JP5078690B2 - Gradation control method for image display device - Google Patents

Description

  The present invention relates to a gradation control method for an image display device in which display elements such as LEDs are arranged in a matrix.

In general, when a gray scale is expressed using a lighting display element (light emitting element) in two states of ON / OFF control such as an LED, the light emitting element is used by a method called pulse width modulation (PWM) control. The current value is controlled to express gradation (gradual change in luminance).
As a gradation expression method (gradation control method), a pulse width addition gradation control method, which is a modification of the PWM control method, is also used.
In this pulse width addition gradation control method, the emission accuracy of the light emission pulse expressing the minimum gradation 1 is limited when the number of gradations is increased or such.
The minimum gradation 1 is limited by response characteristics of a drive circuit / element used in the system (that is, an image display device).
Usually, in order to enhance the image expression capability, the width of the basic light emission pulse is set to a width that can achieve the maximum number of gradations in the system (image display device), that is, the minimum width that can control the gradation with high accuracy. Is done.

One of the important image representations for human visual characteristics is that Mach band-like steps (stair-like light and dark with a large luminance change range) in the gentle image change part on the image in the low gradation part and the intermediate gradation part. This step is invisible.
When the luminance change width between gradations is large, the change between gradations looks like a staircase, which affects the image quality.
In addition, if there is a portion with a large luminance change width, attention is gathered in that portion and it is felt that the image quality is deteriorated, so the change width between the gradations needs to be uniform.

FIG. 6 is a schematic diagram for explaining the pulse width addition gradation control method.
As shown in FIG. 6, in the pulse width addition gradation control method, when the minimum pulse width is 1, the width of each pulse is 2, 4, 8, 16, 32. Control is performed so that the pulse width is doubled for each step.
If the accuracy of this equality is poor, the change width between gradations is not uniform (smooth), leading to image quality deterioration as described above.
For this reason, the minimum pulse width within the range in which the accuracy of equal magnification is maintained is defined as width 1.

Here, the pulse width addition gradation control method is based on a light emission pulse having a basic pulse width that represents the basic minimum gradation 1, and a pulse width that is a multiple of the basic pulse width or a multiple of the basic pulse width is used as a base. This is a method of controlling the time during which the display element emits light by adding the light emission pulses, and obtaining a desired gradation (luminance).
For example, when considering gradation expression of 63 levels of 6-bit gradation, the emission pulse expressing gradation 1 is P0 (pulse width 1), and the time (field) at which the emission pulse P0 emits light is SF0 (SF: sub) The light emission pulse expressing frame 2 and gradation 2 is P1 (pulse width 2), and the emission time of the light emission pulse P1 is SF1. The light emission pulse width of P1 is twice the light emission pulse width of P0.
Thereafter, the emission pulse representing the gradation 4 is P2, the emission time (field) of the emission pulse P2 is SF2, the emission pulse expressing the gradation 8 is P3, and the emission pulse P3 is emission time S.
F3, a pulse representing gradation 16 is P4, a light emission pulse P4 emits time SF4, a pulse representing gradation 32 is P5, and a light emission pulse P5 light emission time is SF5 to express 63 gradations. .

An example of a specific gradation expression method will be described.
For example, when expressing gradation 7, since “7 = 1 + 2 + 4”, there is a light emission pulse only in SF0, SF1, and SF2, and no light is emitted in other portions.
When the gradation 32 is expressed, only “SF5” emits light because “32 = 32”.
In the case of expressing the gradation 63, since “63 = 1 + 2 + 4 + 8 + 16 + 32”, light is emitted at SF0, SF1, SF2, SF3, SF4, and SF5.
In this way, gradation is expressed by controlling the total pulse width obtained by adding the pulse widths of different pulses to a predetermined gradation.
6A shows a case where the gradation 63 is expressed, FIG. 6B shows a case where the gradation 27 is expressed, and FIG. 6C shows a case where the gradation 7 is expressed.
Such a pulse width addition gradation control method is described in, for example, Japanese Patent No. 3756386 (Patent Document 1).

In the currently used image display device, when the number of expression gradations is to be increased, for example, it is necessary to generate a pulse having a width of 0.5 which is a light emission width that is half of the current minimum gradation 1.
However, when the responsiveness of the light emitting system (that is, the light emitting element driving circuit) is poor and the limit of the minimum gradation 1 is large, the control pulse for driving the gradation below the current minimum gradation (driving the display element) When an attempt is made to generate an input pulse), the width of the control pulse (input pulse) and the width of the light emission pulse are not accurately proportional.
Therefore, for example, even if a control pulse having a width of 0.5 is input to the display element, the luminance corresponding to the control pulse width of 0.5 cannot be obtained from the actual light emission pulse.
For example, the display element driving circuit has poor responsiveness, and the light emission luminance is less than, for example, 0.25 equivalent to a signal for lighting a light emission pulse gradation equivalent to 0.5 as a control input. Think.

In terms of control, even when a pulse corresponding to the gradation 0.5 is input, the actual light emission luminance corresponding to the pulse is equivalent to the gradation 0.25, for example.
Ideally, the steps between gradations should be uniform at gradation 0.5, but considering the part where control gradation 0 → 0.5 → 1 → 1.5 → 2 changes, The light emission gradation is equivalent to 0 → 0.25 → 1 → 1.25 → 2, and the change width (brightness change width) between each gradation is 0.25 → 0.75 → 0.25 → 0.75, The range of change between gradations is not uniform.
That is, even if a control pulse (input pulse) having a width of 0.5, which is half the light emission width of the current “minimum gradation 1”, is used to increase the number of expression gradations, the drive circuit for driving the display element is used. Because of the response characteristics, a gradation of 0.5 corresponding to a 0.5-width control pulse cannot be obtained.
Therefore, even if the added pulse width changes by 0.5, a gentle gradation change proportional to the added pulse width cannot be obtained.
Japanese Patent No. 3756386

As described above, in order to increase the number of expression gradations, for example, when gradation control is simply performed using a pulse having a pulse width of 0.5, a gentle gradation change proportional to the added pulse width is obtained. In other words, there are places where the change width between gradations is small and large, which is a problem in visual characteristics.
In addition, when the light emitting element is an RGB three-color LED, if grayscale control is performed using a pulse with a pulse width of 0.5, the response characteristics of the drive circuit for each LED are different, which can lead to a strange white balance. There is sex. From the viewpoint of human visual characteristics, these problems become more apparent as the luminance is lower (that is, the gradation is lower).
The present invention has been made to solve such a problem, and generates a pulse having a light emission width (for example, 0.5 width) smaller than the current minimum gradation 1 and expresses it using this pulse. It is an object of the present invention to provide a gradation control method for an image display apparatus capable of causing a gentle luminance change proportional to the added pulse width even when the number of gradations is increased.

A gradation control method for an image display device according to the present invention is a gradation control method using a pulse width modulation control method or a pulse width addition gradation control method for an image display device in which a plurality of display elements are arranged in a matrix. Generating an input pulse having a minimum pulse width which is smaller than a basic pulse width for expressing the current minimum gradation, and generating the first input pulse having the minimum pulse width and the basic pulse. using the third input pulse having a multiple of the pulse width of the second input pulses or said second input pulse having a width, tone 1 emission luminance of the display device represents the lowest gray level, said first A gradation obtained by causing the display element to emit light by two input pulses and having a luminance higher than the gradation 1 is a pulse in which the second input pulse or the third input pulse is combined with the first input pulse. It is capable to emit light to the display element.

  According to the present invention, even if the number of expression gradations is increased by using an input pulse having a width (for example, 0.5 width) smaller than the input pulse (width 1.0) that obtains the current minimum gradation 1, Thus, it is possible to provide a gradation control method for an image display device capable of causing a gentle luminance change in proportion to the added pulse width after the first gradation 1.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
Embodiment 1 FIG.
FIG. 1 is a block diagram showing an overall configuration example of an image display apparatus to which the present invention is applied.
This figure corresponds to FIG. 12 of Japanese Patent No. 3756386.
As shown in FIG. 1, the image display apparatus receives a display controller 3 for each of R, G, and B colors, a buffer memory 4 that stores image data output from the display controller 3, and information from the buffer memory 4. A plurality of display units 5 arranged in a matrix and a bus 6 for transmitting information of the buffer memory 4 to the display unit 5 are provided.
The display controller 3 scans and samples the image information output from the video system 1 such as a television broadcast receiver or a video playback device, and outputs the image information to the buffer memory 4.
Further, communication control of image information output from the computer system 2 is performed, graphic processing or character information processing is performed, and the image information is output to the buffer memory 4.

The plurality of display units 5 constitute a screen as a whole, and each display unit 5 includes an image memory 5a, a control circuit 5b, a drive circuit 5d, and a display element 5e.
The image memory 5a stores information from the buffer memory 4 and supplies the image data to the drive circuit 5d.
The display element 5e is an element such as a light emitting diode (LED), and is driven by a current supplied from the drive circuit 5d.
A constant current generating circuit for supplying a current having a constant current intensity, that is, a constant current to the display element 5e is provided in the drive circuit 5d.
This constant current generating circuit is a circuit that generates a pulsed constant current waveform in accordance with an external logic signal input and a bias voltage input.
In such a current-driven image display device, the gradation control of the light emission amount is performed by controlling the application time of a constant current flowing through the display element 5e.

The control circuit 5b is a pulse width for driving a pixel (R, G, B) of the display element 5e (LED) with a gradation signal of an external video signal (that is, image data stored in the image memory 5a). Conversion to the addition method is performed, and each display element (LED) is turned on according to the gradation of each pixel.
Here, in the pulse width addition gradation control method according to the present embodiment, in order to obtain a higher quality image, a pulse having a light emission width (for example, 0.5 width) smaller than the current minimum gradation 1 is generated. However, even if the number of expression gradations is increased using this, for example, the minimum value of the current input pulse width can be changed so that a gentle luminance change (gradation change) proportional to the added pulse width can be made. SF0.5 (subframe 0.5) is added to light a pulse having a pulse width that is half that of 1, that is, a pulse corresponding to 1.5 width.
By adding SF 0.5 (subframe 0.5) for turning on the pulse corresponding to 1.5, gradation can be expressed in increments of 0.5, which could not be expressed conventionally, as will be described later. . However, 0.5 which is a pulse having a minimum width of 1 or less cannot be expressed.

FIG. 2 is a diagram for explaining the basic idea of gradation control according to the present invention.
FIG. 2A shows an output response when the display element (LED) is driven with an input pulse having a width of 1 (that is, a change in luminance of the display element), and FIG. 2C shows the output response when driving (LED), FIG. 2C shows the output response when driving the display element (LED) with an input pulse of width 0.5, and FIG. The output response when the display element (LED) is driven with an input pulse of .5 is shown.
Since the display element and its drive circuit have response characteristics, even if an ideal square wave input pulse is applied to the display element, the output waveform is slightly delayed with respect to the input pulse. The rising and falling portions of the output pulse are dull.
Therefore, when an input pulse having a width of 0.5 is simply applied to the display element, as shown in FIG. 2C, the output response is smaller than the expected luminance corresponding to 0.5, and an accurate gradation representation is obtained. Can't do so Note that the product of the light emission time (lighting time) of the display element and the light emission luminance is the brightness perceived by humans.
However, when an input pulse having a width of 1.5 is applied to the display element, an output having a light emission width (light emission time) corresponding to the input pulse having a width of 1.5 is obtained as shown in FIG.

Therefore, in the present embodiment, specifically, the following control method is taken.
In the case of expressing gradation 1, the light emitting element is caused to emit light by an input pulse having a width of 1 as in the past.
In the case of expressing gradation 1.5, the light emitting element is caused to emit light by an input pulse having a width of 1.5 (that is, a pulse obtained by adding an input pulse having a width of 0.5 to an input pulse having a width of 1.0).
In the case of expressing gradation 2, the light emitting element is caused to emit light by an input pulse having a width of 2.
In the case of expressing the gradation 2.5, the light emitting element is caused to emit light using input pulses having a width of 1 and a width of 1.5.
In the case of expressing gradation 3, the light emitting element is caused to emit light using input pulses having widths 1 and 2.
In the case of expressing the gradation 3.5, the light emitting element is caused to emit light using input pulses having a width of 1.5 and a width of 2. (The following gradation expression is omitted.)
Accordingly, gradation control can be performed in increments of 0.5 after gradation 1.

When the responsiveness of the display element (including the drive circuit) is poor, the rising and falling edges of the output pulse waveform with respect to the input pulse are dull. Therefore, the input pulse width corresponds to the input pulse width (proportional) for narrow input pulses of 1 or less However, for an input pulse longer than width 1, an output luminance corresponding to the length of the input pulse can be obtained.
Therefore, for an input pulse with a width of 1.5, which is intermediate between width 1 and width 2, the output luminance when the input pulse width is 1 (gradation 1) and the output luminance when the input pulse width is 2 (gradation) An intermediate output luminance (gradation 1.5) of 2) is obtained.
With such a driving method, the change in the first light emission gradation is 0 → 1, but after gradation 1, smooth gradation control in 0.5 increments becomes possible.
That is, even if the number of expressed gradations is increased by using an input pulse having a width of 1.5, a smooth luminance change proportional to the added pulse width can be made after gradation 1.
Therefore, it is possible to easily increase the number of gradation expressions in the current image display device, and it is possible to obtain a higher quality image display device.

FIG. 3 is a diagram for explaining the effects of the above-described embodiment.
FIG. 3A shows a state of luminance change by the gradation control method in the current image display apparatus, and an equal gradation change with an output change (luminance change) corresponding to the input pulse width of width 1. (Gradation change every 1.0).
FIG. 3B shows a case where gradation control is simply performed using an input pulse having a width of 0.5, and the input pulse having a width of 0.5 corresponds to this because of the response characteristics of the display element. Output (that is, luminance) is not obtained, and uniform gradation change (gradual gradation change) is not made.
For example, when an input pulse having a width of 0.5 is simply used and the output response is 0.2 for an input pulse having a width of 0.5, the first gradation change is 0.2. 0.8, 0.2, 0.8... And the like, and uniform gradation change (gradual gradation change) is not made.
FIG. 3C shows how the luminance changes by the gradation control method of the present embodiment.
In the present embodiment, the first gradation (gradation 1) has a luminance change corresponding to the input pulse width of 1 (that is, gradation change 1.0). This shows that the gradation can be controlled smoothly with a change of 0.5.

FIG. 4 is an example of an internal block of the LED driver IC used in the drive circuit 5e of FIG. Here, the operation of the LED driver IC shown in FIG. 4 will be described.
Data corresponding to ON / OFF of each display element output is set in the lowest F / F (flip-flop) unit using CLOCK and SERIALIN (data).
At the stage where each data set is completed, the LATCH latch pulse is input and the output stage (middle stage) F
Transfer data to / F.
By inputting the Enable pulse after setting the data, the uppermost constant current driver draws the current according to the data.

The width of the enable pulse to be input is a length according to the weight for each bit. If the length of D0 is width 1, the length of D1 is width 2, the length of D2 is width 4, and so on. -D6 has a length of 64.
D0 data is set and a width 1 enable pulse is input, and D1 data is set and a width 2 pulse is input.
Here, the data set is made efficient by setting the next data while the display element is lit.
If the data set is started after the lighting is completed, the next lighting cannot be started until the setting is completed.However, if the lighting time is longer than the set time by starting to set the next data after the data setting, the next lighting is immediately performed after the setting is completed. Lighting can be started.
Even when the lighting time is shorter than the set time, the waiting time can be slightly reduced.
Thus, since there is a relationship in which the lighting time decreases as the set time increases, it is difficult to increase the number of sets.

This is because the reduction in the lighting time leads to a reduction in luminance and an improvement in luminance during lighting (= deterioration of power efficiency) to compensate for the reduction in luminance.
In order to display a moving image, it is necessary to complete one cycle of lighting control in accordance with an external video cycle (usually 16.6 ms in the NTSC system). Is a major element in the control sequence design.
In order to reduce the set time, there is an increase in transmission rate, a large number of transmission bits to be sent in one clock, etc., but it is necessary to use expensive high-speed compatible parts as the transmission rate increases. Increase in EMI noise There are problems such as an increase in the amount of transmission materials used due to an increase in the number of transmission bits, high costs (multi-core cables and connectors), and the need for multi-bit compatible ICs. difficult.

FIG. 5 is a diagram schematically showing the relationship between the set data and the lighting time in the LED driver IC of FIG. 4, and shows a case where four types of lighting pulses D0 to D3 are used.
FIG. 5 shows a lighting sequence when the set time is 5 unit hours and the lighting time of D0 is 1 unit time. (D3 is 8 unit hours because D0 is 1 unit time.)
A set waiting time for setting the data of D3 is generated before D3 is turned on. D2 does not wait for setting, but D1 and D0 have a setting waiting time.
This set waiting time is unlit in light emission control, and the lighting rate deteriorates as this waiting time increases.
Therefore, in order to increase the number of gradation representations, if a short pulse is generated to increase the number of sets, the waiting time increases. Therefore, there is a restriction on increasing the number of sets.

As described above, the gradation control method for the image display device according to the present embodiment is a gradation control method for an image display device in which a plurality of display elements 5e are arranged in a matrix.
An input pulse having a minimum pulse width that is smaller than a pulse width (width 1) serving as a basis for expressing the current minimum gray scale is generated, and the generated first input pulse having the minimum pulse width, A second input pulse having a pulse width equal to or a third input pulse having a pulse width multiple times the width of the second input pulse,
The gradation 1 that is the gradation with the lowest light emission luminance of the display element 5e is obtained by causing the display element 5e to emit light by the second input pulse, and the gradation with the luminance higher than the expression gradation 1 is the first input pulse. It is obtained by causing the display element 5e to emit light with a pulse that is a combination of the second input pulse or the third input pulse.
For example, the width of the first input pulse is ½ of the width of the second input pulse.
Therefore, according to this embodiment, the number of gradations to be expressed using an input pulse having a width (for example, 0.5 width) smaller than the input pulse (width 1.0) that obtains the current minimum gradation 1 Even if is increased, the luminance change (gradation change) can be made gentle in proportion to the added pulse width after the first gradation 1.

Embodiment 2. FIG.
In the first embodiment described above, the number of expression gradations is increased by using a pulse having a light emission width (0.5 width) that is half of the light emission width 1 expressing the current minimum gradation 1, and is proportional to the added pulse width. Although the gradation control method capable of performing a gentle luminance change has been described, in the present embodiment, at the same time that a 1.5-width pulse is added, for example, a gradation expression of width 4 is expressed as a width 1.75 and a width. Realized by 2.25 total pulses.
In other words, the pulse widths used are 1, 1.5, 1.75, 2, 2.25, 8, 16, and 32, respectively.
For the gradations 1, 1.25, 1.5, 1.75, 2, 2.25, 2.5, 2.75, 3,... → pulse width), 1 → 1, 1.25 → 1, 1.5 → 1.5, 1.75 → 1.75, 2 → 2, 2.25 → 2.25, 2.5 → 1 + 1. 5, 2.75 → 1 + 1.75, 3 → 1 + 2, 3.25 → 1 + 2.25, 3.5 → 2 + 1.5, 3.75 → 2 + 1.75, 4 → 1.75 + 2.25, and so on. It can be expressed.
The gradations 1 and 1.25 both become 1, but after that, gradation expression can be made in increments of 0.25.

In the case of the additive pulse gradation control method, it is necessary to set data corresponding to the gradation before each pulse emission, so it is desirable to represent the gradation with as few pulse types as possible.
Simply adding pulses with widths of 1.25, 1.5, and 1.75 enables light emission in 0.25 increments, but the pulse type is 1, 1.25, 1.5, There are nine types of 1.75, 2, 4, 8, 16, 32.
In the method according to the present embodiment, 1, 1.5, 1.75, 2, 2.25, 8, 16, 32.
The number of pulses can be saved.
Although there is a drawback that the gradation of the lowest part does not appear, there is an advantage that the gradation expression ability in the intermediate part which is visually important can be improved with a small number of pulse types.
A simple method leads to an increase in pulse type = non-lighting time, and conversely, reducing the number of pulse types is a method that leads to an improvement in the lighting rate in terms of lighting control.
According to the present embodiment, the width of the first input pulse is 1/4 or 3/4 of the width of the second input pulse. Gradation can be expressed.

Embodiment 3 FIG.
In the present embodiment, in addition to the pulse having a width of 1.5 in the first embodiment, a pulse having a width of 0.5 used only for the lowest level (that is, only for the minimum gradation) is added.
Specifically, at the same time as adding a pulse having a width of 1.5, a pulse having a width of 0.5 that is responsible for only the gradation representation of the gradation 0.5 is added.
However, the gradation expressing ability of a pulse having a width of 0.5 is less than 0.5 because of the response characteristics of the display element.
For example, even when only a gray level of 0.3 is obtained with respect to a pulse with a width of 0.5, before adding a pulse with a width of 0.5, when considering a change from gray level 0 to 1, the display element Since the emission luminance of was changed from 0 to 1, there was a level difference of “1” in the first gradation.

However, when a gradation of 0 → 0.5 → 1.0 is considered by adding a pulse having a width of 0.5, the change in emission luminance is 0 → 0.3 → 1.0.
That is, in the stage of gradation 1, when a pulse having a width of 0.5 is not added, there is a difference in luminance of “1.0”, but the luminance of “0.3” and “0.7”. It becomes a level difference and a gentle brightness change.
Thus, according to the present embodiment, the first expression gradation 1 has a minimum pulse width (for example, a width smaller than the basic pulse width (width 1) for expressing the current minimum gradation (for example, Since the display element is caused to emit light by the first input pulse having a width of 0.5), the luminance difference between gradation 0 where the display element is in a non-light emitting state and light emission start (gradation 1) can be reduced. A change in luminance between 0 and the interval 1 can be suppressed, and the luminance changes more gently than in the first embodiment.
It should be noted that this embodiment is also effective when the responsiveness is poor and a gradation expression higher than that for an input with a width of 0.5 is obtained due to overcorrection or the like.
For example, even when a gradation equivalent to 0.7 is obtained with respect to an input having a width of 0.5, the use of this embodiment can suppress the magnitude of the luminance change and obtain a gentle gradation change.
That is, when 0 → 0.5 → 1.0 is considered as the gradation change, the change in emission luminance is 0 → 0.7 → 1.0, and the luminance steps are “0.7” and “0”, respectively. .3 ", which is a gentle gradation change.

  The present invention provides a gradation control method capable of changing the luminance gently in proportion to the added pulse width even if the number of expression gradations is increased by using a pulse having a light emission width smaller than the current minimum gradation. Useful for realization.

It is a block diagram which shows the example of whole structure of the image display apparatus with which this invention is applied. It is a figure for demonstrating the basic idea of the gradation control by this invention. It is a figure for demonstrating the effect by this Embodiment 1. FIG. It is an example of the internal block of LED driver IC used for a drive circuit. It is a figure which shows typically the relationship between the set data in LED driver IC, and lighting time. It is a schematic diagram for demonstrating the pulse width addition gradation control method.

Explanation of symbols

5 Display unit 5b Control circuit 5d Drive circuit 5e Display element P0 Width 1 input pulse P1 Width 2 input pulse P2 Width 3 input pulse

Claims (4)

A gradation control method using a pulse width modulation control method or a pulse width addition gradation control method of an image display device in which a plurality of display elements are arranged in a matrix,
Generate an input pulse with a minimum pulse width that is smaller than the basic pulse width for expressing the current minimum gradation,
Using the generated first input pulse having the minimum pulse width, the second input pulse having the basic pulse width, or the third input pulse having a pulse width multiple times the width of the second input pulse,
The gradation 1, which is the gradation with the lowest light emission luminance of the display element, is obtained by causing the display element to emit light by the second input pulse, and the gradation having a luminance higher than the gradation 1 is the first input. A gradation control method for an image display device, wherein the display element is made to emit light with a pulse in which the second input pulse or the third input pulse is combined with a pulse.   2. The gradation control method for an image display device according to claim 1, wherein the width of the first input pulse is ½ of the width of the second input pulse.   2. The gradation control method for an image display device according to claim 1, wherein the width of the first input pulse is 1/4 or 3/4 of the width of the second input pulse. The first expression gradation obtained by causing the display element to emit light with an input pulse having a minimum pulse width that is smaller than the basic pulse width for expressing the current minimum gradation, used only for the minimum gradation. The gradation control method for an image display device according to claim 1, wherein 1 is further used .

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