JPS58221575A - Gamma compensating circuit of video signal - Google Patents

Abstract

PURPOSE:To set optionally a gamma compensating curve to the input signal voltage, by using a gain control circuit using a Gilbert circuit, a diode and an external input voltage source to control the negative feedback quantity to the gain control circuit which uses an image pickup signal as an input. CONSTITUTION:The input signal given from an image pickup tube is supplied to a gamma compensating gain control circuit 1 from a terminal 5. The output of the circuit 1 is fed to a differential amplifier 2, and the output of the amplifier 2 is extracted from a terminal 6 and at the same time fed negatively back to the circuit 1 via a trap circuit 3 of chroma signals and a negative feedback gamma quantity control circuit 4. This circuit 1 consists basically of a Gilbert type multiplier, and the feedback input voltage given from a quantity control circuit 4 is set at VNP. In such a case, the amplification characteristics are expressed as (A-B.NNP) where A and B are constants. Thus a gamma compensating curve can be set optionally to the input signal voltage Vx.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a γ (gamma) value correction circuit for correcting the photoelectric conversion characteristics of an image pickup tube in a color television camera.

Color television cameras require a linear relationship between the light input to the picture tube and the actual intensity of the light at the picture tube.

However, the relationship between the input signal from the image pickup tube and the video output signal is not a linear relationship, but is usually proportional to the γ power of the input (approximately 0.7 to 1), and the γ characteristics vary for each image pickup tube. indicate different values. Therefore, in order to faithfully perform so-called γ correction, which corrects the γ characteristics of
Although a polygonal line approximation was used, it was necessary to change the dividing resistance value and make adjustments for each image pickup tube.

The present invention improves these points, and provides a γ correction circuit that can realize arbitrary γ characteristics by appropriately setting the DC voltage of the external terminal according to the γ characteristics of each image pickup tube. It is something. FIG. 1 shows a block diagram of the γ correction circuit of the present invention. That is, the circuit shown in FIG. 1 includes a gain control circuit 1, an amplifier 2, a chroma signal trap circuit 3, and a negative feedback γ amount control circuit 4, and the input signal from the image pickup tube is input to the gain control circuit 1 having a terminal 5. The output of the control circuit 1 is connected to the amplifier 2, and the output of the amplifier 2 is taken out from the terminal 6, and the gain is output through the chroma signal trap circuit 3 and the negative feedback γ amount control circuit 4. It has a configuration in which it is connected in feedback to the control circuit 1. However, the negative feedback γ amount control circuit 4 has an external input terminal 7.

FIG. 2 is a flowchart showing the basic principle of the function of the γ correction circuit configuration shown in FIG. The input signal from the image pickup tube (abbreviated as Y signal) consists of a luminance signal a and a chroma signal, and its alternating current component is expressed as Δvx. Is the gain of gain control circuit 1 G (Y)? The gain of amplifier 2 is GA, and the video output signal voltage is V. ut + binding, and ■. The signal waveform of ut is the amplified luminance signal component a. ut 1 same Roma signal. Consists of ut.

This output signal voltage■. The chroma signal voltage of 3.58 MHz was removed from ut by the trap circuit 3.
Y signal voltage Y. U.

, through the negative feedback γ amount control circuit 4, a part of it (C”
Yout=”NF) is added to the γ correction gain control circuit 1 on the input side. These relationships are expressed by the following equation.

vout ”” G (Y)・GA−Δvx(1)G
(Y) = A −B VNF (2
)vNF=C-You,=C-vou, (3) However, (
2) Equation OG (Y)it: When VNF is -0, Al has a constant gain A, and when feedback is applied by adding the negative feedback signal vNF, the gain decreases linearly with the relationship A -B VNF. It means to do. Note that in equation (2), A and B are coefficients related to the amplification factor and negative feedback of the gain control circuit. It follows from equations (1) to (3).

Therefore, the video output voltage V. U changes linearly with respect to the external input AC signal ΔvX at C-0 (repeated feedback), but tends to be saturated at C=1, so by selecting the negative feedback amount C, the imaging The gamma correction curve can be selected depending on the photoelectric conversion characteristics of the tube.

Next, an embodiment of the gain control circuit 1 and the amplifier 2 is shown in FIG. The base terminal 35 of the NPN) transistor 22 is used as an input terminal, and its emitter is connected to the ground via the constant current source 26, and its collector is connected to the common emitter terminal of the differential pair consisting of the NPN) transistors 16 and 17. Connect to. Each collector of transistors 16 and 17 is
They are connected to the power supply terminal 31 via resistors 20 and 21, respectively.

On the other hand, the NPN transistor 2 has the same configuration as the circuit consisting of the transistors 22, 16.17 and the constant current source 25.
3.18.19 and a constant current 26 are used to form a pair therewith. However, it is assumed that the base terminal 36 of the transistor 230 is connected to a predetermined reference voltage (vref-a). Further, the emitters of transistors 22 and 23 are balancedly connected via a resistor 24, and transistors 17 and 18 are connected in a balanced manner through a resistor 24.
and between the bases of the transistors 16 and 19 and the spaces of the transistors 16 and 19, respectively. ! , transistor 16
and 18, and the collectors of transistors 17 and 19, respectively, are connected in common, and the collector point signals of these pairs are input to the differential amplifier 27. The output terminal 37 of the amplifier 27 serves as a video signal output terminal, and this signal is output through the terminal 30 to the trap shown in the first diagram.

Supplied to circuit 3. The output J from the trap circuit 3 passes through the negative feedback γ amount control circuit 4, and then
, p11 is connected to the base terminal 33 of the inlet J.

On the other hand, the base terminal 34 of the transistor 120 is connected to a reference voltage (vref-b), and the emitters of the transistors 11 and 12 are commonly connected to the constant current source 15 via resistors 26 and 29, respectively. On the other hand, the collectors of the transistors 11 and 12 are connected to the emitters of NPN transistors 13 and 14, whose collectors and bases are commonly connected and connected to a power supply terminal 31 and a direct current source terminal 32, respectively. be done. Also, transistor 1
1.1.120 each collector is connected to both bases of transistors 17.18 and to both bases of transistors 15.19, respectively.

FIG. 4 shows an outline of the negative feedback γ control circuit 4 in the configuration shown in FIG. The base terminal 48 of the NPN transistor 40 is connected to the luminance signal output voltage Y. As the input terminal of ut,
The emitter of the transistor 40 is connected to the base of another (NPN) range metal 410, and the emitter of the transistor 41 has a constant current source 42, a resistor 43, 44, a diode 46 connected in the forward direction, and a constant voltage source 46 input from an external input. The bias circuit system is connected in parallel with the ground, and the resistor 43
, 44 has a terminal 49 for taking out an output voltage as a feedback signal to the gain control circuit 1 in FIG.

Next, the operation of the gain control circuit and the negative feedback γ amount control circuit will be explained based on FIGS. 3 and 4. Figure 3 shows a basic Gilbert-type multiplier. The difference between the circuit using FIG. 3 and the Gilbert type circuit is as follows. That is, the input voltage of the negative feedback input terminal 33 is vNF, and the input signal voltage of the photoelectric conversion AC input signal terminal 35 is Δ■Engine high output terminal 7
If the video signal output voltage ΔV is Δ■. 8. teeth,
In a general guitar-rubert type multiplier, Δv = @vNF11ΔvX (5) ut However, in the circuit shown in Fig. 3, the formula (2) % formula % (6 ).However, K, A, and B are constants determined by the circuit.In other words, in the circuit of FIG. By setting R28<R29 with different values, even when the negative feedback input voltage vNF=0, the output voltage is
Δ■. The circuit is configured so that it does not become ut-〇.

As a result, the multiplier in FIG. 3 increases by 1 as vNF increases. A negative feedback amplifier is formed in which the gain decreases from linear and saturates.

Next, referring to FIG. 3, it will be briefly explained that the above equation (6) holds true. Now, when AC human power vNF is applied to terminal 33, the current ratio flowing through transistors 11 and 12 is 11.
If 12 is ``. (7) 1, then it becomes equal to the ratio of the currents flowing through the transistors 18 and 19 according to the principle of Gilbert form.

Therefore, when the alternating current signal Δvx is input to the terminal 36, the signal current AE changes the resistance value of the resistor 24 to R2420, 21
Let the resistance values of R2o and R21 be the resistances R2゜ and R2
The current I3.1 flowing through I3. I4 is expressed as follows.

I3= IE−y;) Ie (8) However, I
E is the current value flowing through the constant current source 25.26. formula(
8) From (9), the output signal voltage Δ■ of the Gilbert circuit. In the case of R20-R21, ut is Δ■Ou''=v
cc-I3R2o-(vc':C-14R21), i.e.
By making the value of m variable, ΔvOut can be controlled.

Next, find the relationship between (-) and vNF.

m+1 If the current of the constant current source 16 in Fig. 3 is 10, then the terminal 33
If there are two people with an AC signal vNF 75, the current ratio m
is given by the following equation.

Therefore, ΔvOut in the R0 equation is (b) In the θ equations, Equation (2) is expressed as (1:1 Δ■out ”'(A”'NF ) Δvx, so It can be seen that this is a negative feedback multiplier of the type expressed by equation (6).

Next, the operation of the negative feedback γ amount control circuit shown in FIG. 4 will be explained. In this circuit, when the AC input voltage at the input terminal 48 increases, the voltage at the output terminal 49 increases to a control voltage equivalent to the potential of the constant voltage source 46, that is, the voltage v7 applied to the external input terminal 7 in FIG. Until this happens, the voltage at the output terminal 49 increases linearly because the diode 46 is still not fully charged, but this voltage further exceeds the control voltage v7 and current flows through the diode 45. Then, the resistance becomes 43.
The voltage value divided by 44 is output to terminal 49. Therefore, the amount of feedback to the gain control circuit 1 is the external input voltage ■7
A limit effect is applied and the amount of feedback is limited.

As described above, in order to obtain a gamma correction curve, a gain control circuit using a Gilbert circuit, a diode, and a predetermined external input voltage source is used to create a gain control circuit that receives an image pickup tube signal as input. The negative feedback amount can be controlled and the γ correction curve for the input signal voltage can be set arbitrarily, so correction of the γ characteristics due to differences in the photoelectric conversion characteristics of individual image pickup tubes is a function of the negative feedback circuit. This is achieved automatically and under precise control.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of the present invention, FIG. 2 is a block diagram showing the operation of each component of the present invention, and FIG. 3 is a block diagram showing the γ correction gain control circuit which is the main part of the present invention. Embodiment FIG. 4 is a schematic diagram of a negative feedback γ amount control circuit. 1...Gain control circuit for γ correction, 2, 27...
... Differential amplifier, 3 ... Trap circuit, 4.
... Negative feedback γ amount control circuit, 11.12, 13, 1
4,16°17,18,19,22,23,
41...NPN transistor, 20, 21.2
4,28,29゜43.44...Resistance, 15
, 25, 26, 42...constant current source, 46.
...Diode, 46... Constant voltage source.

Claims (2)

[Claims] (1) A saturated negative feedback gain control circuit to which a video signal is applied; a limiter control circuit capable of controlling the feedback amount of the output of this gain control circuit by an external input signal; and the saturated negative feedback gain control circuit. A gamma correction circuit for video signals, characterized in that it has a feedback circuit configuration for feeding back. (2) When the feedback input voltage input to the saturated negative feedback gain control circuit is set to VNF, the amplification characteristic is (A-B・VN
2. The gamma correction circuit for video signals according to claim 1, wherein the gamma correction circuit is represented by F) (A and B are constants).