JPH0294790A - White balance control circuit - Google Patents

Abstract

PURPOSE:To attain highly accurate control by controlling the level of color difference signals R-Y, B-Y inputted to a white level detection circuit so as to vary the level in response to a change in a color temperature thereby preventing the spread of a white level signal to a low or high color temperature and a white level under special light. CONSTITUTION:A luminance signal Y is subtracted from inputted R, B signals via GCA 1, 2 at subtractors 3, 4 and difference signals R-Y, B-Y are inputted to clamp circuits 22, 24 via capacitors 19, 21 respectively and inputted to a white level detection circuit 5 via GCA 26, 27. The circuit 5 detects a white signal area and outputs the detection signal to gates 6, 7, 25. Thus, the color difference signals R-Y, B-Y are inputted to comparators 11, 12 via LPFs 8, 10 and compared with a reference voltage from an LPF 9. An H or L signal is outputted depending whether the signal level is higher or lower than the reference voltage to count up or count down counters 13, 14. The count value is inputted to the GCA 1, 2, 26, 27 via D/A converters 15, 16 and amplifiers 17, 18 to control the color difference signals R-Y, B-Y to be zero.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a white balance control circuit used in color television cameras and the like.

(Prior Art) A so-called full-auto white balance circuit that performs automatic color temperature tracking has been installed in home video cameras and is one of the important circuits. This fully automatic white balance circuit uses an external color temperature sensor method and an internal method that determines the color temperature from the image being captured. Each of these methods has its own advantages and disadvantages, but since the present invention relates to the inner method, the problems with the prior art of this method will be explained below.

Since the inner method controls white balance based on the video signal obtained from the lfR image, it is possible to achieve a more accurate white balance than the external sensor method, and is expected to improve stability. Can be done. In addition, compared to the external sensor method, there is no need to provide a new sensor, so there are advantages such as making it easier to miniaturize the camera head. However, on the other hand, the biggest drawback is that the white balance tends to change depending on the state of the video signal obtained by imaging. - Generally, in the inside method, it is desirable to extract only the portion corresponding to white from the video signal to obtain white balance, but it is difficult to extract only the white signal from the video signal that changes over time. Furthermore, since the color temperature cannot be directly determined, it is difficult to accurately determine and extract a white portion with intermittent color temperature changes from orange to blue. For this reason, in simple video cameras that use single-focal-length lenses that do not have a zoom-up function, the idea has been that if the entire screen is averaged, the image will be dispersed around white.

However, even if such a concept is adopted, there will still be severe color changes when taking a close-up photo of monochromatic light, and to avoid this, an algorithm for determining white areas is required.

As one of these algorithms, the idea of a white detection fully automatic white balance control method has been proposed, in which only the part corresponding to the color temperature locus of the white part is extracted and the white balance is controlled using this signal. This white detection fully automatic white balance control method will be briefly mentioned below.

This method is based on the idea that when only the brightness of objects of the same color changes, the color difference signal that serves as the video camera's color information and the R intensity signal that distinguishes brightness and darkness are approximately proportional. The divided value can be used as a measure to express the color regardless of brightness, and this method is used to extract the portion corresponding to the change in white color temperature using this measure to obtain white balance.

Figure 4 shows (R-Y)/Y on the vertical axis and (B-Y)/Y on the horizontal axis.
It is a diagram plotting the locus of movement due to changes in illumination light at the positions indicated by each color light of the color bar on a two-dimensional coordinate plane formed by . It has become From this coordinate plane, extract the signal in the range corresponding to the color temperature change of the white signal 1
If you use that signal to perform white balance, you can eliminate the effects of highly saturated objects and perform more accurate white balance control. For this purpose, the following relational expression is derived from FIG. 4 as a white signal region.

(B-Y) /Y>a ・・・・・・・・・・・・・・・
・・・・・・・・・・・・(1)(R-Y) /Y>b
・・・・・・・・・・・・・・・・・・・・・・・・
(2) 1/bx (RY)/Y+ 1/aX (B-Y)/Y>1-4-(3) 1/cx
(RY)/Y+ 1/dx (B-Y)/Y<1 (4
〉If the above equations (1) to (4) are rewritten, they become as follows.

1/a (B-Y)>Y ・・・・・・・・・・・・
・・・・・・・・・(5) 1/b (RY)>Y
・・・・・・・・・・・・・・・・・・・・・(6〉1/
b (RY) +1/a (B-Y) >Y-...
・・・・・・・・・・・・・・・(7) 1/C(RY)
+1/d (B-Y)<Y・・・・・・・・・・・・
(8) The area where all of these formulas (5) to (8) hold true becomes the white signal area, and detection of this area can be realized using a plurality of comparison circuits.

FIG. 5 is a block diagram showing an example of a conventional white balance circuit created in consideration of the above.

R (red) obtained directly from the camera circuit not shown
, B (blue) signals are input to subtracters 3 and 4 via gain control amplifiers 1 and 2.

Since the Y (luminance) signal obtained directly from the camera circuit is separately input to the subtracters 3 and 4, these subtracters perform subtraction of R-Y and B-Y. From R-
The Y signal is input to the clamp 22 via the capacitor 19, and the BY signal from the subtracter 4 is input to the clamp 24 via the capacitor 21. Clamps 22 and 24 clamp each input signal and then output these signals to gate 6.7. Therefore, the RY signal is input to the gate 6, and the BY signal is input to the gate 7. On the other hand, the white cane output circuit 5 receives the R-Y signal from the subtracter 3, the B-Y signal from the subtracter 4, and the Y signal from the camera circuit directly. As mentioned above (
A case in which all of equations 5) to (8) hold true is detected, and this detection signal is output to the gate 6.7°25. Note that this circuit 5 can be constructed by a combination of simple comparison circuits. Therefore, the gate 6°7.25 is opened by the detection signal outputted from the white cane output circuit 5, and the gate 6 extracts the portion corresponding to white from the RY signal and outputs it to the low-pass filter 8. Similarly, the gate 7 extracts a portion corresponding to white from the BY signal and outputs it to the low-pass filter 10. Further, since the clamp 23 receives a reference voltage (earth level in this case) via the capacitor 20, it generates a predetermined reference voltage and outputs it to the gate 25. As a result, when the gate 25 receives the detection signal from the white cane output circuit 5, it outputs this reference signal to the low-pass filter 9 for integration, and this integrated reference signal is sent to the comparison circuit 11.12.
is applied to one input terminal of On the other hand, the signal passing through the gate 6.7 is integrated by the low-pass filters 8 and 10, and becomes an average DC voltage, and the comparator circuit it,
It is input to 12 other input terminals. Comparison circuits 11 and 12
compares the DC voltage input from the low-pass filters 8 and 10 with the reference voltage input from the low-pass filter 9, and sets the output to a high level if the input DC voltage is higher than the reference voltage, If it is smaller, it is set to low level, and these output signals are input to up/down counters 13 and 14, respectively. up/down counter 1
3.14 increases the count value when the output of the comparator 11.12 becomes high level, and decreases the count value when it becomes low level. These count values are each converted into a Luli pressure by a D/A converter 15.16, and then applied to the control terminals of the gain control amplifiers 1 and 2 via an amplifier 17.18. The gain control amplifiers 1 and 2 reduce the gain by the control voltage when the count value of the up/down counters 13.14 (input) is increasing, and when the count value of the up/down counters 13.14 is decreasing. Control is performed to increase the gain.As a result, the levels of the color difference signals R-Y and B-Y are converged to zero and white balance is maintained.The reason why the color difference signals are integrated by the low-pass filters 8 and 10 is as follows. This is to make the average value of the color difference signal close to the color difference signal at the time of achromatic color imaging.

However, in the circuit shown in Figure 5 above, the area for detecting white is determined by the color difference signal and luminance signal, which are white-balanced by the two gain control amplifiers 1 and 2, so the white balance is determined at each color temperature. This inevitably has the drawback that there will be a large difference in the surrounding area that is judged to be white.

Figure 6 (A>, (B), and (C) are the same as the two-dimensional coordinate planes shown in Figure 4, with 3000 K and 450 K, respectively.
0 and 6000 are shown, and the white detection area is set to include all of them. As is clear from the figure, due to the white balance control, there are considerable unnecessary areas in the low color temperature region A in FIG. 6 (A>) and in the high color temperature region C in the same figure (C>. In Fig. 6(C) as well, it was actually necessary to set a detection range considerably wider than the color temperature change range of the smallest white part shown in Fig. 4.Of course, the above-mentioned white balance control circuit Now, the above change can be alleviated by adding upper and lower limits to the control voltage applied to the gain control amplifier 1.2, but the chromatic colors included in the wide area enter the signal gated for white detection. This causes the white balance to change, making it impossible to always perform proper white detection and making it impossible to perform accurate white balance control.

(Problem to be Solved by the Invention) As described above, only the signal existing in the white area is extracted from the two color difference signals R-YSB-Y, and the R and B signals are adjusted so that the signal becomes zero. In a circuit that limits the gain to obtain white balance, when the white balance pull-in operation is performed for white under low or high color temperature or special light sources, the white detection range expands, making it difficult to properly detect white. This has the disadvantage that white balance control becomes less accurate. Therefore, the present invention aims to eliminate the above-mentioned drawbacks, and by preventing the white detection area from expanding even for white at low or high color temperatures and under special light sources, it is possible to always perform appropriate white detection. It is an object of the present invention to provide a white balance control circuit that can perform highly accurate white balance control.

``Creation of the Invention (Means for Solving the Problems)'' The white balance control circuit of the present invention is characterized by the fact that the input red color (
The luminance signal (Y) is subtracted from the first gain control amplification means that independently varies the levels of the R) and blue (B) signals, and the R and B signals obtained from the first gain control amplification means.
subtraction means for producing color difference signals of (RY) and (B-Y); and (R-Y) and (B-Y) outputted from this subtraction means.
A white signal is generated from the (RY), (B-Y) signals output from the second gain control amplification means and the Y signal, which are output from the second gain control amplification means and the Y signal. white detection means for detecting an area and outputting a white detection signal;
During the period when the white detection signal output from the white detection means is input, the subtraction means outputs (RY), (
B-Y) signal is input and the (R-Y) corresponding to that level is input.
) side and (B-Y) side control signal, and a (R-Y) side control signal outputted from the white balance control means in order to vary the level of the input R signal. Also, the (B-Y) side control signal is fed back to the first gain control amplification means to vary the level of the input B signal, and the (R-Y) side control signal is fed back from the subtraction means. In order to vary the level of the (RY) signal outputted from the subtracting means, and to vary the level of the (B-Y) signal outputted from the subtracting means, the control signal on the listening (B-Y) side It has a configuration comprising a signal feedback means for feeding back to the second gain control amplification means.

(Function) In the white balance control circuit of the present invention, the first gain control amplification means independently varies the levels of the input red (R) and blue (B) signals, and then outputs these signals to the subtraction means. do. The subtraction means subtracts the luminance signal (Y) from the input R and B signals to create (RY) and (B-Y) color difference signals, and these color difference signals are transmitted to the second gain control amplification means. and a white balance control means. Second
The gain control amplification means of is inputted ('R-Y), (B-
Y) The levels of the signals are varied independently and these signals are output to the white detection means. The white detection means is input (
A white signal area is detected from the Y signal and the Y signal, and a white detection signal is output to the white balance control means. The white balance $lIwJ means outputs (RY), (B-Y) from the subtraction means during a period when the white detection signal output from the white detection means is input.
) signal, generates (RY) side and (BY) side control signals corresponding to the level, and outputs them to the signal feedback means. The signal feedback means is output from this white balance control means (R-
A control signal on the (Y) side is fed back to the first gain control amplification means in order to vary the level of the input R signal, and a control signal on the (B-Y) side is fed back to the first gain control amplification means in order to vary the level of the input B signal. At the same time, in order to vary the level of the (RY) signal outputted from the subtracting means, the control signal on the same (RY) side is outputted from the subtracting means. (B-Y) signal is fed back to the second gain control amplification means to vary the level of the (B-Y) signal.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings, in which the same parts as those of the conventional example are denoted by the same reference numerals. FIG. 1 is a block diagram showing an embodiment of a white balance control circuit according to the present invention. 1, 2, 26, and 27 are gain control amplifiers whose gains can be varied (G, C, A>, 3 and 4 are subtractors that perform subtraction between two signals, 5 is the two input color difference signals R-Y, B
6.7.25 is a gate that opens when the white output circuit 5 detects a white area and allows the signal to pass; 8.9゜1
0 is a low-pass filter (LPl") that integrates the input signal
, 11.12 is a comparator that compares the input signal with a reference signal and outputs the result; 13.14 is an up/down counter that counts up when the input signal is high level and counts down when it is low level; 15 .16 is a D/A converter that converts digital signals into analog signals; 1
7.18 is an amplifier that amplifies the input signal to a predetermined level;
19.20.21 is a coupling capacitor, 22.23.24
is a clamp that clamps the input signal.

Next, the operation of this embodiment will be explained. R and B signals inputted from a camera circuit etc. (not shown) are inputted to subtracters 3 and 4 via gain control amplifiers 1 and 2, and the luminance signal Y from the camera circuit etc. is inputted to the subtracters 3 and 4. Therefore, from these subtracters 3 and 4, R-YSB-Y
color difference signals are output respectively. The color difference signal R-Y is input to the clamp 22 via the capacitor 19, and is also input to the white peach output circuit 5 via the gain control amplifier 26. Further, the color difference signal B-Y is input to the clamp 24 via the capacitor 21, and is also input to the white peach output circuit 5 via the gain control amplifier 27. The white peach output circuit 5 detects a white signal region from the input R-Y and B-Y color difference signals, and outputs a detection signal to a gate 6°7.25. Therefore, the low-pass filter 8°10 receives the color difference signals R-Y, B-
The signals of the white portion of Y are input, and these signals are integrated by each low-pass filter to become an average DC voltage, which is input to comparators 11 and 12 and output from the low-pass filter 9 as a reference voltage. be compared. If the input DC voltage is larger than the reference voltage, the comparators 11.12 output a high level signal to cause the up/down counter 13.14 to count up, and if it is smaller, they output a low level signal to count up the up/down counters 13.14. Down counter 13
．． Make them count down to 14. The count value of the up/down counter 13.14 is converted into a control voltage by a D/A converter 15.16, and then sent to the gain control terminals of gain control amplifiers 1, 2, and 26°27 via amplifiers 17.18. applied. The gain control amplifier 1.2 controls the gains of R and F3M@ with respect to the luminance signal Y using the gain control voltage so that the color difference signals of R-Y and B-Y become zero,
So-called white balance control is performed.

Here, the self-signal region in the Hakuto output circuit is as described above (5
) to (8), but if white balance control is performed at each color temperature, it is necessary to reduce unnecessary areas and reset to appropriate areas. This resetting is performed using the gain coefficient a− of the color difference signal in equations (5) to (8) above.
This can be done by varying d. Therefore, in this example, the level of the R-Y and B-Y color difference signals input to the white peach output circuit 5 is varied by the gain control amplifiers 26 and 27 to perform the above resetting. That is, when the color temperature is low, in order to narrow the detection range in the low color temperature region, (
5) In addition to controlling to increase the coefficient a in the equation (6), i.e., decreasing the gain of the gain control amplifier 27, in order to widen the detection range in the high color temperature region, control to reduce the coefficient a in the equation (6), i.e. Control is performed to increase the gain of the gain control amplifier 26.

Also, when the color temperature is high, the reverse control to the above is performed,
The gain of the gain control amplifier 26 is lowered, and the gain of the gain control amplifier 27 is lowered.
Control is performed to increase the gain of.

Figure 2 shows the above (5) when the control voltage applied to the gain control amplifier 1.2.26.27 corresponds to the color temperature coefficient.
~ (8) In order to determine the coefficients a to d of formula 8, an example of the ratio of the color difference signal to the luminance signal is shown. By giving the color difference signal a variable gain characteristic as shown in this figure,
As shown in FIG. 3, it is shown that even if a white-balanced color difference signal is used in a specific color temperature state, the area can always be reset to an appropriate white detection game 1.

Furthermore, as is clear from Fig. 3, even in a special light source region (for example, under a fluorescent lamp) that exists approximately parallel to the color temperature change curve obtained from equations (7) and (8) above, the color temperature at that time changes. It is also possible to track by changing the slope according to the state curve.

According to the present embodiment, the gain control amplifiers 26 and 27 control the levels of the color difference signals R-Y and B-Y input to the white peach output circuit 5 in response to changes in color temperature.
Since it is possible to prevent the white signal area from expanding, it is possible to always perform appropriate white detection and perform highly accurate white balance control.

[Effects of the Invention] As described above, according to the white balance control circuit of the present invention, even when the white color temperature is low or high, or under a special light source, the white signal area is prevented from expanding, so that the white balance control circuit always maintains This has the effect of performing appropriate white detection and performing highly accurate white balance control.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing an embodiment of the white balance control circuit of the present invention, and Figure 2 is a gain control that controls the level of the color difference signal when the gain control voltage in Figure 1 is converted to color temperature. Figure 3 is a characteristic diagram showing the gain change of the amplifier. Figure 3 is a diagram showing the white detection gate area using a color difference signal whose gain is controlled using the circuit in Figure 1, using color temperature as a parameter.
The figure is a color temperature locus diagram on the RY/Y and B-Y/Y planes, Figure 5 is a block diagram showing an example of a conventional white balance control circuit, and Figure 6 is a white balance diagram in the circuit of Figure 5. FIG. 7 is a diagram showing a white detection gate region during balance control using color temperature as a parameter. 1, 2.26.27... q control amplifier 3.4...
・Subtractor 5...White peach output circuit 6, 7.25...Gate 8, 9.10...Low pass filter 13, 14...Up/down counter 15, 16...
・D/A converter diagram γ

Claims (1)

[Claims] A first gain control amplification means that independently varies the levels of input red (R) and blue (B) signals, and a luminance signal (Y
) to create color difference signals of (R-Y) and (B-Y);
), (B-Y) signals, and a second gain control amplification means for independently varying the levels of the (R-Y) and (B-Y) signals output from the second gain control amplification means. Said Y
A white detection means detects a white signal area from the signal and outputs a white detection signal, and during a period when the white detection signal output from the white detection means is inputted, the white detection signal is output from the subtraction means (RY ), (B-Y) signal and generates control signals on the (R-Y) side and (B-Y) side corresponding to the level thereof; the first gain control amplification in order to use a control signal on the (RY) side to vary the level of the input R signal, and a control signal on the (B-Y) side to vary the level of the input B signal; In order to feed back the (RY) side control signal to the subtracting means and to vary the level of the (RY) signal output from the subtracting means,
side control signal is output from the subtraction means (B-Y)
A white balance control circuit comprising: signal feedback means for feeding back to the second gain control amplification means in order to vary the level of the signal.