JP3307848B2 - Gamma correction circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gamma correction circuit in a video signal processing circuit when displaying a video signal using a liquid crystal display device.

[0002]

2. Description of the Related Art A liquid crystal display device performs display by changing the voltage applied to liquid crystal and controlling the amount of transmission of backlight light disposed behind a liquid crystal sealing plate. The conventional gamma correction circuit is suitable for correction of a cathode ray tube, and since the luminance of the cathode ray tube is proportional to the anode current, the circuit has the signal voltage-anode current characteristic of the cathode ray tube shown in FIG.
To correct this characteristic, the input signal voltage-output signal voltage characteristic shown in FIG. 7 is required. FIG. 5 shows a typical circuit for this purpose, and FIG. 8 shows an input signal voltage-output signal voltage characteristic of the circuit in FIG.

In FIG. 5, 1 is a positive power supply terminal, 2 is a negative power supply terminal, 3 is an input terminal of a first voltage signal (video signal before correction) in a conventional gamma correction circuit, and 4 is a conventional gamma correction circuit. Is an output terminal of the second voltage signal (corrected video signal). 31 and 36 are transistors, 3
Reference numerals 4 and 35 denote constant current sources, 37 denotes a constant voltage source for generating a constant voltage V ₀ , and 11 denotes a resistor having a resistance value R ₁ . The transistors 31 and 36, the constant current sources 34 and 35, the constant voltage source 37, and the resistor 11 constitute a differential amplifier 30.

Reference numeral 21 denotes a resistor having a resistance value R ₂ , and 22 denotes a resistance value R
₃ resistance. 23 and 25 transistor 24 is a constant current source, 26 is a constant voltage source for generating a constant voltage V _1.
The resistors 21 and 22, the transistors 23 and 25, the constant current source 24, and the constant voltage source 26 form an impedance circuit 20 having voltage dependency.
The load is 0.

Next, the operation of the gamma correction circuit will be described. When a first voltage signal (a video signal before correction) is input from the input terminal 3 to the differential amplifier 30, the difference voltage (V) between the first voltage signal V _IN and the constant voltage V ₀ of the constant voltage source 37 is output. _IN− V ₀ ) is converted into a current by the resistor 11 having the resistance value R ₁ and sent to the impedance circuit 20 as a collector current of the transistor 36. This output current is converted into a second voltage signal (corrected video signal) V _OUT by the impedance circuit 20 having voltage dependency.

In the impedance circuit 20, a transformer
The base voltage of the transistor 23 is V₁-V_BE(Transistor
25 base-emitter voltage), the output terminal
Voltage, that is, the second voltage signal V_OUTAnd the constant voltage source 26
Voltage V₁And V_OUT<V₁Under the conditions of Transis
23 is cut off and the impedance circuit 2
The impedance of 0 is represented by the resistor 21 only, and R_TwoTona
You. Next, the second voltage signal V_OUTAnd the constant voltage source 26
Voltage V₁And V_OUT> V₁Then, the transistor 2
3 is turned on, and the emitter terminal of the transistor 23 is
The child will be AC grounded. In other words, Impey
The impedance of the dance circuit 20 is
Expressed in parallel, (R_Two× R_Three) / (R_Two+ R_Three)When
Becomes V _OUT<V₁Impedance is lower than when
I do.

As described above, the differential amplifier 30 and the impedance
When connected to the dance circuit 20, the folding circuit shown in FIG.
Line characteristics. That is, input / output gain
Is the output signal voltage, that is, the second voltage signal V_OUTIs constant voltage
V₁In the lower region (A), R _Two/ R₁And the second
Voltage signal V_OUTIs constant voltage V₁In the higher area (B)
(R_Two× R_Three) / ｛R₁× (R_Two+ R_Three)｝
The input / output gain of the area (B) is the input / output gain of the area (A).
R for_Three/ (R_Two+ R_Three) Will be compressed twice
You.

In the above description, a circuit having only one break point is shown in FIG. 5, and the description has been made based on the circuit.
In order to bring the characteristic closer to that shown in FIG.
It is sufficient to connect a plurality of impedance circuits 20 composed of the elements No. to No. 26 to the resistor 21 in parallel. In this way, a characteristic having a plurality of break points can be obtained, and the characteristic can be made closer to the characteristic shown in FIG.

[0009]

However, in the liquid crystal display device, since the transmittance of the liquid crystal is a characteristic as shown in FIG. 2, the gamma correction characteristic needs to be substantially as shown in FIG. Therefore, the gamma correction characteristics for the conventional CRT display device cannot be said to be sufficient. Therefore, an object of the present invention is to provide a gamma correction circuit capable of obtaining a gamma correction characteristic suitable for a liquid crystal display device.

[0010]

In order to solve this problem, a gamma correction circuit according to the first aspect of the present invention comprises a first gamma correction circuit.
A first transistor having electrical characteristics and first and second resistors;
Having a first predetermined value at the base of the first transistor.
Between the emitter of the first transistor and ground.
And a first impedance circuit in which a second resistor is connected in cascade, and a first voltage signal input from the outside
Having a second transistor of a given second conductivity characteristic
The emitter of the second transistor is connected to the first and second
Connect to the resistor connection to connect to the first impedance circuit
A flowing current signal is output from the collector of the second transistor.
A voltage-current conversion circuit for power, a third tiger first conductive properties
A third transistor having a transistor and third and fourth resistors.
A second predetermined value is given to the base of the transistor, and a third
Third and fourth resistors between the emitter of the transistor and ground
A second impedance circuit connected in cascade ,
A signal is applied to the connection of the third and fourth resistors to provide a second power supply.
A pressure signal is generated and output.

According to this configuration, the first impedance circuit for converting the externally input first voltage signal into a current signal has the first voltage dependency, and the current signal is converted into the second voltage signal. Since the second impedance circuit for conversion has the second voltage dependency, by setting the first and second voltage dependencies to inverse characteristics, the input / output characteristics, that is, the second voltage dependency with respect to the first voltage signal is obtained. In the characteristic of the voltage signal of No. 2, a concave portion and a convex portion are provided, and by appropriately setting the positions and angles of the concave portion and the convex portion, the signal voltage-transmission of the liquid crystal display device is obtained. Abbreviation of rate characteristic S
It is possible to create a substantially inverted S-shaped characteristic for correcting the character characteristic. As a result, gamma correction characteristics suitable for a liquid crystal display device can be obtained.

According to a second aspect of the present invention, there is provided a gamma correction circuit according to the first aspect.
When the voltage signal exceeds a first predetermined value, the first impedance circuit has a first voltage dependency in which the impedance decreases, and when the second voltage signal exceeds a second predetermined value. A second voltage dependency for decreasing the impedance is given to the second impedance circuit;
The first and second predetermined values are set so that the second voltage signal becomes higher than the second predetermined value when the first voltage signal becomes the first predetermined value.

According to this configuration, the first voltage dependency is set to a characteristic that the impedance decreases when the first voltage signal exceeds the first predetermined value, and the second voltage dependency is set to the second voltage dependency. The impedance is reduced when the voltage signal exceeds a second predetermined value, and the second voltage signal becomes higher than the second predetermined value when the first voltage signal becomes the first predetermined value. Since the first and second predetermined values are set as described above, a concave portion and a convex portion are provided in the characteristic of the second voltage signal with respect to the first voltage signal, and the signal voltage of the liquid crystal display device is increased. A substantially inverse S for correcting a substantially S-shaped characteristic of the transmittance characteristic;
Character characteristics can be created. As a result, gamma correction characteristics suitable for a liquid crystal display device can be obtained.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a circuit diagram of a gamma correction circuit according to an embodiment of the present invention. The gamma correction circuit according to this embodiment is suitable for correction of a liquid crystal display device.
FIG. 4 shows the output signal voltage characteristics.

In FIG. 1, 1 is a positive power supply terminal, 2 is a negative power supply terminal, and 3 is a first gamma correction circuit in the gamma correction circuit of the embodiment.
The input terminal 4 for the voltage signal (video signal before correction) is the output terminal for the second voltage signal (video signal after correction) in the gamma correction circuit of the embodiment. 11 of the resistance the resistance value R _1, 12 is the resistance of the resistance value R _4. 13 and 15 transistor 14 is a constant current source, 16 is a constant voltage source for generating a constant voltage V _2. These resistors 11, 12, transistors 13, 15, constant current source 14, and constant voltage source 16
Are connected to the input terminal 3 to constitute a first impedance circuit 10 having a first voltage dependency.

Reference numerals 31 to 34 denote transistors, and reference numeral 35 denotes a constant current source. When these transistors 31 to 34 and the constant current source 35 apply a first voltage signal V _IN input from the outside to the first impedance circuit 10,
A voltage-current conversion circuit 30 for extracting a current signal flowing through the impedance circuit 10 of FIG. 21 is the resistance value R
₂ of the resistor 22 is the resistance of the resistance value R _3, 23, 25 transistors, 24 is a constant current source, 26 is a constant voltage source for generating a constant voltage V _1. These resistors 21 and 22, transistors 23 and 25, constant current source 24 and constant voltage source 26
A second impedance circuit 2 having a second voltage dependency and outputting a second voltage signal V _OUT corresponding to the current signal when the current signal is supplied from the voltage-current conversion circuit 30.
0.

In this case, when the first voltage signal V _IN exceeds a first predetermined value (constant voltage V ₂ ), the first voltage dependency, in which the impedance decreases, is set to the first impedance circuit 1.
0, and the second impedance circuit 20 has a second voltage dependency in which the impedance decreases when the second voltage signal V _OUT exceeds a second predetermined value (constant voltage V ₁ ), and When the first voltage signal V _IN becomes the first predetermined value (constant voltage V ₂ ), the first voltage signal V _OUT becomes higher than the second predetermined value (constant voltage V ₁ ). And a second predetermined value (constant voltages V ₁ , V ₂ ).

According to this configuration, the first impedance circuit 10 for converting the externally input first voltage signal V _IN into a current signal has the first voltage dependency, and the current signal is converted to the second signal. Since the second impedance circuit 20 for converting to the voltage signal V _OUT has the second voltage dependency, by setting the first and second voltage dependencies to inverse characteristics, the input / output characteristics, that is, the second A concave portion and a convex portion are provided in the characteristics of the second voltage signal V _OUT with respect to the first voltage signal V _IN , and the positions and angles of the concave portion and the convex portion are appropriately set. Is used to correct a substantially S-shaped characteristic of the signal voltage-transmittance characteristic of the liquid crystal display device.
Character characteristics can be created. As a result, gamma correction characteristics suitable for a liquid crystal display device can be obtained.

More specifically, the first voltage dependency is a characteristic in which the impedance decreases when the first voltage signal V _IN exceeds a first predetermined value (constant voltage V ₂ ). 2, the impedance is reduced when the second voltage signal V _OUT exceeds a second predetermined value (constant voltage V ₁ ), and the first voltage signal V _IN is a first predetermined value. (constant voltage V ₂₎ a second voltage signal when a V _OUT is the second predetermined value so as to be higher than the (constant voltage V ₁₎ first and second predetermined value (constant voltage V _1, V ₂ ), the _second voltage signal V _{IN with} respect to the first voltage signal V _{IN
Since the} concave portion and the convex portion are provided in the _OUT characteristic, it is possible to create a substantially inverted S-shaped characteristic for correcting the substantially S-shaped characteristic of the signal voltage-transmittance characteristic of the liquid crystal display device. As a result, gamma correction characteristics suitable for a liquid crystal display device can be obtained.

That is, the first voltage-dependent impedance circuit 10 is used as a current conversion element of the voltage / current conversion circuit 30, and the input voltage signal V _IN is converted into a current signal, and the converted current signal is converted. Is supplied to a second voltage-dependent impedance circuit 20 and the voltage signal V
When converting to _OUT , the first impedance circuit 10
If the impedance of I / O decreases, the input / output gain increases,
If the impedance of the second impedance circuit 20 decreases, the input / output gain decreases. Therefore, by setting the first and second impedance change points respectively, the video signal required for the gamma correction circuit of the liquid crystal display device can be adjusted. A gamma correction characteristic for extending the gains on the black side and the white side is obtained.

The operation of the gamma correction circuit configured as described above will be described below with reference to FIGS. In the first impedance circuit 10, the transistor 15 forms an emitter follower circuit by the constant current source 14, and the base voltage of the transistor 13 becomes V ₂ −V _BE (base-emitter voltage of the transistor 15). First
Input voltage of the impedance circuit 10 (transistor 3
1 emitter voltage) in the case of the constant voltage V ₂ or less, since the transistor 13 is cut off, the impedance of the first impedance circuit 10 becomes R _1.

Further, the first voltage signal exceeds the constant voltage V _2, the transistor 13 is turned on operation, the emitter of the transistor 13 is AC-grounded. Therefore,
The impedance of the first impedance circuit 10 is R
₁ and R ₂ are connected in parallel, and R ₁ × R ₄ / (R ₁ +
R ₄ ). Also, the second impedance circuit 2
0 can be similarly considered, and the input voltage (=
The second voltage signal V _OUT which is the output voltage of the gamma correction circuit)
There is lower than the constant voltages V _1, the impedance is R ₂
When the voltage is higher than the constant voltage V ₁ , R ₂ × R ₃ /
It is represented by (R ₂ + R ₃ ). That is, it can be said that the impedances of the first and second impedance circuits 10 and 20 have voltage dependency.

Next, the operation of the voltage / current conversion circuit 30 will be described.
Will be described. Input from input terminal 3 of gamma correction circuit
Generated first voltage signal V_INAre transistors 34 and
Via an emitter follower circuit composed of a constant current source 35
Then, it is input to the base terminal of the transistor 31. Tiger
Base-emitter voltage V of transistors 34 and 31 _BE
Are considered to be substantially equal to each other.
The transmitter voltage is a first voltage signal V which is an input voltage._INEqual
I can say that. The emitter current of the transistor 31 is the first
The value of the current flowing through the impedance circuit 10 becomes
The current value is the input voltage, that is, the first voltage signal V_INAnd proportionality
In charge. In addition, the current amplification factor of the transistor 31 is sufficient.
When the current is high, the emitter current is equal to the collector current.
The current flowing through the first impedance circuit 10 is a transistor.
Current mirror circuit composed of
Is supplied to the second impedance circuit 20 and the second
Voltage signal V_OUTWill be returned to. That is, the first
Voltage signal V_INAnd the second voltage signal V_OUTIn between, the first
And impedance of second impedance circuits 10 and 20
To Z_IN, Z_OUT, Then V_OUT= (Z_OUT/ Z
_IN) × V_INIs established.

FIG. 4 shows the input / output characteristics of the gamma correction circuit of FIG.
Shown in In FIG. 1, each area of (A), (B), and (C)
The area will be described. Region (A) is V_OUT<V₁so
And V_IN<V_TwoIs the area where the condition of
The impedance of the impedance circuit 10 is Z_IN= R₁,
The impedance of the second impedance circuit 20 is Z
_OUT= R_TwoIt is. Therefore, the input / output gain is R_Two/
R₁It is.

Region (B) is where V _OUT > V ₁ and V _IN <
This is the area where the condition of V ₂ is satisfied, Z _IN = R ₁ , Z _OUT
= R ₂ × R ₃ / (R ₂ + R ₃ ). Therefore, the input / output gain is R ₂ × R ₃ / {R ₁ × (R ₂ + R ₃ )}, and is compressed R ₃ / (R ₂ + R ₃ ) times as compared with the area (A). . The region (C) is a region where the conditions of V _OUT > V ₁ and V _IN > V ₂ are satisfied, and Z _IN = R
₁ × R ₄ / (R ₁ + R ₄ ), Z _OUT = R ₂ × R ₃ / (R
₂ + R ₃ ). Therefore, the input / output gain is R ₂
× R ₃ × (R ₁ + R ₄ ) / ｛R ₁ × R ₄ × (R ₂ +
R ₃ )｝, and (R ₁ + R ₄ ) in comparison with the region (B).
/ R will be amplified by a factor of ₄ .

As described above, according to the embodiment, the first and second voltages having the first and second voltage dependencies on both the input side and the output side of the current-voltage conversion circuit 30 are provided. By providing the impedance circuits 10 and 20, the extension characteristics can be obtained in the regions (A) and (C) with respect to the region (B).
A gamma correction characteristic of a substantially inverted S-character characteristic for correcting the character characteristic can be obtained.

FIG. 1 shows a gamma correction circuit in which one input-side and one output-side impedance circuit having voltage dependency are connected to the input side and the output side, respectively. In order to make the impedance close to the above, a plurality of impedance circuits having different characteristics may be connected to the input side, and a plurality of impedance circuits having different characteristics may be connected to the output side. In this way, by connecting a plurality of impedance circuits having voltage dependency, it becomes possible to approximate an ideal gamma correction characteristic.

[0028]

As described above, according to the gamma correction circuit of the present invention, the first impedance circuit having the first voltage dependency is provided on the input side of the current-voltage conversion circuit, and the first impedance circuit is similarly provided on the output side. Since the second impedance circuit having the voltage dependency of 2 is provided, gamma correction characteristics suitable for a liquid crystal display device can be obtained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram illustrating a configuration of a gamma correction circuit according to an embodiment of the present invention.

FIG. 2 is a characteristic diagram of transmittance versus signal voltage of liquid crystal in a liquid crystal display device.

FIG. 3 is an input signal voltage-output signal voltage characteristic diagram desired for a gamma correction circuit for a liquid crystal display device.

FIG. 4 shows an input signal voltage in the gamma correction circuit of FIG. 1;
FIG. 4 is an output signal voltage characteristic diagram.

FIG. 5 is a circuit diagram showing a configuration of an example of a conventional gamma correction circuit for a CRT display device.

FIG. 6 is an anode current characteristic diagram of a cathode ray tube.

FIG. 7 is an input signal voltage-output signal voltage characteristic diagram for a CRT display device.

8 shows an input signal voltage in the gamma correction circuit of FIG.
FIG. 4 is an output signal voltage characteristic diagram.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Positive power supply terminal 2 Negative power supply terminal 3 Input terminal 4 Output terminal 10 First impedance circuit 20 Second impedance circuit 30 Voltage-current conversion circuit 11,12,21,22 Resistance 13,15,23,25,31-33 , 34 Transistor 14, 24, 35 Constant current source 16, 26 Constant voltage source

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yuji Amano 1006 Kazuma Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-5-37822 (JP, A) JP-A-5- 264960 (JP, A) JP-A-4-351072 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04N 5/202 G09G 3/36 H04N 5/66 102

Claims (2)

(57) [Claims] A first transistor having a first conductive property and a second transistor having a first conductive property;
A first transistor having a first resistor and a second resistor.
Giving a first predetermined value to the base of the first
The first and second resistors are connected between the emitter of the star and the ground.
A first impedance circuit connected in cascade and a first voltage signal input from the outside are applied to a base.
A second transistor having a second conductive characteristic,
The emitters of the two transistors are connected to the first and second resistors.
A first impedance circuit connected to a first connection
A current signal flowing to the collector of the second transistor.
, A voltage-current conversion circuit, a third transistor having a first conductive characteristic, and a third and a fourth transistor.
Having a resistor and a third transistor connected to the base of the third transistor.
2 and the emitter of the third transistor
And the third and fourth resistors are cascaded between the ground and the ground.
And a second impedance circuit, given the current signal to the connecting portion of the third and fourth resistor
And generating and outputting a second voltage signal.
That gamma correction circuit. 2. A method according to claim 1, wherein when the first voltage signal exceeds a first predetermined value, the first impedance circuit has a first voltage dependency of decreasing impedance when the first voltage signal exceeds a first predetermined value. A second voltage dependency, in which the impedance is reduced when the value exceeds a predetermined value, is given to a second impedance circuit, and the second voltage signal when the first voltage signal becomes a first predetermined value 2. The gamma correction circuit according to claim 1, wherein the first and second predetermined values are set so that the first and second values are higher than the second predetermined value.