JPH10313417A - Digital gamma correction circuit, liquid crystal display device using the same and electronic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a digital gamma correction circuit that corrects received digital image data into digital image data suitable for an applied voltage versus transmittivity characteristic of a liquid crystal display section at a low gradation region and a high gradation region. SOLUTION: A digital gamma correction circuit 32 has an area discrimination section 110, a correction section 32A using a RAM 102 and a straight line approximation arithmetic section 32B. A utilizing area discrimination section 110 discriminates to which of a 1st area A with a low gradation area, a 2nd area C with a high gradation area or a 3rd area B between the 1st and 2nd areas the gradation of the received digital image data belong. The RAM 102 stores correction data corresponding to the received digital image data, and correction data corresponding to the received digital image data are received from the RAM 102. A linear approximation arithmetic section 32B applies a linear approximation to the received digital image data belonging to the 3rd area according to at least one straight line with a prescribed gradient and an offset to correct the image data.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital gamma correction circuit for correcting input digital image data into digital image data having an applied voltage-transmittance characteristic of a liquid crystal display section, and a liquid crystal display device using the same. And electronic devices.

[0002]

2. Description of the Related Art As an image display unit of an electronic device, a thin liquid crystal display panel has been widely used in place of a conventional relatively large CRT. In the liquid crystal display panel, as shown in FIG. 13, the TV characteristic indicated by the relationship between the applied voltage V and the transmittance T is not linear. In particular, the change in the transmittance T with respect to the change in the applied voltage V is small near the black level where the gradation value is low. Therefore, near the black level, there is little change in the gradation (light transmittance T) with respect to the change in the image data (applied voltage V), and the resolution in this region is reduced. Correcting this so as to obtain an appropriate resolution in all regions is called gamma correction in a liquid crystal display device.

On the other hand, even in a CRT including a television receiver, there is a similar phenomenon that the input signal voltage and the light emission output are not linear. CR
Gamma correction for T is performed. Therefore, gamma correction is not required on the side of the television receiver using the CRT.

[0004] Here, it is known to perform gamma correction in a photographing camera digitally. Japanese Patent No. 25428 discloses an example in which a gamma correction is performed by performing a linear approximation calculation using a photographing camera.
No. 64 and JP-A-8-32837. Japanese Patent Application Laid-Open No. H2-230873 discloses that digital gamma correction is performed by a photographic camera by using both a linear approximation operation and a memory.

Here, in order to display an image on a liquid crystal display panel based on a television signal, gamma correction for a CRT is unnecessary, and finally gamma correction is performed in accordance with the TV characteristics of the liquid crystal display panel. Must be implemented.

To perform gamma correction when an image is displayed based on a television signal by a projector using a liquid crystal display panel as a light valve is disclosed in Japanese Patent Application Laid-Open No. 8-18 / 1994.
No. 6833. However, in this publication,
There is no clear disclosure of gamma correction for a television signal that has been subjected to gamma correction for CRT in advance, and since gamma correction at the subsequent stage is performed in analog, a liquid crystal drive circuit including a gamma correction circuit cannot be integrated into an IC. Was.

The analog gamma correction is performed by using a diode or the like and using a gamma correction characteristic of one-point break as shown in FIG.

[0008] However, since the characteristics vary among the individual diodes, adjustment for uniform characteristics in each liquid crystal display device is complicated. Further, in a case where a total of three liquid crystal display panels for R, G, and B are used in the same device, such as a color projector, adjustment between the three liquid crystal display panels is necessary, which is complicated. Met.

Further, with the gamma correction characteristic of one-point break as shown in FIG. 15, only the black level region of the TV characteristic shown in FIG. 13 can be corrected, and the correction in the black level region is performed by linear approximation. For this reason, there has been a limit in securing accurate correction suitable for TV characteristics.

An object of the present invention is to perform digital processing including gamma correction in accordance with the TV characteristics unique to each liquid crystal display panel, and to cope with an area where the TV characteristics unique to the liquid crystal display panel changes in a curve. It is another object of the present invention to provide a digital gamma correction circuit capable of performing gamma correction with higher accuracy and a liquid crystal display device and an electronic device using the same.

Another object of the present invention is to increase the contrast ratio on the liquid crystal display screen by expanding the driving range of the TV characteristics inherent in the liquid crystal display panel toward a region having a high transmittance. An object of the present invention is to provide a digital gamma correction circuit capable of increasing the N ratio, and a liquid crystal display device and an electronic device using the same.

Still another object of the present invention is to reduce the storage capacity of a memory table for digital gamma correction,
It is an object of the present invention to provide a liquid crystal display device and an electronic device in which power consumption is reduced and a liquid crystal driving circuit can be integrated into an IC.

[0013]

According to a first aspect of the present invention, there is provided a digital gamma correction circuit for correcting input digital image data into digital image data suitable for display driving in a liquid crystal display unit. Which one of the first area on the low gradation area side, the second area on the high gradation area side, and the third area between the first and second areas belongs to the tonal value. An area determination unit for determining, a memory table storing correction data corresponding to the input digital image data belonging to the first and second areas, and reading correction data corresponding to the input digital image data;
A linear approximation calculating unit that corrects the input digital image data belonging to the third area by performing a linear approximation according to at least one straight line having a predetermined inclination and an offset.

According to the first aspect of the present invention, when the input image data belongs to the first region where the gradation value is low and the second region where the gradation value is high, Gamma correction can be performed by reading correction data corresponding to the applied voltage-transmittance curve of the display panel from the memory table. For this reason, in the first and second regions, gamma correction can be performed with high accuracy even if the rate of change between the applied voltage and the transmittance of the liquid crystal display panel varies. On the other hand, the first and second
In a third area between the areas, the applied voltage of the liquid crystal display panel is reduced by
Since the change rate of the transmittance is almost constant, gamma correction is performed by linear approximation. Therefore, the capacity of the memory table can be reduced as compared with the case where the gamma correction of all gradation areas is performed using only the memory table.

Further, as compared with the case where the gamma correction is performed by linear approximation in the second region, the gamma correction region is expanded to a higher transmittance region where the rate of change between the applied voltage and the transmittance of the liquid crystal display panel is not uniform. However, highly accurate gamma correction can be performed using the memory table. For this reason, the driving range of the liquid crystal display panel is expanded, the contrast ratio can be increased, and the S / N ratio also increases.

According to a second aspect of the present invention, in the first aspect, the linear approximation unit shifts the input digital image data by bits, and converts the input digital image data to 2 ⁿ or 1/2 ⁿ (where n is a natural number). A) a plurality of bit shifters, a selector for selecting and outputting at least one of the outputs of the plurality of bit shifters in accordance with the inclination data of the at least one straight line, and an output of the selector.
A first calculator for adding or subtracting the offset data of the at least one straight line.

According to a second aspect of the present invention, when the linear approximation operation unit is configured by using a plurality of bit shifters for bit-shifting the input image data, a circuit configuration for multiplying the inclination of the straight line is simplified, and the outputs of the plurality of bit shifters are output. By making the selector select, one of a plurality of types of inclinations can be selected and a linear approximation operation can be performed. The offset data is added to or subtracted from the selector output by the first computing unit.

According to a third aspect of the present invention, in the second aspect, the straight-line approximation calculating section approximates a straight line for each of a plurality of straight-line approximation sections using a plurality of types of straight lines in the third area. The straight-line approximation unit further includes a register that stores a plurality of pieces of inclination data and a plurality of offset data, and the area determination unit calculates boundary data of each straight-line approximation section and input digital image data. And comparing and reading out the inclination data and the offset data of the corresponding linear approximation section from the register.

According to the third aspect of the present invention, a straight line approximation operation can be performed using different straight lines for each of the plurality of straight line approximation sections in the third region, and the gamma correction accuracy in the third region is further improved. .

According to a fourth aspect of the present invention, in the third or fourth aspect, the gamma correction data of the first and second areas are respectively located on at least one reference straight line.
Divided into respective reference straight line data in a second region and respective difference data in the first and second regions to be added to or subtracted from each of the at least one reference straight line data; Stores the difference data in each of the first and second areas, the straight-line approximation calculating section outputs each of the reference straight-line data in the first and second areas, A second calculator for adding or subtracting the difference data output from the table and the reference straight line data output from the straight line approximation calculator is further provided.

According to the fourth aspect of the present invention, since only the difference data is stored in the memory table, the capacity of the memory table can be further reduced.

Each of the inventions of claims 5 and 6 corresponds to claims 1 to 4
A liquid crystal display device and an electronic device including a digital gamma correction circuit according to any of the above, and a liquid crystal display driving circuit for driving an image to be displayed on the liquid crystal display unit based on the image data on which the digital gamma correction has been performed. ing.

According to the fifth and sixth aspects of the invention, a high quality image can be displayed on the liquid crystal display by accurately compensating for the applied voltage-transmittance in the liquid crystal display.

[0024]

Embodiments of the present invention will be described below with reference to the drawings.

(Overall Description of Data Processing Circuit) FIG. 1 is a block diagram schematically showing a data processing / liquid crystal display driving circuit for driving a liquid crystal display panel. In this embodiment shown in FIG. 1, three liquid crystal display panels are respectively R, G, B
The present invention is applied to a projector used as a light valve. In this embodiment, the three liquid crystal display panels are constituted by active matrix substrates using TFTs as switching elements, but other liquid crystal display substrates can be used.

In FIG. 1, this projector is roughly divided into a signal processing board 10 used for data processing of each color R, G, B, and a liquid crystal display dedicated board 30R provided for each color R, G, B. , 30G, 30B, and a liquid crystal display panel 50 functioning as three light valves, respectively.
R, 50G, and 50B.

The signal processing board 10 is an overall control board on which elements and circuits for realizing the following functions are mounted in addition to various circuits (not shown) for a projector, which is an electronic apparatus of the present embodiment. You can also. First, as input terminals for image data, a first input terminal 12 for inputting an analog television signal such as NTSC or PAL, and a second input terminal 14 for inputting a digital image signal such as a computer output or a CDROM output. Having. Here, the analog television signal input to the first input terminal 12 is C
Although gamma correction is performed in consideration of the characteristics of the RT, the digital image signal input to the second input terminal 14 is not gamma corrected. It is also possible to provide another terminal for inputting a digital image signal subjected to gamma correction for a CRT, such as a CCD camera output.

The first input terminal 12 has an AD converter 1
6 are connected to perform analog-to-digital conversion of the television signal. Further, a digital decoder 18 is connected to the AD converter 16. This digital decoder 18
Calculates the luminance signal Y and the color difference signals U and V in the television signal,
The signal is decoded into R, G, and B signals of three colors.

A frame memory 20 is provided downstream of the digital decoder 18. The data input through the first input terminal 12 is stored in the frame memory 20 via the AD converter 16 and the digital decoder 18 as one.
It is written for the frame. Digital R, G, and B data input through the second input terminal 14 is directly written into the frame memory 20. The liquid crystal display panel 50
When the interlaced scanning is performed in R, 50G, and 50B, one frame of R, G, and B data is read out of the frame memory 20 in two fields in the order of odd lines and even lines. It is.

A primary gamma correction circuit 24 is connected to the subsequent stage of the frame memory 20 via a switch 22. When the data from the frame memory 20 is data input through the first input terminal 12, the switch 22 outputs the data to the primary gamma correction circuit 24. R, G, B which are the above-mentioned CCD camera outputs
Similarly, the primary gamma correction circuit 2
4 is input. On the other hand, when the data from the frame memory 20 is data input through the second input terminal 14, the switch 22 does not lead the data to the primary gamma correction circuit 24 but directly passes through the bypass line 26. To the liquid crystal display dedicated boards 30R, 30G, 30G.
The details of the primary gamma correction circuit 24 will be described later.

Next, the liquid crystal display dedicated board 30R and the liquid crystal display panel 50R will be described with reference to FIG. FIG. 5 is a block diagram of a liquid crystal driving IC mounted on the liquid crystal display dedicated board 30R. The configuration of the liquid crystal driving IC for the other colors G and B is the same as the configuration of the color R.

A secondary gamma correction circuit 32 is provided on the liquid crystal display dedicated board 30R. The details of the secondary gamma correction circuit 32 will also be described later.

At a stage subsequent to the secondary gamma correction circuit 32, a phase expansion circuit 34 is provided. This phase expansion circuit 34
Data phase expansion is performed to reduce the driving frequency of the liquid crystal display panel 50R. For this purpose, as shown in FIG. 5, a shift register 200 and a latch circuit 202 are provided. In FIG. 5, for convenience of explanation, 4 in the case of N = 4
This is an example of phase development. The phase expansion circuit 3 of FIG.
Operation 4 will be described with reference to the timing chart of FIG.

Data of each pixel is input to the phase expansion circuit 34 in a time series in accordance with the dot clock of FIG. As an output from the shift register 200, FIG.
As shown in FIG. 2, the output line of the R color data is divided into N lines, the data of the pixels 0, 0 + N, 0 + 2N,... Are assigned to the first output line, and the second output line is 1, 1 + N, 1 + 2N. ,
.. Are assigned, and similarly, pixel data is assigned to the remaining two output lines and output. In this case,
The data time of the pixel on each output line can be N times the original data time. This is called N-phase development. As described above, since the data time of each pixel becomes longer, the sampling frequency at the time of data sampling in the liquid crystal display panel 50R becomes 1 / N, and especially in the case of a liquid crystal display panel having a high pixel density, the response of the switching element is reduced. The combined sampling frequency can be used. When the liquid crystal display panel 50R has a high pixel density called XGA, unless phase expansion is performed, the data sampling frequency in the liquid crystal display panel becomes as high as 65 MHz, and the TFT cannot respond. Thus, a 12-phase expansion with N = 12 is performed to reduce the sampling frequency to a value that can be responded by the TFT. In the case of VGA and SVGA having a lower pixel density than this, a sampling frequency responsive to the TFT can be obtained by the six-phase expansion where N = 6.

In this embodiment, data of four output lines in the case of four-phase development is latched by the latch circuit 202 at the same timing. As a result, the output of the latch circuit 202 is as shown in FIG. 12, and the data phases of the output lines are aligned. Without providing the latch circuit 202, the data of the four output lines may be sampled at different timings or at the same timing in the liquid crystal display panel 50R.

A polarity inverting circuit 36 is provided at a stage subsequent to the phase expanding circuit 34. The polarity inversion circuit 36 is provided for inverting the polarity of the electric field applied to the liquid crystal of each pixel of the liquid crystal display panel 50R at a predetermined cycle to drive the polarity inversion. In this embodiment, since the switching element of the liquid crystal display panel is constituted by a TFT, the polarity of the data potential supplied to the pixel is inverted with respect to the potential of the common electrode formed on the substrate facing the TFT substrate. It is driven.

The digital data processing for the polarity inversion may be performed by inverting the logic of the digital data. Therefore, as shown in FIG. 5, the polarity inversion circuit 36 includes four inverters 210A to 210D for inverting the data logic of the four output lines and four inverters for selecting and outputting one of the data before and after the inversion. Selectors 212A-212D
Having. When polarity inversion driving is performed for each pixel, for example, data before inversion is selected by the first and third selectors 212A and 212C, and the second and fourth selectors 212B and 212B are selected.
At 212D, the inverted data is selected.

A DA converter 38 having four converters 38A to 38D is provided downstream of the polarity inversion circuit 36, and performs digital-to-analog conversion of the polarity-inverted data of the N lines that have undergone phase development. This analog signal is the output of the liquid crystal display drive IC.

The liquid crystal display drive IC is provided with a timing generation circuit 220, and the above-described phase expansion circuit 34, polarity inversion circuit 36, and DA converter 38 generate necessary timing signals based on the image synchronization signal. You.

As shown in FIG. 1, the liquid crystal display dedicated board 30R is further provided with an amplifier 40 and a buffer 42. The data on which the bias voltage corresponding to the positive and negative polarity inversion drive is superimposed by the amplifier, for example, the operational amplifier 40, is supplied to the liquid crystal display panel 50R via the buffer 42, and the liquid crystal display panel 50R is controlled based on the data. The polarity inversion driving is performed at a predetermined cycle such as one dot or one line.

(Relationship between Primary Gamma Correction and Secondary Gamma Correction) In this embodiment, gamma correction is performed in two steps. The first gamma correction is called primary gamma correction, and the second correction is called secondary gamma correction. However, in this embodiment, since all of the corrections are digital corrections, the same result can be obtained even if the correction order is reversed. However, as in this embodiment, the boards 30R, 30G, 3
When the secondary gamma correction circuit 32 is mounted on the 0B side, the adjustment process of the liquid crystal display panel, which will be described later, is simplified, and the circuit mounted on the liquid crystal display dedicated board can be integrated into an IC.

Here, the primary gamma correction of this embodiment is mainly performed by converting the image data subjected to the CRT gamma correction
The purpose is to return to the original data before the gamma correction for RT. Therefore, the correction data can be determined independently of the characteristics of the individual liquid crystal panels, which is different from the secondary gamma correction described later. In the case where the primary gamma correction is intended only for canceling the CRT gamma correction, and when image data that has not been subjected to the CRT gamma correction is input, the bypass line 26 is used as described above. There is no need to pass through the primary gamma correction circuit 24. Instead, for example, when image data that has always been subjected to CRT gamma correction is input, the primary gamma correction circuit 24 supplies a partial area of the TV characteristic shown in FIG. Other functions such as correction according to the above may be added. Since the primary gamma correction circuit 24 of this embodiment uses a RAM table, it can cope with the correction data stored in the RAM table only by adding those functions.

On the other hand, the secondary gamma correction circuit 32
The main object of the present invention is to perform gamma correction suitable for TV characteristics of individual liquid crystal display panels shown in FIG. This TV
Since the characteristics are different for each liquid crystal display panel, they are different from the primary gamma correction in that they always need to be adjusted. in this way,
Since it is necessary to change gamma correction data according to each liquid crystal display panel, it is necessary to change the gamma correction data separately from the correction (primary gamma correction) mainly for canceling the gamma correction for the CRT, which does not need to be changed. High correction contents are implemented as secondary gamma correction. Moreover, the secondary gamma correction circuit 3
The adjustment process can be simplified by mounting the liquid crystal display 2 on a boat dedicated to liquid crystal display and forming an integral structure with the display panel. further,
By performing the secondary gamma correction that needs to be changed in this way separately from the primary gamma correction, the calculation at the time of changing the secondary gamma correction data is simplified. Can be implemented.

(Description of Primary Gamma Correction Circuit) Next, details of the primary gamma correction circuit 24 will be described with reference to FIG.

FIG. 2 shows an example of the conversion characteristics of the primary gamma correction performed by the primary gamma correction circuit 24. The horizontal axis represents the input data and the vertical axis represents the output data in 256 gradations (8 bits). Represents. As described above, the purpose of the primary gamma correction is that the gamma correction for the CRT (the dashed line in FIG. 2) performed on the television signal input through the first input terminal is performed. 2 is to perform primary gamma correction and substantially return to the original data (linear characteristics indicated by broken lines in FIG. 2) before gamma correction for CRT.

The primary gamma correction circuit 24 has an address generator 24A for generating an address based on the input image data, and a RAM table 24B in which correction data is stored in advance in association with the address. That is, when data X on the horizontal axis of FIG. 2 is input, data Y stored in advance in association with an address generated according to the input data X is read from the RAM table, and primary gamma correction is performed. Is done. As a result, the image data after the primary gamma correction has a substantially linear characteristic as shown by a broken line in FIG.

Here, the reason why the primary gamma correction circuit 24 is mounted on the signal processing board 10 is as follows.
That is, since the purpose of the primary gamma correction is as described above, this primary gamma correction can be carried out independently of the characteristics of the liquid crystal display panel, and production and inspection are performed ignoring the characteristics of the individual liquid crystal panels. Because it is possible.

However, in this embodiment, after the signal processing board 10 and the three liquid crystal display boards 30R, G, B are electrically connected, the individual liquid crystal display panels 50R, G,
Due to the relationship with the characteristic B, the data in the RAM table constituting the primary gamma correction circuit 24 can be rewritten. The rewriting of the data in the RAM table can be performed in the adjustment process at the factory before the device is shipped, or may be performed by the user operating the operation unit. The data rewriting of the RAM table will be described later.

(Description of Secondary Gamma Correction Circuit) An example of the secondary gamma correction circuit 32 shown in FIG. 1 is shown in FIG. Also,
FIG. 6 shows the correction characteristics of the secondary gamma correction performed by the secondary gamma correction circuit shown in FIG. Here, when the actual TV characteristics of the liquid crystal display panel are as shown in FIG. 13, ideal secondary gamma correction characteristics are as shown in FIG.
Therefore, the secondary gamma correction characteristic of FIG. 6 is more effective than the secondary gamma correction characteristic of FIG. 15 in the case of the conventional analog correction.
The characteristics are close to ideal.

FIG. 6 shows input data of 256 gradations (8 bits) on the horizontal axis and output data of 512 gradations (9 bits) on the vertical axis. As described above, in the secondary gamma correction, by outputting a bit number larger than the bit number of the input data, it is possible to express different gradations even in an area with a small change rate. In this case, the number of gradations of the output data is doubled to 512 gradations for the input data. However, if necessary, the number of gradations may be quadrupled to 1024 gradations.

If the number of bits of the output data is an integral multiple of the number of bits of the input data, if all the output data is stored in the RAM in the same way as the primary gamma correction, the capacity of the RAM increases. Power consumption increases and R
It is difficult to incorporate the AM into the IC. Therefore, in this embodiment, as described below, the output data of only the areas A and C in FIG. 6 is stored in the RAM to reduce the capacity.

In FIG. 5, the secondary gamma correction circuit 3
Reference numeral 2 denotes a correction unit 32A using a RAM used for secondary gamma correction of the hatched areas A and C in FIG.
And a linear approximation correction calculation unit 32B used for the secondary gamma correction of the area B in FIG. In region B of FIG. 6, straight line approximation is performed using, for example, three straight lines.

Here, the straight line Y = a.X + in the area B in FIG.
To explain using b as an example, a is referred to as inclination data, and X
The value b of Y when = 0 is referred to as offset data. Therefore, the data a, d, and g shown in FIG. 6 become the inclination data, and the data b, e, and h become the offset data. The data c, f, i and j located at the boundaries between the regions A, B and C and the linear approximation section are referred to as boundary data.

As shown in FIG. 5, the secondary gamma correction circuit 32 includes an address generation section 100 and a R section as a correction section 32A for performing the secondary gamma correction in the areas A and C.
AM102. The address generator 100 generates an address based on the input image data X, and reads out the correction data Y in the RAM 102 corresponding to the address. Since the boundary data c and j are input to the address generator 100, no address is generated from the address generator 100 when c <X <j.

On the other hand, as a linear approximation correction operation unit 32B for performing the secondary gamma correction in the area B of FIG. 6, for example, three bit shifters 1 for shifting the input image data by bits
04A, 104B, and 104C, and a first selector 106 for selecting at least one bit shifter output based on the set inclination data a, d, and g, and a first selector 1
Offset data b added to or subtracted from the output
e and h are stored in the first selector 106
And the adder 108 for adding or subtracting the output of the register 122 and the offset data from the register 122 to calculate, for example, Y = a × X + b.

The bit shifter 104A shifts the input image data X by one bit to the upper side and outputs a value of 2 · X. The bit shifter 104B shifts the input image data X by one bit to the lower side and outputs a value of (（) · X. The bit shifter 104C shifts the input image data X by 2 bits to the lower side and outputs a value of (（) · X.

In the first selector 106, the inclination data a,
d and g are “１ ／”, “１ ／”, “３”,
When one of “2”, “2 +＋ １”, and “2 +＋” is selected, one or more corresponding outputs of the bit shifters 104A to 104C are selected.

The area determining section 110 compares the value of the input image data with the boundary data c, determines that the area is A if X ≦ c, and determines that the area is B if c <X <m. , X ≧
If j, it is determined that the area is C. Based on the result of the determination by the area determination unit 110, the second selector 112 selects the output of the RAM 102 in the areas A and C, and selects and outputs the output of the adder 108 in the area B.

Further, based on the boundary data c, f, i, and j, the area judging section 110 determines that when c <X ≦ f,
The register 122 outputs the slope data a and the offset data b. Also, the region determination unit 110 determines that f <X ≦ i
At this time, the inclination data d and the offset data e are output from the register 122. The region determining unit 110 further includes:
When i <X <j, the register 122 outputs the inclination data g and the offset data h.

In this embodiment, the areas A and C where the rate of change of the curve is not uniform in the correction characteristics of FIG. 6 are determined using the RAM 102, and the area B where the rate of change of the curve is almost constant and has a characteristic close to a straight line. , Correction data is obtained by linear approximation calculation.

One of the objectives of the secondary gamma correction is that the TV curve of FIG. 13 showing the correlation between the applied voltage V of the liquid crystal display panel and the light transmittance T is obtained by changing the curve of FIG. In the area A, the change in the transmittance is small with respect to the change in the applied voltage, and the object is to prevent a decrease in resolution in an area near the black level caused by the change.

Further, as another purpose of the secondary gamma correction, the following can be mentioned. In the vicinity of the upper limit of the transmittance and the range exceeding the upper limit in the first driving range of the related art shown in FIG. 13, the change in the transmittance with respect to the change in the applied voltage is small. For this reason, a range exceeding the upper limit of the conventional first drive range cannot be linearly approximated, and the vicinity of the upper limit cannot be accurately corrected by the linear approximation. In the present embodiment, the second driving range wider than the conventional first driving range is secured by performing the secondary gamma correction using the RAM table even in the white level area. Further, even if the first drive range shown in FIG. 13 is expanded to the second drive range toward the low-voltage drive region having a large curvature on the white level side shown in FIG. 13, the correction can be performed in accordance with the curvature. There is.
That is, the driving range can be expanded to a range that cannot be realized by the linear approximation. As described above, by expanding the drive range on the white level side, the upper limit of the transmittance is widened, and the contrast ratio is further increased. Thereby, even if the power of the backlight is reduced, the same brightness as before can be ensured, and the power consumption can be reduced accordingly. Further, since the drive voltage range is widened, there is also an advantage that the step of the voltage per gradation is widened and the S / N ratio is increased.

For this reason, in this embodiment, since only the correction data of the areas A and C are stored in the RAM 102,
The power consumption is reduced by reducing the capacity of the AM 102,
The AM 102 can be built in an IC.

The present embodiment can be similarly applied to the case where gamma correction is performed based on the secondary gamma correction characteristic shown in FIG. 7 in which the area B is approximated by one straight line.

(Change of secondary gamma correction data)
Since the characteristics of the individual liquid crystal display panels are different from each other, it is necessary to adjust the gamma correction data according to the characteristics of the individual liquid crystal display panels at least before shipment in a factory. For this purpose, as shown in FIG. 8, an operation unit 300 for inputting data for adjustment, a storage unit for storing TV characteristics of individual panels, for example, a PROM 302, and an operation unit 300 and a PROM 302 Based on the information,
A CPU 304 for calculating various adjustment data. The operation input unit 300 and the PROM 302
The CPU 304 is incorporated in an adjustment device installed in the factory when such adjustment can be performed only at the factory shipment stage, and when the user can make adjustments, the overall control board 10 and the liquid crystal display It is mounted on the substrate 30R or another built-in substrate. These operations will be described in different cases.

In the adjustment process before shipment of the apparatus, the TV characteristics of the individual liquid crystal display panels 50R, 50G, 50B are measured and stored in the PROM 302, respectively.
Thereafter, a predetermined pattern is formed on the liquid crystal display panels 50R, 50R.
G, 50B to display on the panel or R, G, B
Is visually inspected and inspected, for example, on a projector screen on which is synthesized.

As a result of this inspection, in order to change the secondary gamma correction data in the area A in FIG. 6, the contents of the RAM 102 and the data a to j supplied to the linear approximation operation unit may be changed. For example, a case will be described in which a command to increase the gradient of the area A in FIG. In this case, the CPU 3
04 calculates the correction data in the RAM 102 in the second gamma correction unit 32 based on the TV characteristics in the PROM 302, and rewrites the correction data in the RAM 102 based on the calculation result. Further, the CPU 304 changes the correction data at least between the boundaries c and f of the area B in accordance with the change of the correction data of the area A. This change is performed by changing and setting the inclination data a and the offset data b. If necessary, other linear approximation sections may be changed. Further, the boundary position between the regions A, B, and C or the boundary position between the straight-line approximation sections can be changed based on a command from the operation input unit 300. In this case, the CPU 304
May change the boundary data c, f, i, j.

(Change of primary gamma correction data)
In the present embodiment, by changing the primary gamma correction data for the entire screen, it is possible to adjust the contrast ratio and the brightness over the entire screen.

The adjustment of the contrast ratio is performed by operating, for example, a rotary knob for adjusting the contrast ratio of the operation input unit 300. For example, the primary gamma correction characteristic of the solid line in FIG. 3 can be changed to the primary gamma correction characteristic of the broken line in FIG. Thus, the correction data of the RAM table in the primary gamma correction circuit 24 is
Rewriting to include the contrast ratio adjustment data increases the contrast ratio.

On the other hand, the brightness adjustment is performed by operating, for example, a rotary knob for brightness adjustment of the operation input unit 300. For example, while maintaining the gradient of the primary gamma correction characteristic of the solid line in FIG. 4, the whole can be shifted so as to become the primary gamma characteristic of the broken line in FIG. As described above, by rewriting the correction data of the RAM table in the primary gamma correction circuit 24 so as to include the brightness adjustment data,
The brightness of the entire screen decreases.

As described above, the contrast ratio of the entire screen,
In order to adjust the luminance, the contents of the primary gamma correction RAM table storing the primary gamma correction data for the entire area, instead of the secondary gamma correction RAM table 102 storing the correction data for some areas, are rewritten. Can be easily handled.

(First Modification of Secondary Gamma Correction Circuit) FIG. 9 shows still another modification of the secondary gamma correction circuit. The secondary gamma correction circuit of FIG.
5 shows an improved circuit to reduce the capacitance of M102. In the circuit of FIG. 9, the first reference straight line Y ′ in the region A and the second reference straight line Y ″ in the region C are shown in FIG. 10 showing characteristics similar to the secondary gamma correction characteristics of FIG. Assuming each of them, the RAM 102 of FIG. 10 stores only the difference data between the first and second reference straight lines Y ′, Y ″ and the final correction data. This will be described with reference to FIG. 11 which is a partially enlarged view of the region A in FIG. 10. Data on the first reference straight line Y ′ in the region A is obtained by a straight line approximation operation, and is added or subtracted from the data. Are stored in the RAM 102 only. The data on the second reference straight line Y ″ in the area C is also obtained by a straight-line approximation operation, and only the difference data added or subtracted from the data is stored in the RAM 102.

Therefore, in the secondary gamma correction circuit shown in FIG. 9, for example, three bit shifters 104A to 104C
And the first and second reference straight lines Y ′,
Third selector 130 for selecting data matching Y ″
And an adder 132 for adding or subtracting the output of the third selector 130 and the difference data from the RAM 102. That is, three bit shifters 104
The outputs of A to 104C are used for the linear approximation calculation in the region B of FIG. 10 and also for the linear approximation calculation using the first and second reference straight lines Y ′ and Y ″ in the regions A and C. The fixed data input to the third selector 130 is
The first or second reference straight line Y ′, Y ″ is used alone when it is parallel to the X axis, that is, when the inclination is zero, or is added to the operation results of the three bit shifters 104A to 104C. It is used as offset data for the reference straight lines Y ′, Y ″.

According to the circuit of FIG. 9, it is determined that the input image data X belongs to the area A or C of FIG.
In the third selector 130 to which the determination signal is input, the first reference straight line Y ′ in FIG.
From the three bit shifters 104A to 104B and the fixed data,
Select one or more of them. 11 is output from the RAM 102 based on the address generated by the address generator 110 in correspondence with the input image data X. These are added or subtracted by the adder 132, and the output of the adder 132 is output to the second selector 112.
Is selected.

In this case, the number of bits of the difference data is smaller than the number of bits of the correction data in FIG. 5, so that the capacity of the RAM 102 in FIG. 9 can be smaller than that in FIG.

The number of reference straight lines Y 'set in the areas A and C is not limited to one, and a plurality of reference straight lines can be set. In this case, the area determination unit 110 determines which section the image data X belongs to using a different straight line, and based on the determination result, determines whether the third selector 1
At 30, data corresponding to the corresponding reference straight line may be selected in the same manner as described above.

(Explanation of Electronic Apparatus) An electronic apparatus using the liquid crystal display device of the above-described embodiment includes a display information output source 1000, a display information processing circuit 1002, a display drive circuit 1004, a liquid crystal panel shown in FIG. Display panel 10 such as
06, clock generation circuit 1008 and power supply circuit 1010
It is comprised including. The display information output source 1000 is RO
M, a memory such as a RAM, a tuning circuit for tuning and outputting a television signal, and the like.
Based on the clock from 008, display information such as a video signal is output. The display information processing circuit 1002 processes and outputs display information based on the clock from the clock generation circuit 1008. This display information processing circuit 1002
Is constituted by the data processing board 10 described above. The display driving circuit 1004 is a board for exclusive use of the liquid crystal display described above.
The liquid crystal panel 1006 includes a scanning side driving circuit and a data side driving circuit in addition to the 0R, 30G, and 30B.
Is driven for display. The power supply circuit 1010 supplies power to each of the above circuits.

As an electronic apparatus having such a configuration, FIG.
And a personal computer (PC) compatible with multimedia shown in FIG.

The liquid crystal projector shown in FIG. 17 is a projection type projector using a transmission type liquid crystal panel as a light valve, and uses, for example, a three-plate prism type optical system.

In FIG. 17, in a projector 1100, projection light emitted from a lamp unit 1102 of a white light source is divided into R, G, and B light by a plurality of mirrors 1106 and two dichroic mirrors 1108 inside a light guide 1104. Three liquid crystal panels 1110R, 1110 that are divided into primary colors and display images of each color
G and 1110B. The light modulated by each of the liquid crystal panels 1110R, 1110G and 1110B is applied to a dichroic prism 1112.
Is incident from three directions. Dichroic prism 11
In 12, the red R and blue B lights are bent by 90 ° and the green G light goes straight, so that the images of the respective colors are synthesized, and a color image is projected on a screen or the like through the projection lens 1114.

The personal computer 12 shown in FIG.
00 is a main body 1204 having a keyboard 1202
And a liquid crystal display screen 1206.

The present invention is not limited to the above embodiment, and various modifications can be made within the scope of the present invention.

[0083]

[Brief description of the drawings]

FIG. 1 is a block diagram of a data processing / liquid crystal display driving circuit for driving a liquid crystal display panel according to an embodiment in which the present invention is applied to a projector.

FIG. 2 is a characteristic diagram of primary gamma correction data stored in a RAM table of a primary gamma correction circuit.

FIG. 3 is a characteristic diagram for explaining primary gamma correction data including contrast ratio adjustment data rewritten in a RAM table of a primary gamma correction circuit.

FIG. 4 is a characteristic diagram for explaining primary gamma correction data including luminance adjustment data rewritten in a RAM table of a primary gamma correction circuit.

FIG. 5 is a block diagram of a liquid crystal display driving IC mounted on the liquid crystal exclusive board shown in FIG. 1;

6 is a characteristic diagram of secondary gamma correction data stored in a RAM table of the secondary gamma correction circuit shown in FIG.

FIG. 7 is a block diagram showing a modified example of the secondary gamma correction circuit of FIG. 5 for performing a linear approximation operation using one straight line.

FIG. 8 is a block diagram showing a configuration for rewriting data in a RAM table of the primary and secondary gamma correction circuits.

FIG. 9 is a block diagram showing a modified example of the secondary gamma correction circuit of FIG. 5 in which only difference data is stored in a RAM.

FIG. 10 is a characteristic diagram showing a secondary gamma correction characteristic used in the circuit of FIG. 9;

FIG. 11 is a characteristic diagram for explaining a relationship between difference data and a reference straight line by partially enlarging FIG. 10;

FIG. 12 is a timing chart showing the operation of the phase expansion circuit shown in FIGS. 1 and 5;

FIG. 13 shows the relationship between applied voltage and transmittance (T-
It is a characteristic view which shows V) characteristic.

14 is a characteristic diagram showing an ideal secondary gamma correction characteristic for compensating the TV characteristic of FIG.

FIG. 15 is a characteristic diagram showing an analog gamma correction characteristic using a conventional one-point broken linear approximation.

FIG. 16 is a block diagram of an electronic device of the present invention.

FIG. 17 is a schematic sectional view of a color projector which is an example of the electronic apparatus of the invention.

FIG. 18 is a schematic perspective view of a personal computer as an example of the electronic apparatus according to the invention.

[Explanation of symbols]

 Reference Signs List 10 signal processing board 12, 14 input terminal 16 AD converter 18 digital decoder 20 frame memory 22 switch 24 primary gamma correction circuit 24 RAM 30R, 30G, 30G liquid crystal display dedicated board 32 secondary gamma correction circuit 34 phase expansion circuit 36 polarity inversion Circuit 38 DA converter 40 Amplifier 42 Buffer 50R, 50G, 50B Liquid crystal display panel 100 Address generator 102 RAM (memory table) 104A to 104B Bit shifter 106 First selector 108 Adder (first arithmetic unit) 110 Area judging unit 112 2 selector 122 register 130 third selector 132 adder (second arithmetic unit)

Claims (6)

[Claims] 1. A digital gamma correction circuit for correcting input digital image data to digital image data suitable for display driving in a liquid crystal display unit, wherein a gradation value of the input digital image data is lower than a gradation value of a lower gradation region. 1 area, a second area on the high gradation area side,
An area determining unit that determines which of a third area and a second area belongs to the second area, and correction data corresponding to the input digital image data belonging to the first and second areas, A memory table from which correction data corresponding to the input digital image data is read, and correcting the input digital image data belonging to the third area by linear approximation according to at least one straight line having a predetermined inclination and offset A digital gamma correction circuit, comprising: a linear approximation calculation unit; 2. The method according to claim 1, wherein the linear approximation operation unit shifts the input digital image data by 2 ⁿ or 1/2 ⁿ (n
Is a natural number) times, a selector that selects and outputs at least one of the outputs of the plurality of bit shifters in accordance with the slope data of the at least one straight line, A first arithmetic unit for adding or subtracting straight line offset data; and a digital gamma correction circuit. 3. The straight-line approximation unit according to claim 2, wherein the straight-line approximation unit approximates a straight line for each of a plurality of straight-line approximation sections using a plurality of types of straight lines in the third area. Further includes a register for storing a plurality of inclination data and a plurality of offset data, wherein the area determination unit compares boundary data of each straight-line approximation section with input digital image data, and the register A digital gamma correction circuit for reading and controlling the inclination data and offset data of the corresponding linear approximation section. 4. The first and second gamma correction data according to claim 2, wherein the gamma correction data of the first and second regions are respectively located on at least one reference straight line.
Each of the reference straight line data in the region, and each of the difference data in the first and second regions to be added or subtracted from each of the at least one reference straight line data, The memory table, The difference data in each of the first and second regions is stored, and the straight-line approximation unit outputs each of the reference straight-line data in the first and second regions. The output difference data;
A digital gamma correction circuit further comprising a second calculator for adding or subtracting the reference straight line data output from the straight line approximation calculator. 5. A liquid crystal display, comprising: a digital gamma correction circuit according to claim 1; and driving an image on the liquid crystal display based on image data on which digital gamma correction has been performed. A liquid crystal display device comprising: a liquid crystal display driving circuit. 6. An electronic apparatus comprising: the liquid crystal display device according to claim 5; and a power supply circuit that supplies power to the liquid crystal display device.