JP2804665B2 - Nonlinear conversion 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 non-linear conversion circuit for obtaining an output signal which is non-linear with respect to an input signal, such as gamma correction of a video signal.

[0002]

2. Description of the Related Art A non-linear conversion circuit used for gamma correction of a video signal is generally constituted by an amplifier circuit whose gain changes in accordance with the level of an input signal. FIG. That is, the output characteristic is such that a desired characteristic curve is approximated by a plurality of straight lines having different slopes. A method of approximating a curve showing a certain characteristic by a combination of a plurality of straight lines is disclosed in, for example, JP-A-57-173239.

By the way, digitization of a signal processing circuit for handling a video signal has been promoted, and a nonlinear conversion circuit by digital processing has been considered in response to this.
FIG. 4 is a block diagram showing a configuration of a nonlinear conversion circuit based on digital processing. Analog value input signal A-IN
Are input to the A / D converter 1, are converted into n-bit digital data D−1, and are supplied to the decoder 2. In this decoder 2, input digital data D-1
And the digital data subjected to the non-linear conversion processing are associated with each other, so that n-bit digital data D-2 showing a non-linear characteristic with respect to the digital data D-1 input from the A / D converter 1 is output. Is done. The digital data D-2 subjected to the non-linear conversion processing is sequentially stored in the memory 3, and other necessary digital processing, for example, processing such as insertion of a sub-screen or superimpose is performed. Then, the digital data D-3 read from the memory 3
Is again converted into an analog value by the D / A converter 4,
The output signal A-OUT is supplied to the next circuit. The control circuit 5 controls the capture of the input signal A-IN, determines the operation cycle of the A / D converter 1 and the D / A converter 4, and controls the signal processing operation in the memory 3. I do. As a result, the output signal A-OUT becomes the input signal A
-IN is output as a video signal of a desired format in association with -IN.

FIG. 5 is an output characteristic diagram when gamma correction of a video signal is performed. In general TV cameras,
Since the light emission output is not proportional to the input signal of the CRT for displaying the playback screen, so-called gamma correction is performed to correct the level of the video signal so that a light emission output proportional to the light input to the television camera is obtained. . This gamma correction follows the equation of Y = X ^r (X: input signal, Y: output signal), and is usually set to r = 0.45. Therefore, as shown in FIG. 5, the rate of change of the output signal A-OUT with respect to the input signal A-IN gradually decreases as the level of the input signal A-IN increases. In this case, the maximum value and the minimum value of the output coincide with the input.

When the decoder 2 has a 4-bit configuration, digital data D-1 input to the decoder 2 and digital data D-2 output from the decoder are associated with each other as shown in Table 1. .

[0006]

[Table 1]

That is, as shown in Table 1, the decoder 2
It is configured to output each digital data D-2 for each of the digital data D-1, thereby obtaining an output characteristic as shown in FIG. here,
Although the resolution is low and a smooth output curve cannot be obtained due to the 4-bit configuration, an ideal output curve can be obtained by increasing the number of bits.

[0008]

However, the conversion circuit based on digital signal processing has a large circuit size such as the A / D converter 1 and the decoder 2, and the system as a whole is expensive. In particular, A / D converters and D / A converters that handle video signals require high-speed operation and have a special circuit configuration, so that it is difficult to simplify the circuit configuration. Further, in order to obtain the resolution required for realizing the ideal gamma correction, it is necessary to use digital data of 9 bits or more. To simplify the circuit configuration of the A / D converter and the decoder, furthermore, It will be difficult.

Accordingly, an object of the present invention is to simplify the circuit configuration and to construct an inexpensive nonlinear conversion circuit.

[0010]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and is characterized by first and second P-channel MOS transistors connected in parallel to a power supply. First and second N-channel MOS transistors are respectively connected in series to the transistor, and the gates of the first and second P-channel MOS transistors are connected to the drain of the first P-channel MOS transistor; First and second N-channel type M
The source of the OS transistor is grounded to form a current mirror circuit. The drain of the second P-channel MOS transistor is connected to a power supply via a third N-channel MOS transistor, and the third P-channel MOS transistor is connected to a power supply. An object of the present invention is to obtain an output signal from the drain of the second N-channel MOS transistor in response to an input signal that is grounded via a channel-type MOS transistor and is applied to the gate of the first N-channel MOS transistor.

[0011]

According to the present invention, the polarity of the P-channel type and the N-channel type of the complementary MOS transistor whose current-voltage characteristic shows the square characteristic is reversed and connected to the output side of the current mirror circuit. The output of the current mirror circuit is 1 /
An output showing a square characteristic can be obtained. Therefore, the output signal with respect to the input signal exhibits a 乗 power characteristic.

[0012]

FIG. 1 is a circuit diagram of a nonlinear conversion circuit according to the present invention. Two P-channel MOS transistors T _P1 ,
T _P2 is connected in parallel to the power supply, the gates of these MOS transistors T _P1 and T _P2 are connected to the drain of the MOS transistor T _P1 , and the two MOS transistors T _N1 and T _N2 are connected to the MOS transistors T _P1 and T _N2. Each is connected in series to _P2 . Further, MOS transistors T _N1 and T _N1
The source of _N2 is grounded via MOS transistors T _N3 and T _N4 to which a constant control voltage V _CS is applied to the gate, and is connected to each other via a resistor R1 to form a current mirror circuit. Further, to the gate of the MOS transistor T _N2, divided power supply voltage by two resistors R2, R3 are applied, the input signal A-IN to the gate of the MOS transistor T _N1 is applied.

Then, the MOS transistor T _P2 and the MOS
The connection point of the transistor T _N2, the source of the MOS transistor T _P5 and the MOS transistors T _N5 is connected,
The drain of the MOS transistor T _P5 is connected drains of the MOS transistors T _N5 to the power supply is grounded. The gates of these MOS transistors _TP5 and _TN5 have a P-channel type MO having a gate connected to a drain.
Two voltages obtained by dividing the power supply voltage by the S transistor T _P6 , the N-channel MOS transistor T _N6, and the two resistors R3 and R4 are applied. These two voltages are set close to the threshold _values of the MOS transistors T _P5 and T _N5 , respectively, and turn on / off the MOS transistors T _P5 and T _N5.
It is in the intermediate state of off.

In such a non-linear conversion circuit, when the level of the input signal A-IN decreases and the MOS transistor T _N1 operates to turn off, the MOS transistor T _N1 is turned off.
The potential on the drain side of _N1 increases, and the MOS transistors T _P1 and T _P2 are operated in a direction to turn off. At this time, M
Since the gate voltages of the OS transistors T _N2 and T _N4 are fixed, these MOS transistors T _N2 and T _N4
, A current flows through the MOS transistor T _N5 . Conversely, when the level of the input signal A-IN rises and the MOS transistor T _N1 operates to turn on,
The potential on the drain side of the MOS transistor T _N1 becomes lower, and the MOS transistors T _P1 and T _P2 are operated in a direction of turning on. Similarly, MOS transistors T _N2 , T _N4
Since the gate voltage of is fixed, the current flowing through the MOS transistor T _P2 flows to the ground through the MOS transistor T _P5. Therefore, since the MOS transistors T _P5 and T _N5 whose current-voltage characteristics have a squared characteristic are connected to the output side with the power supply side and the ground side reversed,
From the MOS transistor T _N5 to the MOS transistor T _N2 ,
MOS for the current flowing to _TN4 or the current flowing from MOS transistor _TP2 to MOS transistor _TP5
A change in the potential on the source side of the transistors T _P5 and T _N5 , that is, a change in the level of the output signal A-OUT has a ２ power characteristic with respect to a change in the current. Therefore, MO
Control voltage V applied to the gates of S transistors T _N3 and T _N4
If you adjust the _CS changes in the current flowing through the MOS transistor T _P5, T _N5 to be proportional to the change in the input signal A-IN, an output signal according to the characteristics of Y = X ^0.5 for an input signal A-IN A-OUT can be obtained.

By the way, in the case of the conversion circuit shown in FIG.
= 0.5, but in the case of gamma correction of a video signal, ideally r = 0.45, so that as shown in FIG.
= 0.9 is used in combination with the conversion circuit. That is, the output A-OUT1 of the conversion circuit formed of the analog circuit shown in FIG. 1 is set as the input of the conversion circuit formed of the digital circuit, and the output A-OUT2 is set as the gamma correction output. As shown in FIG. 3, the conversion in which r = 0.9 is closer to a linear conversion (r = 1) than the conversion in which r = 0.45. Can be reduced. For this reason, even when a combination of an analog circuit and a digital circuit is used, the circuit configuration is simplified as compared with a conversion circuit that is entirely composed of digital circuits.

In practice, the conversion characteristic when r = 0.5 is quite close to the conversion characteristic when r = 0.45, so that there is no great visual difference in image quality, and so high image quality is not required. In a surveillance camera or the like, FIG.
Can be used as it is. In this case, the signal processing circuit can be greatly simplified, which is effective in reducing the cost of the camera.

[0017]

According to the present invention, since a non-linear conversion circuit can be realized by a conversion circuit that combines MOS transistors and performs analog operation, the circuit configuration is significantly reduced as compared with a non-linear conversion circuit using a decoder such as a ROM. it can. When used as a video signal gamma correction circuit, the configuration of the video signal processing circuit can be simplified, and the cost of video equipment employing the video signal processing circuit can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a nonlinear conversion circuit according to the present invention.

FIG. 2 is a block diagram of a correction circuit for realizing ideal gamma correction.

FIG. 3 is a diagram illustrating conversion characteristics of nonlinear conversion.

FIG. 4 is a block diagram of a conventional nonlinear conversion circuit.

FIG. 5 is a diagram illustrating conversion characteristics of a nonlinear conversion.

[Description of Signs] 1 A / D converter 2 Decoder 3 Memory 4 D / A converter 5 Control circuit T _{P1 to} T _P6 P-channel MOS transistors T _{N1 to} T _P6 N-channel MOS transistors R1 to R5 Resistance

Claims (2)

(57) [Claims] 1. A first and a second N-channel MOS transistor connected in parallel to a power supply.
The channel-type MOS transistors are connected in series, respectively, and the gates of the first and second P-channel-type MOS transistors are connected to the drain of the first P-channel-type MOS transistor, and the first and second N-channel-type MOS transistors are connected. The source of the MOS transistor is grounded to form a current mirror circuit, and the drain of the second P-channel MOS transistor is connected to a third N-channel MOS transistor.
Connected to a power supply via a transistor, grounded via a third P-channel MOS transistor,
A non-linear conversion circuit characterized in that an output signal is obtained from a drain of the second N-channel MOS transistor in response to an input signal supplied to a gate of the first N-channel MOS transistor. 2. A source of the second N-channel MOS transistor is grounded via a control N-channel MOS transistor having a gate to which a constant control voltage is applied. A non-linear conversion circuit as described.