JPH021472B2 - - Google Patents

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 this γ characteristic differs depending on the image pickup tube. Show value. Therefore, to correct this γ characteristic, so-called γ
In order to perform the correction faithfully, conventionally, blocks consisting of diode limiters and divided resistors were arranged in several stages in parallel to perform a polygonal approximation, but this required adjustment by changing the divided resistor value for each image pickup tube. It was hot.

The present invention improves these points,
The present invention provides a γ correction circuit that can realize arbitrary γ characteristics by appropriately setting the DC voltage of an external terminal according to the γ characteristics of each image pickup tube. 1st
The figure shows a block diagram of the gamma 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. The output of the control circuit 1 is input connected to an amplifier 2, and the output of the amplifier 2 is taken out from a terminal 6, and the gain is 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 function in the γ correction circuit configuration shown in FIG. The input signal from the image pickup tube consists of a luminance signal a and a chroma signal b, and the input level (alternating current component) of the video signal with respect to the black level (horizontal retrace period) of the luminance signal is expressed as ΔVx. The gain of gain control circuit 1 is G(Y), and the gain of amplifier 2 is
Let G _A be the video output signal voltage V _put , and this V _put
The signal waveforms of are the amplified luminance signal components, respectively.
It consists of a _put and the same chroma signal b _put . The signal voltage Y _put is obtained by removing the 3.58MHz chroma signal voltage from this output signal voltage V _put by the trap circuit 3.
is applied to the γ correction gain control circuit 1 on the input side via the negative feedback γ amount control circuit 4 (C·Y _put =V _NF ). These relationships are expressed by the following formula.

V _put = G(Y)・G _A・ΔV _x (1) G(Y)=A−BV _NF (2) V _NF =C・Y _put C・V _put (3) However, (1), (2 ) and (3), V _put , V _NF ,
Like ΔX, the magnitude of Y _put is defined based on the black level of the video signal. Also, G(Y) in equation (2) has a certain gain A when V _NF = O, and when the negative feedback signal V _NF is added and feedback is applied, the gain becomes A - BV _NF . means linearly decreasing in the relationship. Note that in equation (2), A and B are coefficients related to the amplification factor and negative feedback of the gain control circuit. From equations (1) to (3), V _put =A・G _A・ΔV _x /1+B・CG _A ΔV _x (4).

Therefore, when C=0 (no feedback), the video output voltage V _put varies linearly with respect to the externally input AC signal ΔV _x , but when C=1, it tends to be saturated, so the amount of negative feedback C By selecting , it is possible to select a γ correction curve according to the photoelectric conversion characteristics of the image pickup 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, its emitter is connected to the ground point via the constant power current 25, and its collector is connected to the common emitter terminal of the differential pair consisting of the NPN transistors 16 and 17. do. The collectors of transistors 16 and 17 are connected to power supply terminal 31 via resistors 20 and 21, respectively.
On the other hand, the same circuit configuration as the circuit consisting of the transistors 22, 16, 17 and constant current source 25 is configured as a pair using NPN transistors 23, 18, 19 and constant current 26. However, it is assumed that the base terminal 36 of the transistor 23 is connected to a predetermined reference voltage (V _ref-a ). Further, the emitters of the transistors 22 and 23 are balancedly connected through a resistor 24, and the bases of the transistors 17 and 18 and the bases of the transistors 16 and 19 are directly connected, respectively. Also, each collector of transistors 16 and 18, and transistor 1
Two of the collectors 7 and 19 are commonly connected, and the collector point signals of each pair are input to the differential amplifier 27. Output terminal 37 of amplifier 27
constitutes a video signal output terminal, and on the other hand, this signal is supplied through a terminal 30 to the trap circuit 3 shown in the first diagram. The output from the trap circuit 3 is connected to the base terminal 33 of the NPN transistor 11 via the negative feedback γ amount control circuit 4 . On the other hand, the base terminal 34 of the transistor 12
is connected to the reference voltage (V _ref-b ), and the emitters of transistors 11 and 12 are connected to resistors 28 and 2, respectively.
It is commonly connected to the constant current source 15 via 9. 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 DC bias power supply terminal 32, respectively. Further, the collectors of the transistors 11 and 12 are connected to both bases of the transistors 17 and 18, and the collectors of the transistors 16 and 12, respectively.
Connected to both bases of 19.

FIG. 4 shows an outline of the negative feedback γ amount control circuit 4 in the configuration shown in FIG. The base terminal 48 of the PNP transistor 40 is used as the input terminal for the luminance signal output voltage Y _put , and the emitter of the PNP transistor 40 is connected to
Connect the base of another NPN transistor 41,
A constant current source 42 is connected to the emitter of the transistor 41.
, resistors 43 and 44, and forward-connected diode 4.
5 and a bias circuit system having a constant voltage source 46 from an external input are connected in parallel between the ground and the resistor 4.
It has a terminal 49 for taking out an output voltage as a feedback signal to the gain control circuit 1 in FIG. 1 from between 3 and 44.

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 is based on a 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 set to V _NF , and the input voltage of the optical conversion AC input signal terminal 35 is
_{ΔV put is the} input signal _{voltage ΔV} However, in the circuit shown in Figure 3, as stated in equations (1) and (2) above, it is set so that ΔV _put = (A-B・V _NF ) ΔV _x (6) There is. However, K, A, B
is a constant determined by the circuit. In other words, in the circuit shown in Figure 3, the resistance value _R28 of resistors 28 and 29 is
By changing the value of R ₂₉ and setting it so that R ₂₈ < R ₂₉ , the circuit is configured so that the output voltage does not become ΔV _put = 0 even when the negative feedback input voltage V _NF = 0. ing. As a result, the multiplier in FIG. 3 forms a negative feedback amplifier whose gain decreases from linearity and saturates as V _NF increases.

Next, referring to FIG. 3, it will be briefly explained that the above equation (6) holds true. Now, input AC to terminal 33.
When V _NF is applied, if the ratio of currents I ₁ and I ₂ flowing through transistors 11 and 12 is I ₂ /I ₁ = m (7), then according to the principle of Gilbert form, the ratio of currents flowing through transistors 18 and 19 is be equal. Therefore, the signal current I _x when the AC signal ΔV _x is input to the terminal 35 is expressed as I _x = ΔV _x /R ₂₄ where the resistance value of the resistor ₂₄ is R 24 . If the resistance values are R ₂₀ and R ₂₁ , then the resistance R ₂₀ ,
The currents I ₃ and I ₄ flowing through R ₂₁ are respectively expressed as follows.

I ₃ = I _E - (m+1/m-1) I _x (8) I ₃ = I _E - (m-1/m+1) I _x (9) However, I _E flows through constant current sources 25 and 26 It is the current value. According to equations (8) and (9), the output signal voltage ΔV _put of the Gilbert circuit is, when R ₂₀ = R ₂₁ , ΔV _put = V _cc −I ₃ R ₂₀ −(V _cc −I ₄ R ₂₁ ) = 2 (m-1/m+1)I _x =2(m-1/m+1) ^Vx
_R24 (10) That is, by varying the value of m, ΔV _put can be controlled.

Next, find the relationship between (m-1/m+1) and _VNF .

Assuming that the current of the constant current source 15 in FIG. 3 is _I0 , when the alternating current signal _VNF is input to the terminal 33, the current distribution ratio m is given by the following equation.

mI ₀ R ₂₉ −V _NF /I ₀ R ₂₈ +V _NF (11) Therefore, ΔV _put in equation (10) is ΔV _put =2R ₂₉ −R ₂₈ /R ₂₈ +R ₂₉ −2V _NF /I ₀ (R ₂₈ +R
₂₉ ) ΔV _x /R ₂₄ (12) In formula (12), A=2R ₂₉ −R ₂₈ /R ₂₈ +R ₂₉ 1/R ₂₄ B=22/I ₀ (R ₂₈ +R ₂₉ ) 1/R ₂₄ Then, equation (12) can be expressed as ΔV _put = (A-BV _NF ) ΔV _x (13), so it can be seen that it is a negative feedback multiplier of the form 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 becomes a control voltage equivalent to the potential of the constant voltage source 46, that is, the voltage V ₇ applied to the external input terminal 7 in FIG. Since the diode 45 has not yet risen sufficiently, the voltage at the output terminal 49 increases linearly, but this voltage further exceeds the control voltage V ₇ and current flows through the diode 45. Then, resistance 4
The voltage value divided by 3 and 44 is output to terminal 49. Therefore, the amount of feedback to the gain control circuit 1 is limited by the external input voltage _V7 , and the amount of feedback is limited.

As described above, in order to obtain a gamma correction curve, by using a gain control circuit using a Gilbert circuit, a diode, and a predetermined external input voltage source, a negative By controlling the amount of feedback, γ for input signal voltage is
Since the correction curve can be set arbitrarily, correction of γ characteristics due to differences in photoelectric conversion characteristics of individual image pickup tubes can be performed by the 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 gamma 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... Correction gain control circuit, 2, 27... Differential amplifier, 3... Trap circuit, 4... Negative feedback γ amount control circuit, 11, 12, 13, 14, 16, 1
7, 18, 19, 22, 23, 41...NPN transistor, 20, 21, 24, 28, 29, 4
0...PNP transistor, 43,44...resistance,
15, 25, 26, 42... constant current source, 45...
Diode, 46... Constant voltage source.

Claims (1)

[Claims] 1. A saturated negative feedback gain control circuit to which a video signal is applied, an amplifier to which the video signal is input via the saturated negative feedback gain control circuit, and an output generation point of the amplifier. a trap circuit that receives an attached output terminal and a signal generated at the output terminal and removes a chroma signal voltage in the signal; and a trap circuit that is provided between the trap circuit and the saturated negative feedback gain control circuit, A gamma correction circuit for video signals, comprising a negative feedback gamma amount control circuit that controls a negative feedback gamma amount. 2 When the feedback input voltage input to the saturated negative feedback gain control circuit is _VNF , the amplification characteristics are (A-B,
2. The gamma correction circuit for video signals according to claim 1, wherein the gamma correction circuit is represented by V _NF ) (A and B are constants).