JPH066820A - Gamma correction circuit for liquid crystal display device - Google Patents

Abstract

PURPOSE:To improve the contrast of a display picture on a liquid crystal display device by implementing white level expansion and black level expansion corresponding respectively to a high APL and a low APL. CONSTITUTION:In the gamma correction circuit in which gamma correction data are read out of a gamma correction memory based on a video signal (e.g. R, G, B signals) and the read data are used for display data to a liquid crystal display device, the gamma correction memory consists of 1st gamma correction memories 32r,... storing white level expansion and 2nd gamma correction memories 42r,... storing black level expansion, and an average level (APL) of a video signal (e.g. luminance signal) is detected by an APL detection circuit 40, a discrimination circuit 42 discriminates whether or not the detected level is larger than a set level and a selection means 50 selects either of the 1st gamma correction memories 32r,... or the 2nd gamma correction memories 34r,... based on the discrimination output.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is an improvement of a gamma correction circuit for correcting a display signal supplied to a liquid crystal display so that the input / output characteristics (for example, voltage-transmittance characteristic) of the liquid crystal display are in a proportional relationship. It is about.

[0002]

2. Description of the Related Art Generally, when a receiving side is a CRT (cathode ray tube) display, a video signal output from a TV camera or the like on the transmitting side is subjected to a gamma correction characteristic (eg, γ = -2) as shown in FIG. .2) is used to perform gamma correction, and the input / output characteristics of the CRT itself on the receiving side (eg, γ = 2.2) as shown in FIG.
And the corrected total characteristic is corrected so that the input / output characteristic has a proportional relationship (γ = 1) as shown in (c) of FIG. In FIG. 4A, Ai represents the input signal voltage corresponding to the brightness (brightness), and Ao represents the output signal voltage of the comma correction circuit on the transmission side.

However, when the display is a liquid crystal display, the input / output characteristics of the liquid crystal display itself are as shown in FIG. 6 (c) (for example, γ corresponds to about 3.5), and FIG. Since the input / output characteristics of the CRT display shown in b) are different, conventionally, by providing the receiving side with the gamma correction circuit 10 shown in FIG. 5, the gamma correction shown in FIG. Then, the corrected total characteristics are corrected so that the input / output characteristics have a proportional relationship (γ = 1) as shown in FIG. In FIG. 6, Bi is the input signal voltage to the gamma correction circuit 10, Bo
Is the output signal voltage, Ci and Di are the input signal voltage, Co
And Do represent the transmittance, and Vm represents the maximum value of Bi.

The gamma correction circuit 10 includes R, G, and B color signals (hereinafter simply referred to as R, G and B) obtained by color demodulating an analog video signal.
Input terminal 12 for inputting G and B signals)
r, 12g, 12b, and these input terminals 12r, 1
LPFs 14r, 14g, and 1 for 2g and 12b, respectively
4b, A / D (analog / digital) converter 16r, 1
6g, 16b, gamma correction memory (hereinafter simply referred to as γ correction memory) 18r, 18g, 18b, D / A (digital / analog) converter 20r, 20g, 20b, LP
F22r, 22g, 22b and output terminals 24r, 24
It is configured by sequentially connecting g and 24b.

The gamma correction memories 18r, 18g and 18b are
For example, it is formed in a table (list) format and stores gamma correction data that satisfies the gamma characteristic as shown in FIG. 6B using digital R, G, B signals as a kind of address. Then, the input terminals 12r, 12
The R, G, and B signals input to g and 12b are input to the LPF 14
A / D converters 16r, 16 via r, 14g, 14b
g, 16b to be converted into digital R, G, B signals, and based on the digital R, G, B signals, the corresponding gamma correction data is read from the γ correction memories 18r, 18g, 18b, and D / A converter 20r, 20g, 20b
Converted into analog signal with LPF 22r, 22g, 2
2b to output terminals 24r, 24g, 24b, and the R, G, B signals output from the output terminals 24r, 24g, 24b are supplied to the liquid crystal display for display.

[0006]

However, in the conventional gamma correction circuit 10 shown in FIG. 5, the APL (Average Picture Level) which is the average level of the video signal for a predetermined period (for example, one field of one screen display period).
Is higher or lower than the set range, the gamma correction data in the γ correction memories 18r, 18g, and 18b does not change, and the gamma characteristic is constant.
When the value is higher or lower than the set range, the variable range of the brightness level of the video signal becomes narrow, and the linear drive range of the liquid crystal display may be narrow, which causes a problem that the contrast of the display image on the liquid crystal display becomes low. .

The present invention has been made in view of the above-mentioned problems. Even if the APL of a video signal fluctuates to a higher or lower side,
An object of the present invention is to provide a gamma correction circuit for a liquid crystal display, which can extend the brightness level by widening the variable range of the brightness level of the video signal in the changed range and improve the contrast of the display image on the liquid crystal display. To do.

[0008]

According to the present invention, there is provided a gamma correction circuit for a liquid crystal display, wherein the gamma correction data is read from a gamma correction memory based on a video signal and used as display data for the liquid crystal display. The memory is formed by k (k is an integer of 2 or more) kinds of γ correction memories, and each of the k kinds of γ correction memories divides the entire range of the brightness level that the video signal can take into k individual ranges. A gamma correction data obtained by expanding the brightness level of the corresponding individual range at that time is stored, and an APL detection circuit for detecting an average level (APL) of the video signal for a predetermined period, and a detection value of the APL detection circuit are A determination circuit that determines which of the k individual ranges belongs to, and a corresponding γ complement of the k types of γ correction memories based on the determination output of the determination circuit. Those characterized by comprising comprises a selection means for selecting a memory.

[0009]

The APL detection circuit is an average level A for a predetermined period (for example, one screen display period of one field) of the video signal.
PL is detected, and the selecting means sets the γ correction memory (for example, the kth γ correction memory that extends the high brightness level) corresponding to the individual range (for example, the kth individual range having a high brightness level) to which the detected value belongs. select. Then, a γ correction memory (for example, the kth γ correction memory) based on the video signal
Since the gamma correction data obtained by expanding the corresponding luminance level (for example, white level) is read out and used as the display data on the liquid crystal display, even if the APL varies, the variable range of the luminance level of the video signal is within the varied range. Get wider

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a gamma correction circuit for a liquid crystal display according to the present invention will be described below with reference to FIGS.
1 to 3, the same parts as those in FIGS. 4 to 6 are designated by the same reference numerals. In FIG. 1, reference numerals 12r, 12g, 12b are input terminals for inputting R, G, B color signals (hereinafter simply referred to as R, G, B signals) obtained by color demodulating an analog video signal. Is. The input terminals 12r, 12g, 12
Each of b has LPFs 14r, 14g, 14b and A / D (analog / digital) converters 16r, 16
g and 16b are sequentially coupled, and the A / D converters 16r and 1
One of the selector switches 30 is provided on the output side of 6g and 16b.
The common terminals of r, 30g, and 30b are connected.

The changeover switches 30r, 30g, 30b
To each of the two individual terminals of the white level expansion first gamma correction memory (hereinafter simply referred to as the first γ correction memory) 32r, 32g, 32b and the second black level expansion second gamma correction memory.
The input sides of the gamma correction memories (hereinafter simply referred to as second γ correction memories) 34r, 34g, 34b are connected, and the output sides of the first γ correction memories 32r, 32g, 32b are connected to the other changeover switches 36r, 36g, 36b. One of the individual terminals is connected and the second γ correction memories 34r, 34g,
On the output side of 34b, the changeover switches 36r, 36g,
The other individual terminal of 36b is connected, and the changeover switch 3
The D / A converter 20 is connected to the common terminals of 6r, 36g, and 36b.
r, 20g, 20b, LPFs 22r, 22g, 22b and output terminals 24r, 24g, 24b are sequentially coupled.

The first γ correction memories 32r, 32g, 3
In 2b, gamma correction data that satisfies the gamma correction characteristics shown in FIG. 2B is stored in advance. The gamma correction characteristic shown in (b) of FIG.
When i is in the range of 0 to Vm / 2, the characteristics are the same as the conventional gamma correction characteristic shown in FIG. 6B, and the input signal voltage Bi is Vm.
In the range of / 2 to Vm, the proportional relationship of γ = 1 is established and configured.

The second γ correction memories 34r, 34g, 3
Gamma correction data that satisfies the gamma correction characteristics shown in FIG. 3B is stored in advance in 4b. The gamma correction characteristic shown in (b) of FIG.
When i is in the range of 0 to Vm / 2, a proportional relationship of γ = 1 is established, and when the input signal voltage Bi is in the range of Vm / 2 to Vm, as shown in FIG.
It is the same as the conventional gamma correction characteristic shown in (b), and the overall characteristic after correction is configured to be the black level expansion.

Reference numeral 38 denotes a luminance signal input terminal, and an APL detection circuit 40 for detecting an APL which is an average level of the luminance signal for a predetermined period (for example, one field of one screen display period) is coupled to the luminance signal input terminal 38. ing. The APL
A switch control circuit 44 is coupled to the output side of the detection circuit 40 via a determination circuit 42. The determination circuit 42 is A
The switch control circuit 44 is mainly configured by a comparator 48 that compares PL with a reference voltage set by a reference power supply 46.
Is the changeover switch 30r, 30g, 30b, 36
It is configured to control the switching of the movable pieces of r, 36g, and 36b.

The switch control circuit 44, changeover switches 30r, 30g, 30b and 36r, 36g, 36b.
Constitutes the selection means 50. This selection means 50
Is not limited to the above-mentioned configuration. For example, the determination circuit 42
When the determination output of H is H, the first γ correction memories 32r, 32
g and 32b may be enabled, and when L, the second γ correction memories 34r, 34g, and 34b may be enabled to make them readable and writable.

Next, the operation of the above embodiment will be described. (A) The APL detection circuit 40 detects the APL which is the average level of the luminance signal of the analog video signal for a predetermined period (for example, one screen display period of one field), and the determination circuit 4
2 determines whether the detected value APL is larger or smaller than the set value.

(B) The case where the average brightness of the screen during one screen display period is larger than the set value and whitish corresponds to the case where APL is larger than the set value. At this time, the output of the comparator 48 becomes the H level, Switch control circuit 44
Are changeover switches 30r, 30g, 30b and 36.
The movable pieces r, 36g, and 36b are attached to the first γ correction memory 32.
The first γ correction memories 32r, 32g, 32b are selected as the γ correction memories by connecting to the individual terminals on the r, 32g, 32b side.

Therefore, the gamma correction characteristic of the γ correction circuit on the receiving side is a characteristic in which the white level is extended, as shown in FIG. That is, the input signal voltage Bi corresponding to the R, G, and B signals of the analog video signal is 0 to Vm.
In the range of / 2 (the range of low brightness), as shown on the left side of FIG. 2D, the gamma correction characteristic is the same as that of the conventional example, and the corrected total characteristic has a proportional relationship of γ = 1. Although satisfied, in the range of the input signal voltage Bi from Vm / 2 to Vm (high brightness range), as shown on the right side of (d) of FIG.
The variable range of the output (transmittance) is wider than in the case of the proportional relationship of γ = 1 shown by the dotted line, and the white level is extended.

(C) When the average brightness of the screen during one screen display period is smaller than the set value and is dark, it corresponds to when the APL is smaller than the set value. At this time, the output of the comparator 48 becomes the L level, Switch control circuit 44
Are changeover switches 30r, 30g, 30b and 36.
The movable pieces of r, 36g, and 36b are attached to the second γ correction memory 34.
The second γ correction memories 34r, 34g, 34b are selected as the γ correction memories by connecting to the individual terminals on the r, 34g, 34b side.

Therefore, the gamma correction characteristic of the γ correction circuit on the receiving side is a characteristic in which the black level is extended, as shown in FIG. 3B. That is, the input signal voltage Bi corresponding to the R, G, and B signals of the analog video signal is 0 to Vm.
In the range of / 2 (the range of low luminance), the variable range of the output (transmittance) becomes wider than in the case of the proportional relationship of γ = 1 shown by the dotted line, as shown on the left side of FIG. In the range where the black level is extended and the input signal voltage Bi is Vm / 2 to Vm (high brightness range), the gamma correction characteristic is the same as that of the conventional example as shown on the right side of FIG. The corrected overall characteristic has a proportional relationship of γ = 1.

In the above embodiment, two types of γ correction memories are provided, a first γ correction memory that stores gamma correction data for white level expansion and a second γ correction memory that stores gamma correction data for black level expansion. It was formed with a correction memory,
The present invention is not limited to this, and the γ correction memory is formed by three or more types of γ correction memories, and each of these γ correction memories has an individual range of the brightness level that the video signal can take. The gamma correction data obtained by expanding the brightness level of the corresponding individual range when divided into ranges may be stored.

In the above embodiment, the video signal is the received signal of the video signal which has been gamma-corrected for the CRT display in advance on the transmitting side, and the gamma-correction data is read from the γ-correction memory based on the received video signal. The present invention is not limited to this, and it is also possible to use a video signal that is not gamma-corrected on the transmission side as a reception signal and read gamma-correction data from the γ-correction memory based on the received video signal. can do.

In the above embodiment, the case where the present invention is applied to the gamma correction circuit of the liquid crystal display for color display has been described, but the present invention is not limited to this, and the gamma correction circuit of the liquid crystal display for monochrome display is not limited to this. Of course, you can also use

[0024]

As described above, the gamma correction circuit of the liquid crystal display according to the present invention uses the γ correction memory for k types of γ correction memories.
Gamma correction data that is formed by a correction memory and that extends the brightness level of the individual range corresponding to each of the γ correction memories of this type is stored, the APL of the video signal is detected, and the individual range to which the detected value belongs (for example, brightness A γ correction memory (for example, the kth γ correction memory in which the high brightness level is extended) corresponding to the high level kth individual range) is selected, and the γ correction memory (for example, the kth correction memory) selected based on the video signal is selected. The gamma correction data obtained by expanding the corresponding luminance level (for example, the white level) is read from the (th gamma correction memory) and used as the display data on the liquid crystal display. In, the variable range of the brightness level of the video signal can be widened. Therefore, when the displayed image is bright, the white level is extended, and when it is dark, the black level is extended to improve the contrast.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of a gamma correction circuit of a liquid crystal display according to the present invention.

FIG. 2 is an explanatory diagram illustrating gamma correction and white level expansion when APL is larger than a set value in FIG.

FIG. 3 is an explanatory diagram illustrating gamma correction and black level expansion when APL is smaller than a set value in FIG.

FIG. 4 is an explanatory diagram illustrating gamma correction in the case of a CRT display.

FIG. 5 is a schematic configuration diagram showing a gamma correction circuit of a liquid crystal light valve in a conventional example.

FIG. 6 is an explanatory diagram illustrating gamma correction of FIG.

[Explanation of symbols]

12r, 12g, 12b ... Input terminal, 24r, 24
g, 24b ... Output terminal, 32r, 32g, 32b ... First gamma correction memory for white level expansion, 34r, 34g,
34b ... Second gamma correction memory for expanding black level, 38
... Luminance signal input terminal, 40 ... APL detection circuit, 42
... Judgment circuit, 44 ... Switch control circuit, 48 ... Comparator, 50 ... Selection means.

Claims (3)

[Claims] 1. A gamma correction circuit for a liquid crystal display, wherein the gamma correction data is read from the gamma correction memory based on a video signal and is used as display data for a liquid crystal display. Integer number) of the gamma correction memories, and each of the k kinds of gamma correction memories has a corresponding individual range when the entire range of luminance levels that the video signal can take is divided into k individual ranges. An APL detection circuit that stores gamma correction data in which the brightness level is expanded and detects an average level (APL) of the video signal for a predetermined period, and a detection value of the APL detection circuit is one of the k individual ranges. And a corresponding gamma correction memory of the k kinds of gamma correction memories based on the judgment output of the judgment circuit. Gamma correction circuit for a liquid crystal display characterized by comprising comprises a selecting means for selecting the positive memory. 2. A gamma correction memory comprising two types of gamma correction: a first gamma correction memory storing gamma correction data for white level expansion and a second gamma correction memory storing gamma correction data for black level expansion. The determination circuit is formed by a memory, and the determination circuit determines whether the detection value of the APL detection circuit is larger or smaller than the value in the set range, and the selection means determines the first and second gamma corrections based on the determination output of the determination circuit. The gamma correction circuit for a liquid crystal display according to claim 1, wherein one of the memories is selected. 3. A video signal is gamma-corrected in advance on the transmission side to become a video signal, and gamma correction data in the first gamma correction memory is a gamma on the transmission side for video signals whose video signal is below a set range value. The gamma correction data for properly correcting the correction is used as gamma correction data for excessively correcting the gamma correction on the transmitting side when the video signal is larger than the set range value. The data is gamma correction data for properly correcting the gamma correction on the transmitting side when the video signal is equal to or larger than the setting range value, and the transmitting side is set when the video signal is smaller than the setting range value. The gamma correction circuit for a liquid crystal display according to claim 2, wherein the gamma correction data is used as gamma correction data for excessively correcting the gamma correction.