JPS6282794A - Video camera - Google Patents

Abstract

PURPOSE:To normally maintain a black balance by changing a setup voltage in the clamp circuit of a low frequency luminance signal or a green channel signal in accordance with the AGC voltage of an AGC circuit. CONSTITUTION:When switches 34, 3 of a clamp circuit are turned on, the output voltage of an amplifier is equal to the voltage value of a setup voltage source 38 and when the output voltage of the amplifier 33 is higher than the voltage value of the setup voltage source 38, the output voltage of a comparator 37 is reduced. Since the output of the comparator 37 is the input voltage of the amplifier 33, the output voltage of the comparator 37 is reduced, and when the output voltage of the amplifier 33 is lower than the voltage value of the constant voltage source 38, the output voltage of the amplifier 33 is raised. When the switches 34, 35 are turned of, if an input impedance of the amplifier 33 is fully large, a signal inputted from an input terminal 31 is amplified by the amplifier 33, superimposed on the setup voltage and outputted. Thereby, a black balance is maintained.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a signal processing circuit for a frequency separation type color video camera, and is particularly suitable for maintaining a normal black balance at all times from high illumination to low illumination. The present invention relates to a signal processing circuit.

[Background of the invention]

FIG. 4 shows an example of a signal processing circuit of a frequency separation type color camera, and its configuration will be briefly explained. As shown in FIG. 5, from the frequency separation type image pickup tube 1, a luminance signal as a low frequency component 21 is modulated by the recombination frequency 23 of the stripe filter, and a color signal is frequency multiplexed as a high frequency component 22. can get. After this signal is amplified to a predetermined magnitude by the preamplifier 2, the AGC circuit 3 maintains a constant large intensity against changes in illuminance. One of the signals maintained at a constant level is converted into a luminance signal by removing the high-frequency component 22 by the LPF 4. After being given a υ gamma (r) characteristic by the gamma circuit 13, the signal is sent to the encoder 19.
is input. Also, one of the outputs of the AGC circuit 3 is passed through an LPF 5 having the same bandwidth as the BPF 6, which will be described later.
, Create a YL times signal with the same bandwidth as the B signal, Gamma circuit 1
The color difference matrix 17, 18 is given r% property by 4.
supplied to In addition, the output of the AGC circuit 3 is further processed by a BPF 5 whose center frequency is the repetition frequency (modulation frequency of the color signal) of the stripe filter.
is removed to obtain a color signal component as a high frequency component. In the frequency separation method, the R and B signals constituting the color signal have a phase shift of π from each other, and are phase multiplexed such that one advances by π/2 in phase and the other lags behind each horizontal period. The R signal is a leading phase, and the B signal is a lagging phase. BPF6
One of the outputs is input to a π/2 phase delayer 7, and the other one is input to a 1 horizontal period delay line (IH delay line) 8. The R signal is
Since the output of the π/2 phase delayer 7 and the output of the 1H delay line 8 are in the same phase, they are obtained by addition by the adder 9, and the B signal is 180° out of phase, so it is obtained by subtraction by the subtracter 10. It will be done. The R signal and the B signal are converted into low-frequency signals by detectors 11 and 12, respectively, and then sent to a gamma circuit 15.
, 16, the color difference matrix is inputted by IJ x 17.19, and subtracted from the output of the LPF 5.

A color difference signal (R-Y) signal and a (B-Y) signal are generated, input to the encoder 19, and combined with the luminance signal to generate, for example, an NT signal.
This becomes an SC color video signal 20.

Next, we will discuss black balance in such signal processing and its problems. If you have a fake color video camera, "black balance" means that the brightness is 0.
The video signal of the subject part is the luminance component two set up level, the color component = 0 (that is, the color difference signal R-Y = B-Y =
0). Now, since many of the existing image pickup tubes use bias light to reduce residual light, a small amount of light is input to the image pickup tube even if the subject has luminance equal to O.

As a result, both the luminance component and the color component exist at small levels in the image pickup tube output.

Additionally, home color video cameras typically use an auto iris device to maintain a constant video output level even when the illuminance changes.

An AGC circuit is used. (The auto-iris device is omitted in the configuration shown in Figure 4.) Therefore, it is assumed that the image pickup tube output changes depending on the illuminance of the subject.
AGC circuit 3 ensures that a constant level is always maintained.
The amplification degree of the circuit changes. At this time, the output of the AGC circuit 3 for the luminance portion = 00 portion of the subject naturally changes both the luminance component and the color component depending on the amplification degree. Normally, a change in the brightness component is a change in the black level in response to a change in illuminance, so at the AGC circuit input, a negative set is made before the AGC input so that the brightness component = O when 20 brightness minutes are captured. By applying the up-current component (DC component),
The black level is kept constant despite changes in illuminance. However, the color signal of the image pickup tube output of the subject in the brightness part 20 is a high-frequency signal component and cannot be canceled out at the AGC circuit input section.
The F output level changes according to the 5AGC circuit amplification degree as the illuminance of the subject changes. Therefore, the black levels of the R and B signals change depending on the illuminance of the subject. As a result, the part of the subject whose luminance is -0 will be exposed to a certain illuminance, for example, at high illuminance (
AGC amplification degree = minimum), so that the output YL of LPF5 and the R, B signal level are equal to each other, that is, R-Y=B-Y
Even if a set-up is given to the gamma circuits 14 to 16 so that Gamma = 0, the illuminance will change. For example, when the illuminance becomes low, the amplification degree of the AGC circuit will change, and the levels of the R signal and B signal will rise. As mentioned above, in order to maintain a constant level of YL acid component, YL<R.

B (that is, (RY), (B-Yl>0), and a color signal will be generated in the video signal output.In other words, the black part will be colored, and the black balance will not be maintained. This problem occurs in other configurations,
For example, when the gamma input is set to R, G, and B (it is obvious that the same problem occurs even when the gamma input is set to R, G, and B). An example of such a frequency separation type signal processing circuit is Japanese Patent Publication No. 52-1615.

[Purpose of the invention]

An object of the present invention is to realize a signal processing circuit that maintains black balance in order to always achieve good color reproduction under any illuminance.

[Summary of the invention]

Changes in the black balance when the illuminance changes are a matter of constant adjustment under each illuminance, but the cause of the change is that the signal amount at black due to the bias light is A.
This was done based on the finding that this is due to changes in the GC gain, and the set-up in the clamp circuit of the low-range luminance signal or green channel signal is adjusted according to the AGC voltage of the AGC circuit that controls the gain of the image pickup tube output. It is characterized by applying pressure to change the voltage.

[Embodiments of the invention]

The present invention will be explained below using examples.

First, an example of the configuration of a feedback clamp circuit closely related to the present invention is shown in FIG. 2 and will be explained. This is the fourth
This is normally provided within the gamma circuit 15, 15.degree. 16 shown in the figure. In addition.

Here, it is assumed that the amplifier 33 is in positive phase. In this clamp circuit, when the switches 34 and 55 are closed, the output voltage of the amplifier 33 becomes equal to the voltage value of the constant voltage source 38 (referred to as a set-up voltage) by the next operation. Output voltage of amplifier 33 > voltage value of constant voltage source 38 and e comparator! +
7's output voltage decreases.

Since the output of comparator 37 is the input voltage of amplifier 33, the output voltage of amplifier 37 decreases. When the output voltage of the amplifier 33 <the voltage value of the constant voltage source 38, the output voltage of the amplifier 33 increases due to the opposite movement. At this time, the signal input to the input terminal 31 and the amplifier 3 are connected to the capacitor 32.
A charge determined by the voltage difference between the input voltage and the input voltage that gives an output voltage equal to the voltage of the constant voltage source No. 3 is accumulated. Next, when the switches 34 and 35 are open, if the input impedance of the amplifier 330 is sufficiently large, the accumulated charge in the capacitor 32 will be maintained, so that the potential across the capacitor 32 will be the same as the potential at the input terminal 31 when the previous switch was closed. Since the potential difference between the input voltage and the input voltage of the amplifier 360 is maintained, the signal inputted from the input terminal 31 is amplified by the amplifier 33, and is output after being superimposed on the setup voltage. The switch 40 connects the constant voltage source 41 to the amplifier 33 side during the horizontal scanning period and to the constant voltage source 41 during the horizontal blanking period.
The voltage value of the constant voltage source 41 is outputted from the output terminal 42 during the horizontal blanking period, and the output voltage of the amplifier 33 is outputted during the horizontal scanning period.

FIG. 6 shows an example of the waveform of the circuit operation in FIG. 2. The waveform 43 shown in FIG. 6(a) is input to the input terminal 31, and the switches 34 and 35 are connected to When the H section of the waveform 44 shown in FIG. At this time,
47 in the figure is the set-up voltage Vset (constant voltage source 38 (
7) Voltage value) -8 is the reference voltage Vugr (voltage value of the constant voltage source 41), 49 is the set-up amount v3et-=Vset
-It's Viny. In this clamp circuit,
The set-up itr, et can be changed by the set-up voltage Vset.

FIG. 1 is a block diagram showing the main parts of the present invention. A
The GC circuit 52 corresponds to the AGC circuit 3 shown in FIG. 4, and a signal equivalent to the output of the preamplifier 2 shown in FIG. It is supplied to LPF 4, LPF 5, and BPF 6, respectively. 54 is an AGO control circuit, and AGC circuit 52
The AGC control suppression voltage 55 is generated depending on the level of the output signal. The AGC control voltage 55 controls the gain of the AGC circuit 52 and also controls the coefficients of the coefficient multiplier 57.

The coefficient unit 57 calculates the coefficient K(
0≦≦1) and outputs a current KIo. When the AGC circuit gain is minimum, it is 0, and when the AGC circuit gain is maximum, it is 1. 5 is a supply terminal for power supply voltage Vcc, and the resistance values of fixed resistors 59 and 60 are RA and R.

Then, the voltage value of the output terminal 61 is VccXRs/(RA
+RB)+RB XKIo. By using this output voltage as the setup voltage of the clamp circuit provided in the gamma circuit 14 shown in FIG. 4, the following effects can be obtained.

Note that the gamma circuit 13 in FIG. The set-up voltages of the clamp circuits provided in the gamma circuit 15 and the gamma circuit 16 are each given a constant voltage. Gamma circuit 1
4. Setup voltages of gamma circuit 15 and gamma circuit 16 as Vset(y,,) and Vset(R
), Vset(B), and the reference voltages are all the same potential V
REF, each set up *rset
(YL) *rset(R), rset(B) is Pset(YL) = Vset(Y, )-vR
B,, rset(R) = Vsat(R)-VR
IIF + rset(B) = Vset(B)
It becomes Vmzr. Here Vcc X Re ReKI@, so y s e t (YL, )
= ball fu], + RBKI.

−V□r. This means that -5et(YL) is controlled by K, and as the AGC gain increases, K approaches from 0 to 1, and the set value of YL increases. Now, when the AGC gain is minimum, rs et (Y+, )=f
The set-up amount in each gamma circuit is determined so that set(R)=rset(B)=0. In other words, Vs
et(YL)=Vset(R)=Vset(B)=
Let us choose VREF. At this time, the signal levels of YL, R, and H for the black balance, that is, the part of the subject where the luminance component is 0, are naturally z e t(YL)=rset(R)=&Pset(
B), and the black balance is maintained.

Next time the AGC gain increases, as mentioned earlier, there is a small amount of equal sign component due to the bias light, so
As the AGC gain increases, 1. ->The gamma circuit input levels of R and H increase. By the way, in general, the blanking period, that is, the period in which there is no signal, is selected as the period during which the switches 54 and 35 of the clamp circuit are closed, so that the gamma circuit input level for the part of the subject whose luminance is 20 is only R and B (A-1)e.
b increases (A is the AGC circuit gain increase rate + el) since the luminance component = 0 o'clock signal level), this is due to the R-B set-up amount (AI)eb increasing.

Here, by setting KRBIo=(A I )eb, even if the AGC gain increases, the YI of the luminance = 00 part
,.

The R and B signal levels remain equal, which means that the black balance is achieved. Further, although it is difficult to exactly match the tracking of K and A-1, no problem usually occurs.

FIG. 3 shows an example of the coefficient multiplier 57 and the current source 56.

Vec - (Vb@+V people) In this circuit, 0=--1. -It is one and one.

However, vA is the voltage value of the constant voltage source 66, vb8 is the transformer,
64.65 is the 111-way emitter-to-emitter voltage.

Furthermore, it is clear that the same applies to a system in which the Yl input of the gamma circuit is replaced with the G input.

〔Effect of the invention〕

According to the present invention, it is possible to always achieve stable black balance even when illuminance changes, so it is possible to easily add a circuit for low-level signal components that conventionally had to be rendered achromatic by suppressing low luminance or low saturation. Since it can be used without being ignored, it has the effect of expanding the dynamic range of colors and improving color reproducibility.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the main parts of an embodiment of the present invention, FIG. 2 is a block diagram showing an example of a clamp circuit of a signal processing circuit to which the present invention is applied, and FIG. FIG. 4 is a block diagram showing the general configuration of a frequency separation type color video camera, FIG. 5 is a frequency spectrum diagram of the image pickup tube output, and FIG. FIG. 3 is a waveform diagram for explaining the operation of the clamp circuit shown in FIG. 2; 1... Image pickup tube, 3... AGC circuit, 4.
5...LPF, 6...BPF. 11.12... Detector, 13-16... Gamma circuit, 17, 18... Color difference calculator, 19... Encoder, 52... AGC circuit, 53... Output terminal, 54... ...AGC control circuit, 56...current source,
57...Coefficient unit, 5th...Power supply terminal. 1 口 2nd difficulty 4th rf 1 6th concealment

Claims (1)

[Claims] In a color video camera using a frequency separation type image pickup tube and having an AGC circuit and a clamp circuit, a constant current source and a coefficient unit whose output current is changed by an AGC control voltage proportional to the current value of the current source and the AGC circuit gain; A voltage dividing means consisting of two fixed resistors is provided, and the output current of the coefficient multiplier flows into the midpoint of the resistor, and the midpoint potential is used as the setup voltage of the clamp circuit of the low band luminance signal or green signal channel. A video camera featuring: