JPH06253178A - Gamma correction circuit - Google Patents

Abstract

(57) [Summary] [Purpose] To realize a gamma correction circuit with improved frequency characteristics by eliminating the presence of a capacitance such as a PN junction in the signal path. [Structure] A video signal (V
in) is input to the gamma correction circuit, and a clipping circuit (16, 17, 18, 19) for removing a portion of the input signal equal to or lower than a predetermined voltage, and a clip circuit for clipping the signal corresponding to the input video signal. The subtraction circuit (7, 8,
9, 10, 14, 15). Also, a signal inverting circuit (26, 27, 28, 29) for inverting an input video signal to generate an inverted video signal, and a clipping circuit (40, 4) for removing a portion of the inverted video below a predetermined voltage.
1, 42, 43) and the inverted video signal and the signal output from the clipping circuit (33,
34, 35, 36, 44).

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, and more particularly to a technique capable of performing gamma correction on an input video signal without degrading frequency characteristics.

[0002]

2. Description of the Related Art In a television camera or the like, a gamma correction circuit is used to correct the non-linearity of a video signal obtained from an image pickup device. FIG. 5 shows a configuration of a gamma correction circuit which has been conventionally used. The gamma correction circuit in the figure includes resistors 20, 21, 23, 25 and a diode 2.
2 and a transistor 24.

One end of the resistor 20 is connected to the input terminal to which the input video signal Vin is applied, and the other end is connected to the resistors 21 and 2.
3 and the node N1 are connected together. Resistance 2
The other end of 1 is a voltage V of a predetermined potential via the diode 22.
Connected to 3. The other end of the resistor 23 is the transistor 2
4, the collector of the transistor 24 is connected to the positive power supply voltage Vcc, and the emitter of the transistor 24 is connected to the resistor 25.
Is connected to the negative power supply voltage -Vcc via. The emitter of the transistor 24 has a gamma-corrected video signal V
It is connected to the output terminal for taking out out.

In the gamma correction circuit of FIG. 5, the input video signal Vin is divided by a circuit including the resistors 20 and 21 and the diode 22, and the output video signal Vout is obtained from the node N1 via the resistor 23 and the transistor 24. . The transistor 24 and the resistor 25 constitute a so-called emitter follower circuit, and the node N
It serves as an output buffer for one video signal. Since the diode 22 has non-linearity, the resistors 20, 21
The voltage dividing circuit constituted by the diode 22 and the diode 22 becomes a voltage dividing circuit having a non-linear input / output characteristic, whereby a gamma-corrected video signal is obtained at the node N1, and this video signal is supplied to the resistor 23 and the transistor 24. Is output via.

[0005]

However, in such a conventional gamma correction circuit, the P of the diode 22 is
Due to the capacitance of the N-junction, the frequency characteristic of the voltage dividing circuit composed of the resistors 20 and 21 and the diode 22 deteriorates
Therefore, there is an inconvenience that the frequency characteristic of the output video signal Vout is not good.

In view of the above problems in the conventional circuit, an object of the present invention is to provide a gamma correction circuit capable of appropriately performing gamma correction on an input video signal without deteriorating the frequency characteristic. To do.

[0007]

In order to achieve the above object, according to a first aspect of the present invention, there is provided a gamma correction circuit having a nonlinear input / output characteristic and to which a video signal is input. The gamma correction circuit subtracts a clip circuit that removes a portion of the video signal having a voltage equal to or lower than a predetermined voltage, and a signal from the signal corresponding to the input video signal, the signal having a portion of the video signal having a voltage equal to or lower than the predetermined voltage removed. Includes a subtraction circuit.

According to another aspect of the present invention, there is provided a gamma correction circuit which has a non-linear input / output characteristic and to which a video signal is input, and the gamma correction circuit outputs the input video signal. A signal inversion circuit for inverting and generating an inversion video signal, a clip circuit for removing a portion of the inversion video signal generated by the signal inversion circuit which is equal to or lower than a predetermined voltage, and a signal corresponding to the inversion video signal. And a summing circuit for summing the signals from which the portions below the predetermined voltage have been removed by the clipping circuit.

Further, in these gamma correction circuits, each of the clipping circuits has a first transistor and a second transistor whose emitter and collector are connected to each other, and the base of the first transistor receives an input video signal or the above-mentioned video signal. Advantageously, an inverted video signal is applied, a predetermined bias voltage is applied to the base of the second transistor, and the clipped signal is obtained from the commonly connected emitters.

[0010]

In the above gamma correction circuit, a clipping circuit, a subtraction circuit, an addition circuit and the like are used in order to realize a non-linear input / output characteristic for gamma correction. Further, in these circuits, unlike the conventional gamma correction circuit, the capacitance due to the PN junction does not exist between the signal path and the ground, so that it is possible to prevent the frequency characteristic from being deteriorated as in the conventional case. For example, the clipping circuit includes two transistors each having an emitter and a collector connected to each other as described above, and an input video signal is applied to the base of the first transistor to be clipped from the commonly connected emitters. Since a signal is obtained, an operation equivalent to that of the emitter follower circuit is performed on the video signal, and the frequency characteristic is not deteriorated.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of a gamma correction circuit according to an embodiment of the present invention. The circuit shown in FIG. 1 includes a clamp circuit including resistors 1 and 2 and a transistor 3, an emitter follower circuit including resistors 4 and 6 and a transistor 5, a resistor 7,
A subtraction circuit including 8, 10, 14, 15 and the transistor 9, an output buffer emitter follower including the resistors 11, 13 and the transistor 12, a black clip circuit including the resistors 16, 19 and the transistors 17, 18. Composed by.

In the above structure, the clamp circuit including the resistors 1 and 2 and the transistor 3 is well known,
A clamp pulse is applied to one end of the resistor 1 and the other end is connected to the base of the transistor 3. The resistor 2 is connected between the base and the emitter of the transistor 3. A predetermined bias voltage V1 is applied to the emitter of the transistor 3, and the collector of the transistor 3 receives the input video signal Vi.
It is connected to the input line to which n is applied. The input line is also connected to the base of the transistor 5 via the resistor 4. The emitter of the transistor 5 is connected to the negative power supply voltage -Vcc via the resistor 6.

The emitter of the transistor 5 is a resistor 1
It is connected to the base of the transistor 17 via 6. The collectors of transistors 17 and 18 are both connected to positive power supply voltage Vcc. Transistor 17
The commonly connected emitters of and 18 are connected via resistor 19 to the negative supply voltage -Vcc, and also resistor 1
It is connected to the base of the transistor 9 via 4.

The base of the transistor 9 further has a resistor 15
Is connected to ground via. The emitter of the transistor 9 is connected to the emitter of the transistor 5 via the resistor 7, and is connected to the negative power supply voltage -Vcc via the resistor 8. The collector of the transistor 9 is connected to the positive power supply voltage Vcc via the resistor 10 and further connected to the base of the transistor 12 via the resistor 11. The emitter of the transistor 12 is a resistor 1
3 is connected to the negative power supply voltage −Vcc and is also connected to an output terminal for supplying the output video signal Vout.
The collector of the transistor 12 is connected to the positive power supply voltage Vcc.

The operation of the gamma correction circuit thus constructed will be described with reference to FIG. The input video signal Vin is clamped to a predetermined DC potential by a well-known clamp circuit composed of the transistor 3 and the like, and is output from the emitter of the transistor 5 via the resistor 4. FIG. 2A shows an example of the signal of the emitter of such a transistor 5. When this signal is input to the black clip circuit including the transistors 17 and 18, a signal obtained by slicing the lower side of the signal of FIG. 2A as shown in FIG. 2B is obtained. The slice level of such a signal is the bias voltage V applied to the base of the transistor 18.
It can be set to a desired value by 2.

The signal of the emitter of the transistor 5, that is, the signal of FIG. 2 (a), is input to the emitter of the transistor 9 through the resistor 7, while the output signal from the black clip circuit, FIG. 2 (b). Signal is input to the base of the transistor 9 via the resistor 14. by this,
2C is obtained by subtracting the signal of FIG. 2A from the signal of FIG. 2A from the collector of the transistor 9, and this signal is output via the resistor 11 and the transistor 12. It is output as the signal Vout. This signal is a gamma-corrected signal having non-linearity according to the level of the input signal, as is apparent from FIG.

FIG. 3 shows the configuration of a gamma correction circuit according to the second embodiment of the present invention. The circuit shown in FIG.
28, 29 and the transistor 27 are included, and the input video signal V
signal inversion circuit for inverting in and resistors 30, 32
And an emitter follower circuit including a transistor 31, an adder circuit including resistors 33, 35, 36 and 44 and a transistor 34, resistors 40 and 43, and transistors 41 and 4
2 and a black clip circuit including 2 and an emitter follower circuit for an output buffer including resistors 37 and 39 and a transistor 38.

In the above structure, the black clip circuit including the resistors 40 and 43 and the transistors 41 and 42 is the same as that shown in FIG. In addition, resistors 33, 35, 36,
The adder circuit including the transistor 44 and the transistor 34 is configured to supply two signals, which are respectively input via the resistors 33 and 44, to the emitter of the transistor 34 and add the signals.

The operation of the circuit of FIG. 3 will be described with reference to FIG. FIG. 4A shows the input video signal Vin, and the input video signal Vin passes through the resistor 26 and the transistor 27.
Is inverted by the transistor 27, and an inverted video signal as shown in FIG. 4B is obtained at the collector of the transistor 27.

This inverted video signal is applied to the base of the transistor 41 via the resistor 30 and the transistor 31, and further via the resistor 40. The emitter of the transistor 41 is connected to the emitter of the transistor 42,
A predetermined bias voltage V is applied to the base of the transistor 42.
Since 2 is applied, a signal obtained by slicing a portion below a predetermined level is obtained at the emitter of the transistor 41 as shown in FIG. This signal is resistor 44
Is input to the emitter of the transistor 34 via. Further, a signal inverted by the signal inverting circuit and applied through the emitter of the transistor 31, that is, (d) of FIG. 4, is input to the emitter of the transistor 34. Therefore, these signals are added and the transistor 34
A gamma-corrected signal as shown in FIG. 4 (e) is obtained at the collector of, and this signal is output as the output video signal Vout via the resistor 37 and the transistor 38.

..

As described above, according to the present invention, in the gamma correction circuit, the capacitance due to the PN junction or the like does not exist between the signal path and the ground or the power supply, so that the deterioration of the frequency characteristic of the circuit is prevented. A broadband circuit can be realized. Further, according to the second embodiment of FIG. 3, it is possible to perform gamma correction on a video signal having a negative waveform obtained by inverting the positive waveform, and it is possible to greatly improve the degree of freedom in circuit design. It will be possible.

[Brief description of drawings]

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

FIG. 2 is a signal waveform diagram for explaining the operation of the gamma correction circuit in FIG.

FIG. 3 is an electric circuit diagram showing a configuration of a gamma correction circuit according to a second embodiment of the present invention.

FIG. 4 is a signal waveform diagram for explaining the operation of the gamma correction circuit in FIG.

FIG. 5 is an electric circuit diagram showing a schematic configuration of a conventional gamma correction circuit.

[Explanation of symbols]

1,2,4,6,7,8,10,11,13,14,1
5,16,19 resistors 3,5,9,12,17,18 transistors 26,28,29,30,32,33,35,36,3
7, 39, 40, 43, 44 Resistance 27, 31, 34, 38, 41, 42 Transistor

Claims (6)

[Claims] 1. A gamma correction circuit having a non-linear input / output characteristic and to which a video signal is input, the clipping circuit removing a portion of the input video signal having a voltage equal to or lower than a predetermined voltage, and the input video. A gamma correction circuit comprising: a subtraction circuit that subtracts a signal from which a portion having a voltage equal to or lower than a predetermined voltage is removed by the clipping circuit from a signal corresponding to the signal. 2. The clipping circuit has first and second transistors whose emitters and collectors are connected to each other, applies an input video signal to the base of the first transistor, and applies the input video signal to the base of the second transistor. The gamma correction circuit according to claim 1, wherein a predetermined bias voltage is applied and the clipped signal is obtained from the commonly connected emitters. 3. The subtraction circuit includes a transistor in which signals to be subtracted from each other are input to a base and an emitter, and a gamma-corrected signal is obtained from a collector.
Or the gamma correction circuit according to any one of 2 above. 4. A gamma correction circuit having a non-linear input / output characteristic and to which a video signal is input, the signal inversion circuit inverting the input video signal to generate an inverted video signal, A clipping circuit that removes a portion of the inversion video signal generated by the signal inversion circuit that is equal to or lower than a predetermined voltage, and a signal that corresponds to the inversion video signal and a signal that has a portion of the inversion video signal that is equal to or less than the predetermined voltage removed. A gamma correction circuit comprising: an addition circuit. 5. The clipping circuit has first and second transistors whose emitters and collectors are connected to each other, and applies the inverted video signal to the base of the first transistor, and the base of the second transistor. 5. The gamma correction circuit according to claim 4, wherein a predetermined bias voltage is applied to the gamma correction circuit and a clipped signal is obtained from the commonly connected emitters. 6. The adder circuit according to claim 4, wherein the adder circuit includes transistors each of which receives a signal to be added to each other through a resistor to a base to obtain a gamma-corrected signal from a collector. Gamma correction circuit.