JPH0575897A - Gamma correction circuit - Google Patents

Abstract

PURPOSE:To obtain excellent frequency characteristics with high accuracy by adding the output of a plurality of clip circuits by an differential amplifier using an emitter follower and controlling the clip characteristic of each clip circuit in a memory. CONSTITUTION:A clip circuit 13 is composed of transistors 8 and 9 as differential amplifiers realizing clip characteristic using an emitter follower, a transistor 10 for impedance conversion, and each component of emitter resistances 11 and 12. In this case, when an input signal is supplied to an input terminal 1 and the signal level of the input terminal 1 becomes lower than the break point voltage, the transistor 9 is turned ON, and the emitter output of the transistor 10 for impedance conversion does not become lower than the break point voltage. Accordingly, the output of the inversion adder 14 does not become higher than the voltage. Thus, the input/output characteristics of the gamma correction circuit can be changed by changing each break point setting voltage.

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 having good high frequency characteristics, high accuracy and capable of changing input / output characteristics.

[0002]

2. Description of the Related Art A gamma correction circuit is a non-linear compression circuit for image signals. This gamma correction circuit is often used for compensating for non-linear characteristics of input / output characteristics of a television camera, a still video camera, a display, or the like, or for intentionally having the non-linear characteristics.

FIG. 5 shows a conventional gamma correction circuit. In FIG, 1 is output terminal to which an input signal (negative polarity) is supplied, 2 for impedance conversion transistor, 3 emitter resistor for determining the emitter current, 4 _k diodes (k = 1, 2, ... n) , 5, 6 _k (k = 1, 2, ... n)
Is a dividing resistor that determines the amplitude voltage of the output signal, E _1k (k =
1, 2, ..., N) are power supply voltages that set break points of input / output characteristics, and 7 is an output terminal.

Next, the operation will be described. For example, when the input signal as shown in FIG. 4 is supplied to the input terminal 1, the output signal is output from the output terminal 7 through the impedance conversion transistor 2 and the dividing resistor 5 connected to the emitter thereof. At this time, the potential of the output terminal 7 is more than the _break point setting voltage E _{1k of the} input / output characteristics.

[0005]

[Outer 1]

When the potential becomes lower than the low potential, the diode 4 _k becomes conductive and the output from the emitter is divided by the dividing resistors 5 and 6.
_{It is divided} by _k . If there is one set of 4 _k , 6 _k , E _1k ,
The input / output characteristic is a broken line characteristic with one break point. Therefore, the relationship between the input signal and the output signal is as shown in FIG. 6 (the input / output signals are indicated by relative values). The number of break points is equal to the number of 4 _k , 6 _k , E _1k pairs. The accuracy of the gamma correction increases with an increase in the number of break points as the diode 4 _k , the dividing resistor 6 _k , and the break point setting voltage E _1k increase.

[0007]

Since the conventional gamma correction circuit is constructed as described above, in order to perform gamma correction with higher accuracy, more diodes 4 _k are required.
(K = 1, 2, ... N) must be used. Therefore, there is a drawback that the junction capacitance of the die diode becomes a load of the circuit and the frequency characteristic is deteriorated by the AC and DC levels of the input signal.

An object of the present invention is to provide a gamma correction circuit that solves the above problems.

[0009]

A gamma correction circuit according to the present invention comprises a plurality of clipping circuits by a differential amplifier using transistors, an adding means for adding respective output signals of the plurality of clipping circuits, and each of the clipping circuits. And a means for controlling the input / output characteristics of the circuit.

[0010]

According to the present invention, the gamma characteristic is obtained by adding the clipping characteristics of the clipping circuits having different inclinations.

[0011]

EXAMPLE One example of the present invention will be described below.

FIG. 1 is a circuit diagram of a gamma correction circuit to which the present invention is applied. In FIG. 1A, 8 and 9 are transistors as a differential amplifier that realizes a clipping characteristic by using an emitter follower, 10 is a transistor for impedance conversion, and 11 and 12 are emitter resistors of the transistors 8 to 10. I is an input terminal connected to the base of the transistor 8, O is an output terminal connected to the emitter of the transistor 10, and C is a control terminal connected to the base of the transistor 9. A clip circuit is constituted by these respective components of 8 to 12, and this clip circuit is indicated as 13 as one block. FIG. 1 is a circuit diagram of the entire embodiment using a plurality of clip circuit blocks.
It shows in (b).

In FIG. 1B, 14 is an adder and 15
_k (R _k ) (k = 1, 2, ... N), 16 are addition resistors and feedback resistors for adding the outputs of the respective circuit blocks 13 and supplying them to the output terminal 7. Reference numeral 1 denotes an input terminal, which supplies an input signal to the input terminal I of each circuit block 13. E
_2k (k = 1, 2, ... N) is a voltage for setting a break point of the input / output characteristic, which is applied to the control terminal C of each block 13,
The gamma characteristic can be changed by controlling these from the CPU 17. Reference numeral 18 denotes a memory, each clip circuit block 1 for realizing various gamma characteristics.
Digital data corresponding to the breakpoint setting voltage E _2k of 3 is recorded. Reference numeral 17 is a CPU, 19 is a DA converter, and the digital data from the memory 18 selected by the CPU 17 is converted by the DA converter 19 into an analog breakpoint setting voltage E _2k .

Next, the operation will be described. For example, when an input signal as shown in FIG. 4 is supplied to the input terminal 1, when the number of the clipping circuit blocks 13 is one, the relationship between the input signal and the output signal is as shown in FIG. When the symbol level of the input terminal 1 becomes lower than the corner voltage E _2k , the transistor 9 of the differential amplifier is turned on, so that the emitter potential of the transistor 9 does not become lower than E _2k −V _BE .

[0015]

[Outside 2]

Therefore, the emitter output of the impedance conversion transistor 10 does not become lower than E _{2k, and the} output of the inverting adder 14 does not become higher than E _2k . Therefore, the input / output characteristics are as shown in FIG. At this time, the inclination of the clip characteristic is obtained by the gain of the adder 14 = | −R _f / R _k |
Becomes When the broken line setting voltage E _2k and the addition resistance 15 _k (R _k ) are changed, the input / output characteristics of each clip circuit block 13 _k become, for example, b to d in FIG. Therefore, the input / output characteristic of the gamma correction circuit becomes like e obtained by adding a to d.

Further, the input / output characteristics of the gamma correction circuit of the present invention can be variously changed as shown in FIG. 3 by changing each breakpoint setting voltage E _2k . A linear characteristic like f is also possible (the input / output signals in FIGS. 2 and 3 are shown as relative values).

The clip circuit using the emitter follower has a transistor 9 when the transistor 8 is off.
Is ON and the emitter resistance is small, so that the field through due to the base-emitter capacitance of the transistor 8 is negligibly small. Therefore, it has a good high-frequency characteristic, and changes in the frequency characteristic due to the AC and DC levels of the input signal are small.

Further, when the characteristics of the transistors 8 and 9 are matched, the accuracy of the break point is high, and therefore the accuracy of gamma correction is high.

[0020]

As described above, the following effects can be obtained by the present invention.

It has a good high frequency characteristic, and A of the input signal
Little change in frequency characteristics due to C and DC levels.

The accuracy of gamma correction is high.

Since the gamma characteristic can be changed in various ways, the gamma characteristic can be automatically adjusted or remotely controlled.

[Brief description of drawings]

FIG. 1A is a diagram showing a clip circuit block, and FIG. 1B is a circuit diagram of a gamma correction circuit embodying the present invention.

FIG. 2 is a diagram showing basic input / output characteristics of a gamma correction circuit embodying the present invention.

FIG. 3 is a diagram showing various input / output characteristics that can be realized by performing DC control on the gamma correction circuit according to the present invention.

FIG. 4 is a diagram showing an example of an input waveform.

FIG. 5 is a circuit diagram of a conventional example of a gamma correction circuit.

FIG. 6 is a diagram showing input / output characteristics of a conventional gamma correction circuit.

[Explanation of symbols]

 13 Clip Circuit 14 Adder 17 CPU 18 Memory 19 DA Converter

Claims (1)

[Claims] 1. A plurality of clipping circuits by a differential amplifier using transistors, an adding means for adding output signals of the plurality of clipping circuits, and a means for controlling input / output characteristics of each of the clipping circuits. A gamma correction circuit characterized by that.