time adjuster 206 and a gain adjuster 208, and may adjust the reference duty output from the luminescence amount regulator 202 and each of the gains of picture signals.
The luminous time adjuster 206 adjusts a reference duty output from the luminescence amount regulator 202, and outputs an effective duty for practically regulating a luminous time for which each of the luminescence elements of the panel 158 within a unit time. In the following, “effective duty adjustment” refers to adjusting a reference duty and outputting an effective duty by the luminous time adjuster 206.
[First Example of Adjusting Effective Duty: Setting Lower Limit Value]
FIG. 16 is an illustration for illustrating the first way of adjusting an effective duty by the luminous time adjuster 206 according to the embodiment of the present invention. FIG. 16 shows the relation between a reference duty (Duty) output from the luminescence amount regulator 202 and an effective duty (Duty′) output from the luminous time adjuster 206.
With reference to FIG. 16, it can be seen that the reference duty (Duty) output from the luminescence amount regulator 202 and the effective duty (Duty′) output from the luminous time adjuster 206 are in proportional relation of the tilt of 1 basically, and that an lower limit value L1 is provided for the effective duty (Duty′).
As described above, if a duty is small, there is an advantage of lighter “blurred movement” whilst there arises a disadvantage of lower “luminance.” And if a duty is shortened to a certain measure, there also arises a disadvantage that flickers occur (can be obviously noticed). Then, by providing the lower limit value L1 for the effective duty (Duty′), the luminous time adjuster 206 outputs the reference duty as the effective duty if the reference duty (Duty) output from the luminescence amount regulator 202 fulfils L1≦Duty (within the regulation range), and outputs the lower limit value L1 as the effective duty if the reference duty (Duty) fulfils L1>Duty (exceeding the regulation range). By the luminous time adjuster 206 adjusting the effective duty as described above, the appearance of the above-mentioned disadvantages may be controlled so as to avoid deterioration of the display quality.
By adjusting the effective duty as shown in FIG. 16, for example, the luminous time adjuster 206 may avoid deterioration of the display quality of a picture displayed by the display device 100 so as to achieve higher display quality.
Now, the effective duty may be adjusted by comparing the reference duty output from the luminescence amount regulator 202 and the lower limit value L1, which is stored in advance into memory means (not shown) by the luminous time adjuster 206, though adjusting of the effective duty is not limited thereto. Also, the lower limit value L1 may be held in memory means which is included in the luminous time adjuster 206. Now, examples of the memory means included in the luminous time adjuster 206 may include non volatile memories like EEPROMs or flash memories, but are not limited thereto. And also, the lower limit value L1 for use by the luminous time adjuster 206 may be held in memory means included in the display device 100, such as the recorder 106 or the memory 150, and be read by the luminous time adjuster 206 at appropriate occasions.
Also, the lower limit value L1 may be set to a value such that flickers will not be obviously noticed when a picture is displayed on the panel 158. For example, it may be set depending upon the characteristic of the panel 158 (such as the characteristics of the luminescence elements, for example).
[Second Example of Adjusting Effective Duty: Setting Upper Limit Value]
FIG. 17 is an illustration for illustrating the second way of adjusting an effective duty by the luminous time adjuster 206 according to the embodiment of the present invention. As FIG. 16, FIG. 17 shows the relation between a reference duty (Duty) output from the luminescence amount regulator 202 and an effective duty (Duty′) output from the luminous time adjuster 206.
With reference to FIG. 17, it can be seen that the reference duty (Duty) output from the luminescence amount regulator 202 and the effective duty (Duty′) output from the luminous time adjuster 206 are in proportional relation of the tilt of 1 basically, and that an upper limit value L2 is provided for the effective duty (Duty′).
As described above, if a duty is large, there is an advantage of higher “luminance” whilst there arises a disadvantage of heavily “blurred movement.” Then, by providing the upper limit value L2 for the effective duty (Duty′), the luminous time adjuster 206 outputs the reference duty as the effective duty if the reference duty (Duty) output from the luminescence amount regulator 202 fulfils Duty≦L2 (within the regulation range), and outputs the upper limit value L2 as the effective duty if the reference duty (Duty) fulfils Duty>L2 (exceeding the regulation range). By the luminous time adjuster 206 adjusting the effective duty as described above, the appearance of the above-mentioned disadvantage may be controlled so as to avoid deterioration of the display quality.
By adjusting the effective duty as shown in FIG. 17, for example, the luminous time adjuster 206 may avoid deterioration of the display quality of a picture displayed by the display device 100 so as to achieve higher display quality.
Now, the effective duty may be adjusted by comparing the reference duty output from the luminescence amount regulator 202 and the upper limit value L2, which is stored in advance into memory means (not shown) by the luminous time adjuster 206, though adjusting of the effective duty is not limited thereto. For example, by clipping the value of the reference duty output from the luminescence amount regulator 202, the luminous time adjuster 206 may output the effective duty with its upper limit value L2 set.
Also, the upper limit value L2 may be set to a value such that blurred movements will not be obviously noticed when a picture is displayed on the panel 158. For example, it may be set depending upon the characteristic of the panel 158 (such as the characteristics of the luminescence elements, for example).
[Third Example of Adjusting Effective Duty: Setting Lower Limit Value and Upper Limit Value]
The first and second examples of adjusting an effective duty show examples where a lower limit value L1 or an upper limit value L2 is provided for the effective duty, respectively. However, adjusting an effective duty by the luminous time adjuster 206 is not limited to the first and second examples of adjusting. FIG. 18 is an illustration for illustrating the third way of adjusting an effective duty by the luminous time adjuster 206 according to the embodiment of the present invention. As FIG. 16, FIG. 18 shows the relation between a reference duty (Duty) output from the luminescence amount regulator 202 and an effective duty (Duty′) output from the luminous time adjuster 206.
With reference to FIG. 18, it can be seen that the reference duty (Duty) output from the luminescence amount regulator 202 and the effective duty (Duty′) output from the luminous time adjuster 206 are in proportional relation of the tilt of 1 basically, and that a lower limit value L1 and an upper limit value L2 are provided for the effective duty (Duty′). Thus, in the third example of adjusting, the luminous time adjuster 206 outputs the reference duty as the effective duty if the reference duty (Duty) output from the luminescence amount regulator 202 fulfils L1≦Duty≦L2 (within the regulation range). And the luminous time adjuster 206 outputs the lower limit value L1 as the effective duty if L1>Duty (exceeding the regulation range), and outputs the upper limit value L2 as the effective duty if Duty>L2 (exceeding the regulation range).
By providing a lower limit value L1 and an upper limit value L2 for the effective duty (Duty′), the luminous time adjuster 206 controls the appearance of the disadvantages due to the relation of trade off of luminance and blurred movement (the disadvantages shown in the first and second examples of adjusting) so as to avoid deterioration of the display quality. By adjusting the effective duty as shown in FIG. 17, for example, the luminous time adjuster 206 may avoid deterioration of the display quality of a picture displayed by the display device 100 so as to achieve higher display quality.
As shown above by the first to third examples of adjusting, the luminous time adjuster 206 may avoid deterioration of the display quality of a picture displayed by the display device 100 so as to achieve higher display quality by adjusting the effective duty with a lower limit value L1 and/or an upper limit value L2 provided for the effective duty to be output. Besides, the lower limit value L1 and/or upper limit value L2 of the effective duty shown in FIG. 16-FIG. 18 may be preset depending upon the characteristic of the panel 158 included in the display device 100 (such as the characteristics of the luminescence elements, for example), though these are not limited to such a way. For example, the lower limit value L1 and/or upper limit value L2 of the effective duty may be changed in accordance with a user input from the operating unit (not shown).
FIG. 12 is now referred again for explaining the luminous time controller 126. The gain adjuster 208 includes a primary gain adjuster 210 and a secondary gain adjuster 212. The gain adjuster 208 may adjust input R, G, and B picture signals in correspondence with adjusting the effective duty by the luminous time adjuster 206. As shown in Equation 7, a luminescence amount may expressed by the product of a signal level and a luminous time. The gain adjuster 208 adjusts the gain of a picture signal so that the luminescence amount regulated by the reference duty and the gain of a picture signal is kept constant even after the effective duty has been adjusted.
The primary gain adjuster 210 multiplies each of the input R, G, and B picture signals by the reference duty output from the luminescence amount regulator 202.
The secondary gain adjuster 212 divides each of the R, and B picture signals adjusted by the primary gain adjuster 210 by the effective duty (Duty′) output from the luminous time adjuster 206.
As a result of adjusting by the primary gain adjuster 210 and the second gain adjuster 212, the adjusted R picture signal (R′), the adjusted G picture signal (G′), and the adjusted B picture signal (B′) to be output from the gain adjuster 208 can be expressed as following Equation 8-Equation 10.

R′={(R)×(Duty)}/(Duty′)

R′=(R)×{(Duty)/(Duty′)}  (Equation 8)

G′={(G)×(Duty)}/(Duty′)

G′=(G)×{(Duty)/(Duty′)}  (Equation 9)

B′={(B)×(Duty)}/(Duty′)

B′=(B)×{(Duty)/(Duty′)}  (Equation 10)

With reference to Equation 8-Equation 10, it can be seen that the picture signals to be output from the gain adjuster 208 (R′, G′, and B′) depend upon the adjustment ratio of duty for the luminous time adjuster 206: ((Duty)/(Duty′)).
Now, the relationship between the adjustment ratio of duty for the luminous time adjuster 206 and adjustment of the gain of a picture signal by the gain adjuster 208 can be given as following (1)-(3), for example.
(1) for Adjustment Ratio of Duty=1,
Picture Signals Output from Gain Adjuster 208 (R′, G′, B′)=Input Picture Signals (R, G, B); No Change in Gains of Picture Signals.
(2) for Adjustment Ratio of Duty<1 (: if the effective duty is set to the lower limit value L1),
Picture Signals Output from Gain Adjuster 208 (R′, G′, B′)<Input Picture Signals (R, G, B); Gains of Picture Signals Damped.
(3) for Adjustment Ratio of Duty>1 (: if the effective duty is set to the upper limit value L2),
Picture Signals Output from Gain Adjuster 208 (R′, G′, B′)>Input Picture Signals (R, G, B); Gains of Picture Signals Amplified.
Moreover, as shown in Equation 7 and Equation 8-Equation 10, the luminescence amount for one frame period (unit time) regulated with the effective duty (Duty′) output from the adjuster 204 and the picture signals (R′, G′, and B′) does not change through adjusting by adjuster 204. Thus, the adjuster 204 may adjust the effective duty and the gain of a picture signal with the luminescence amount kept constant.
As described above, the display device 100 according to the embodiment of the present invention calculates average luminance by R, G, and B picture signals input during one frame period (unit time; predetermined period), and sets a reference duty depending upon the calculated average luminance. The reference duty according to the embodiment of the present invention is set to a value such that the largest luminescence amount for a predetermined duty equals to the luminescence amount regulated with the reference duty and with the average luminance for one frame period (unit time; predetermined period). Also, the display device 100 may adjust the effective duty and the gain of a picture signal so that the luminescence amount regulated with the reference duty and with the gain of a picture signal is kept constant. Thus, as for the display device 100, because the luminescence amount within one frame period (unit time) will not be larger than the largest luminescence amount for the predetermined duty, the display device 100 may prevent the current from overflowing into each of the pixels (strictly, the luminescence elements of each of the pixels) of the panel 158.
Also, by adjusting the effective duty with a lower limit value L1 and/or an upper limit value L2 provided for the effective duty, the display device 100 may control the appearance of the disadvantages due to the relation of trade off of luminance and blurred movement (the disadvantages shown in the above-described first and second examples of adjusting) so as to avoid deterioration of the display quality. Thus, the display device 100 may achieve higher display quality for a picture displayed on the panel 158.
[Other Example of Luminous Time Controller 126]
As shown in FIG. 12, the luminous time controller 126 may include an average luminance calculator 200 and a luminescence amount regulator 202, and set a reference duty based on the average luminance calculated by the average luminance calculator 200. However, the luminous time controller 126 according to the embodiment of the present invention is not limited the above configuration. For example, the luminous time controller 126 may include, as a component replacing the average luminance calculator 200, a histogram calculator for calculating a histogram value of a picture signal, and the luminescence amount regulator may set a reference duty based on the histogram value. Even with the above-described configuration, as for the display device 100, the luminescence amount for one frame period (unit time) will not be larger than the largest luminescence amount for the predetermined duty, the display device may prevent the current from overflowing into each of the pixels (strictly, the luminescence elements of each of the pixels) of the panel 158.
And, the display device 100 has described for an embodiment of the present invention, though embodiments of the present invention are not limited thereto; for example, embodiments of the present invention may be applied to various machines, such as a self-luminescence type television set for receiving the television broadcasts and displaying pictures, and as a computer, such as a PC (Personal Computer) with display means outside or inside thereof, for example.
Program According to Embodiment of Present Invention
By a program for causing a computer to function as the display device 100 according to the embodiment of the present invention, the luminous time per unit time may be controlled to prevent the current from overflowing into the luminescence elements, and also, the gain of a picture signal may be controlled as well to achieve higher display quality.
Picture Signal Processing Method According to Embodiment of Present Invention
Next, there will be described a method of processing a picture signal, according to an embodiment of the present invention. FIG. 19 is a flow diagram that shows an example of the method of processing a picture signal according to the embodiment of the present invention, where shown is an example of a method related to control on the luminous time per unit time. In the following, the explanation will be provided with assumption that the display device 100 executes the method of processing a picture signal, according to an embodiment of the present invention. And, in the following, the explanation will be provided with assumption that the unit time is one frame period, and that an input picture signal is a signal which corresponds to an image for each one frame period (unit time) and which is provided separately for each colour of R, G, and B.
First, the display device 100 calculates average luminance of picture signals for a predetermined period from input R, G, and B picture signals (S100). Examples of the way of calculating average luminance in step S100 include the arithmetic mean, but are not limited thereto. And, the above-mentioned predetermined period can be one frame period, for example.
The display device 100 sets a reference duty based on the average luminance calculated in step S100 (S102). At this point, for example, the display device 100 may set a reference duty by use of a Look Up Table in which average luminance and reference duties are correlated with each other. Then, in the Look Up Table, reference duties are held such that the largest luminescence amount for a predetermined duty equals to a luminescence amount regulated on the basis of the reference duties and the average luminance. Also, in the Look Up Table, an upper limit value may be provided for the reference duty.
The display device 100 adjusts the respective gains of the input R, and B picture signals, based on the reference duty set in step S102 (S104: Primary Gain Adjustment). At this point, the display device 100 may adjust the gains by multiplying each of the input R, G, and B picture signal by the reference duty set in step S102, for example.
And, the display device 100 determines whether the reference duty set in step S102 is within a regulation range or not (S106). In step S106, the display device 100 may determine that it is within the regulation range in either case of the following (A)-(C).

    
    
        (A) if the reference duty is larger than a lower limit value
        
            (which is corresponding to the first method of adjusting)
        
        
        (B) if the reference duty is smaller than an upper limit value
        
            (which is corresponding to the second method of adjusting)
        
        
        (C) if the reference duty is equal to or larger than the lower limit value, and if the reference duty is equal to or smaller than the upper limit value
        
            (which is corresponding to the third method of adjusting)
        
        
    
    


Besides, the lower limit value and/or the upper limit value for use in step S106 may be a preset value which is to be fixed, or be a value which can be varied at any appropriate occasions by, for example, a user input.
If it is determined in step S106 that the reference duty is within the regulation range, then the display device 100 outputs the reference duty set in step S102 as an effective duty (S108).
And, if it is determined in step S106 that the reference duty is not within the regulation range, then the display device 100 adjusts the reference duty set in step S102 (adjustment of effective duty), and outputs it as an effective duty (S110). At this point, the display device 100 may operate the adjustment of effective duty as following (a)-(c) in the cases of above described (A)-(C), respectively.

    
    
        (a) in the case of (A): output the lower limit value as the effective duty
        (b) in the case of (B): output the upper limit value as the effective duty
        (c) in the case of (C): output the lower limit value or the upper limit value as the effective duty
    
    


The display device 100 adjusts the gains of the picture signals adjusted in step S104, based on the effective duty output in step S108 or step S110 (S112: Secondary Gain Adjustment). At this point, the display device 100 may adjust the gains of the picture signals depending upon the adjustment ratio of the effective duty to the reference duty, as shown in Equation 8-Equation 10. Accordingly, the display device 100 may adjust the gains of the picture signals in step S112 in three manners: to make them “damped,” “amplified,” or “unchanged.”
Moreover, as shown in Equation 7 and Equation 8-Equation 10, the luminescence amounts regulated with the effective duty output in step S108 or step S110 and with the gains of the picture signals adjusted in step S112 will be the same as the luminescence amounts given before the adjustment.
As described above, by the picture signal processing method according to the embodiment of the present invention, a reference duty is output depending upon average luminance of an input picture signal for one frame period (unit time), where the reference duty is set to a value such that the largest luminescence amount for a predetermined duty equals to a luminescence duty regulated with the reference duty and the average luminance for one frame period (unit time; predetermined period).
Moreover, by the picture signal processing method according to the present invention, a lower limit value and/or an upper limit value for the effective duty is provided for the effective duty in order to adjust the effective duty. Accordingly, by use of the picture signal processing method according to the embodiment of the present invention, the display device 100 may control the appearance of the disadvantages due to the relation of trade off of luminance and blurred movement (the disadvantages shown in the above-described first and second examples of adjusting) so as to avoid deterioration of the display quality.
Furthermore, by the picture signal processing method according to the present invention, the effective duty and the gain of a picture signal may be adjusted so that the luminescence amount kept constant, which amount is regulated with the reference duty and with the gain of the picture signal.
Thus, by use of the picture signal processing method according to the present invention, the display device 100 may prevent the current from overflowing into each of the pixels (strictly, the luminescence elements of each of the pixels) of the panel 158. Also, by use of the picture signal processing method according to the present invention, the display device 100 may achieve higher display quality for a picture displayed on the panel 158.
In the above, the preferred embodiments of the present invention have been described with reference to the accompanying drawings, whilst the present invention is not limited the above examples, of course. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alternations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.
For example, with regard to the display device 100 according to an embodiment of the present invention shown in FIG. 1, an input picture signal is explained as a digital signal, though it is not limited thereto. For example, a display device according to an embodiment of the present invention may include an A/D converter (Analogue to Digital converter), convert an input analogue signal (picture signal) into a digital signal, and process the converted picture signal.
And, the above explanation has shown that a program (computer program) is provided for causing a computer to function as the display device 100 according an embodiment of the present invention, whilst a further embodiment of the present invention may provide as well a memory medium in which the above-mentioned program is stored.
The above-mentioned configurations represent exemplary embodiments of the present invention, of course belonging to the technical scope of the present invention.

## Claims

1. A display device comprising:
a display unit having luminescence elements that individually becomes luminous depending on a current amount, the luminescence elements arranged in a matrix pattern;
a luminescence amount regulator that sets a reference duty for regulating a luminescence amount per unit time for each of the luminescence elements, according to picture information of an input picture signal; and
an adjuster that adjusts, based on the reference duty, an effective duty regulating a luminous time for which the luminescence elements become luminous within a unit time so that the effective duty is limited to a predetermined range, and that adjusts a gain of the picture signal so that a luminescence amount regulated with the effective duty and with the gain of the picture signal equals to the luminescence amount regulated with the reference duty.

2. The display device according to claim 1, wherein the adjuster includes
a luminous time adjuster for outputting, as the effective duty, the reference duty adjusted to a predetermined lower or upper limit value if the reference duty set by the luminescence amount regulator is out of the predetermined range, and
a gain adjuster for adjusting the gain of the picture signal, based on the reference duty set by the luminescence amount regulator and on the effective duty output from the luminous time adjuster.

3. The display device according to claim 2, wherein the gain adjuster damps the gain of the picture signal, depending upon an increasing ratio of the effective duty to the reference duty, if the luminous time adjuster has output the effective duty adjusted to the lower limit value.

4. The display device according to claim 2, wherein the gain adjuster amplifies the gain of the picture signal, depending upon a decreasing ratio of the effective duty to the reference duty, if the luminous time adjuster has output the effective duty adjusted to the upper limit value.

5. The display device according to claim 2, wherein the gain adjuster includes
a primary gain adjuster for multiply the input picture signal by the reference duty, and
a secondary gain adjuster for dividing the adjusted picture signal output from the primary gain adjuster by the effective duty output from the luminous time adjuster.

6. The display device according to claim 1, further comprising an average luminance calculator that calculates average luminance of the input picture signal for a predetermined period,
wherein the luminescence amount regulator sets the reference duty, depending upon the average luminance calculated by the average luminance calculator.

7. The display device according to claim 6, wherein the luminescence amount regulator memorises a look-up table, in which luminance of the picture signal and the reference duty are correlated to each other, and uniquely sets the reference duty depending upon the average luminance calculated by the average luminance calculator.

8. The display device according to claim 6, wherein the predetermined period for the average luminance calculator to calculate the average luminance is one frame.

9. The display device according to claim 6, wherein the average luminance calculator includes
a current ratio adjuster that multiplies primary colour signals of the picture signal by respective adjustment values for each of the respective primary colour signals based on a voltage-current characteristic, and
an average value calculator for calculating the average luminance of the picture signal output from the current ratio adjuster for the predetermined period.

10. The display device according to claim 1, further comprising:
a linear converter for adjusting the input picture signal to a linear picture signal by gamma adjustment,
wherein a picture signal input into the luminescence amount regulator is the adjusted picture signal.

11. The display device according to claim 1, further comprising:
a gamma converter for performing, on the picture signal, gamma adjustment according to a gamma characteristic of the display unit on the picture signal.

12. A picture signal processing method of a display device including a display unit having luminescence elements that individually becomes luminous depending on a current amount, the luminescence elements arranged in a matrix pattern, the picture signal processing method comprising the steps of:
setting a reference duty for regulating a luminescence amount per unit time for each of the luminescence elements, according to picture information of an input picture signal; and
adjusting, based on the reference duty, an effective duty regulating a luminous time for which the luminescence elements become luminous within a unit time so that the effective duty is limited to a predetermined range, and adjusting a gain of the picture signal so that a luminescence amount regulated with the effective duty and with the gain of the picture signal equals to the luminescence amount regulated with the reference duty.

13. A non-transitory computer readable storage medium having instructions stored therein, which when executed by a processor in a display device including a display unit having luminescence elements that individually becomes luminous depending on a current amount, the luminescence elements arranged in a matrix pattern, causes the processor to execute the method comprising the steps of:
setting a reference duty for regulating a luminescence amount per unit time for each of the luminescence elements, according to picture information of an input picture signal; and
adjusting, based on the reference duty, an effective duty regulating a luminous time for which the luminescence elements become luminous within a unit time so that the effective duty is limited to a predetermined range, and adjusting a gain of the picture signal so that a luminescence amount regulated with the effective duty and with the gain of the picture signal equals to the luminescence amount regulated with the reference duty.