JPS63115490A - White balance auto control circuit - Google Patents

Abstract

PURPOSE:To substantially reduce color blurring in case an object including a part with strong chroma is photographed by permitting gate circuits to block color difference signals outputted from mixing circuits if they have amplitudes exceeding a prescribed one and insuring the signals against being given to mean circuits. CONSTITUTION:The titled circuit is provided with 1st and 2nd detection circuits 18 and 19 to which the color difference signals are given from the 1st and 2nd mixing circuits 8 and 9, and the 1st and 2nd gate circuits 16 and 17 blocking outputs from the mixing circuits 8 and 9 based on outputs from the detection circuits 18 and 19. If either of the color difference signals R-Y and B-Y exceeds the prescribed amplitude, the detection circuit 18 or 19 gives a detection signal to an OR circuit 20 during the exceeding session. The OR circuit 20 outputs gate pulses to the gate circuits 16 and 17. Without the gate pulses given, the gate circuits 16 and 17 are turned on to output the color difference signals from the mixing circuits 8 and 9 to the mean circuits 12 and 13. With the gate pulses given from the OR circuit 20, the circuits 16 and 17 block the outputs from the mixing circuits 8 and 9 during that session.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to an auto white balance control circuit that adjusts white balance in a video camera or the like so that colors are faithfully reproduced according to changes in color temperature.

<Prior Art> FIG. 4 is a block diagram of a conventional auto white balance control circuit. In the figure, ■ is incident light from the subject, 2 is the photographing lens, 3 is the image sensor, and 4 is the image sensor 3.
A color separation circuit 5 separates and outputs a red (R) signal and a blue (B) signal from the output of the image sensor 3;
) a low-pass filter that separates the signals; 6.7 is a red signal and blue signal gain control circuit that controls the gain of the red and blue signals respectively in response to the gain control signal; 8 is a gain-controlled red signal and low-pass filter; A first mixing circuit mixes the luminance signal from 5 and outputs the R-Y color difference signal, and 9 mixes the gain-controlled blue signal and the luminance signal from the low-pass filter 5 to output the B-Y color difference signal. a second mixing circuit for outputting; 10 a color encoder for outputting a composite color video signal based on the color difference signal and the luminance signal; 11;
is the output terminal of the composite color video signal, 12.13 is R
-The first one, which takes the average value of the level of each color difference signal Y, B-Y;
Second average value circuit, 14.15 is each average value circuit 12.13
The first one amplifies the error amount and outputs it as a gain control signal.
This is a second error amplification circuit.

In this auto white balance control circuit, R-
The average value of the Y and B-Y color difference signals, that is, the first . The white balance is adjusted by controlling the gains of the red signal and the blue signal so that the output of the second average value circuit 12.13 becomes zero.

However, with conventional auto white balance control circuits like this, if there is a highly saturated part of the subject (highly saturated part), color shift occurs and correct color reproduction cannot be achieved. The problem is that it is difficult to do.

For example, as shown in FIG. 5, in order of strength of saturation, a red subject R1 with strong saturation, a red subject R2 with medium saturation,
Considering the case where a subject is photographed in which a red subject R3 with weak saturation is placed at a rate of 1/6 of the imaging surface, and a white subject W is placed in the remaining area, in this case, , the R-Y color difference signal and the B-Y color difference signal corresponding to the subject have a staircase waveform as shown in FIGS. 6(A) and 6(B), respectively.

In the auto white balance control circuit shown in Figure 4, this R
-Y, B-Y color difference signal average value, that is, Fig. 6 (A
) (B) - average value level CI shown by the dotted chain line,
Since C2 is controlled to be zero, the white balanced RY and BY color difference signals after white balance control are as shown in FIG. 6 (CXD). As is clear from FIG. 6 (C) and (D),
The part of the white object W has a negative value in the R-Y color difference signal,
The BY color difference signal takes a positive value, resulting in a muddy cyan color. In addition, red objects R1, R
The amplitude decreases from the 2°R3 portion, and the level of the intercolor difference signal becomes zero, that is, white, in the portion of the weakly saturated red object R3. In addition, in FIG. 6, the amplitude relationship is also shown, and in FIG. 6, IH shows the horizontal scanning period, A shows the effective video period, and B shows the horizontal blanking period.

In the conventional auto white balance control circuit that adjusts the white balance by controlling the average value of the R-Y and B-Y color difference signals to zero, When photographing an object with a large occupancy of a specific color area, the average value level of the color difference signal shifts as a result, resulting in color shift as described above, making it difficult to reproduce colors correctly. I won't be able to do it.

<Object of the Invention> The present invention has been made in view of the above-mentioned points, and aims to reduce the above-mentioned color shift that occurs when photographing a subject having a highly saturated portion, and to reproduce colors correctly. An object of the present invention is to provide an auto white balance control circuit that allows automatic white balance control.

<Structure of the Invention> In order to achieve the above-mentioned object, the present invention includes a red signal gain control circuit, a blue signal gain control circuit, and a red signal and a luminance signal from the red signal gain control circuit that are mixed. R
- a first mixing circuit that outputs a Y color difference signal, and a blue signal and a luminance signal from the blue signal gain control circuit are mixed;
- a second mixing circuit that outputs a Y color difference signal, and a first mixing circuit that outputs an average value of each of the color difference signals. a second average value circuit, and a first average value circuit for error amplifying the outputs of the respective average value circuits.
．． a second error amplification circuit, and controls the white balance by giving the output of each of the error amplification circuits as a gain control signal to each of the corresponding gain control circuits; When the amplitude of each color difference signal to be output exceeds a predetermined amplitude, the first . between the second detection circuit, the first mixing circuit and the first average value circuit;
A first filter is provided between the mixing circuit and the second average value circuit, and cuts off the output of each mixing circuit based on the detection signal from the detection circuit. and a second gate circuit.

<Example> Hereinafter, an example of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram of an embodiment of the present invention,
Portions corresponding to the conventional example in FIG. 4 are given the same reference numerals. In the figure, l is incident light from the subject, 2 is a photographing lens, 3 is an image sensor, 4 is a color separation circuit that separates and outputs a red (R) signal and a blue (B) signal from the output of the image sensor 3; 5 is a low-pass filter that separates the luminance (Y) signal from the output of the image sensor 3; 6.7 is a red signal and blue signal gain control circuit that controls the gains of the red signal and blue signal, respectively, in response to the gain control signal; 8 is a first mixing circuit that mixes the gain-controlled red signal and the luminance signal from the low-pass filter 5 and outputs an R-Y color difference signal; 9 is the gain-controlled blue signal and the luminance signal from the low-pass filter 5; a second mixing circuit that mixes the B-Y color difference signal and outputs the B-Y color difference signal;
0 is a color encoder that outputs a composite color video signal based on the color difference signal and the luminance signal, l! 12.13 is the output terminal of the composite color video signal, and 1.13 is the output terminal of the composite color video signal. The second average value circuit 14.15 amplifies the error of each average value circuit 12.13 and outputs it as a gain control signal. This is a second error amplification circuit.

This configuration is similar to the conventional example, and is RY.

The average value of the B-Y color difference signal, that is, the first . Similarly to the conventional example, white balance is adjusted by controlling the gains of the red signal and the blue signal so that the output of the second average value circuit 12.13 becomes zero.

The auto white balance control circuit of this embodiment has a first
．． The first mixing circuit 8.9 receives each color difference signal from the second mixing circuit 8.9. Second detection circuit 18.19 and rain detection circuit 18
．． 19, and a logical sum circuit 20 which takes the logical sum of the outputs of the first and second mixing circuits 8. and second gate circuits 16 and 17.

1st. The second detection circuit t8.19 outputs a detection signal over a period when the amplitude of each color difference signal exceeds a predetermined amplitude corresponding to a highly saturated subject.

FIG. 2 is a diagram showing the characteristics of this detection circuit 18.about.19, where the horizontal axis shows the input and the vertical axis shows the output. The detection circuits 18 and 19 output positive detection signals by setting VSI as a threshold level for color difference signals of positive polarity, and by setting VS2 as a threshold level for color difference signals of negative polarity.

That is, when the amplitude of the R-Y color difference signal from the first mixing circuit 8 exceeds a predetermined amplitude, the first detection circuit 18 outputs a detection signal to the OR circuit 20 over the period, and similarly, When the amplitude of the BY color difference signal from the second mixing circuit 9 exceeds a predetermined amplitude, the second detection circuit I9 outputs a detection signal to the OR circuit 20 over the period.

The logical sum circuit 20 calculates the logical sum of the outputs of the rain detection circuits 18 and 19 and calculates the first . It is applied as a gate pulse to the second gate circuits 16 and 17. Therefore, when either the R-Y color difference signal or the B-Y color difference signal exceeds a predetermined amplitude, the OR circuit 20 detects the first or second detection circuit 18.
A detection signal is applied from the first . Second gate circuit 16
．． A gate pulse will be output to 17.

between the first mixing circuit 8 and the first average value circuit 12 and the second
The first . When the gate pulse is not applied, the second gate circuits 16.17 turn on and output each color difference signal from each mixing circuit 8.9 to each average value circuit 12.13, while the OR circuit When a gate pulse is given from 20, it turns off in response to the gate pulse,
The output of each mixing circuit 8.9 during that period is cut off.

Therefore, in this embodiment, the RY color difference signal or the B
- When the amplitude of one of the Y color difference signals exceeds a predetermined amplitude corresponding to a highly saturated subject,
During that period, the inter-color difference signal is the first. Second gate circuit 1
Both will be cut off on June 17th.

In this way, when either the R-Y color difference signal or the B-Y color difference signal exceeds a predetermined amplitude corresponding to a highly saturated subject, the inter-color difference signal is cut off for that period, and if the amplitude is below the predetermined amplitude, White balance control is performed by applying only color difference signals corresponding to areas with low saturation to each average value circuit 12 and 13, thereby reducing the influence of objects with high saturation.

Next, the operation of the auto white balance control circuit having the above configuration will be explained by taking as an example the case where the subject shown in FIG. 5 is photographed as in the conventional example. These subjects are arranged in order of strength of saturation: a red subject R1 with strong saturation, a red subject R2 with medium saturation, and a red subject R3 with weak saturation.
/6, and a white object W is placed in the remaining portion. Here, the 'RY color difference signal will be explained. The waveform of the RY color difference signal before white balance control corresponding to this subject is shown in FIG. 3(A). Moreover, FIG. 3(B) shows the waveform of the RY color difference signal after white balance control in the same conventional example as FIG. 6(C).

In the auto white balance control circuit of this embodiment, when the amplitude of the R-Y color difference signal shown in FIG. 3(A) exceeds a predetermined amplitude VSI, the first detection circuit 18 detects the The signal is output to the OR circuit 20, and thereby the OR circuit 20 provides the gate pulse shown in FIG. 3(C) to the first gate circuit 16. In response to this gate pulse, the first gate circuit 16 is turned off, and the color difference signal exceeding the predetermined amplitude VSI is cut off.
The RY color difference signal shown by the solid line in D) is provided, and the white balance is adjusted by gain control so that the average value of this RY color difference signal becomes zero.

Therefore, the waveform of the white-balanced RY color difference signal after white balance control is as shown in FIG. 3(E). As is clear from comparing this Fig. 3 (E) with the conventional example shown in Fig. 6 (C), the subject R with weak saturation
It can be seen that the portion 3 is also slightly balanced with red points, the degree of cyan color created from the white object W portion is reduced, and color shift is significantly improved.

In this auto white balance control circuit, the OR circuit 20 also outputs the B-Y color difference signal from the second mixing circuit 9.
Since color difference signals exceeding a predetermined amplitude are blocked by the second gate circuit 17 due to the gate pulses from the second gate circuit 17, the influence of objects with strong chroma is reduced, similar to the R-Y color difference signal.

In addition, in FIG. 3, the magnitude relationship of the amplitude is also shown, and in FIG. 3, IH is the horizontal scanning period,
A indicates an effective video period, and B indicates a horizontal blanking period.

In the embodiment described above, each gate circuit 16.17 is controlled by the output of the OR circuit 20, but in another embodiment of the present invention, the OR circuit 20 is omitted and each detection circuit 18. Each gate circuit 16.1 with the output of 19
7 may be controlled respectively.

<Effects of the Invention> As described above, according to the present invention, when the amplitude of each color difference signal output from each mixing circuit exceeds a predetermined amplitude,
Since it is blocked by a gate circuit and is not applied to each average value circuit, the color shift that occurs when photographing a subject with a highly saturated area is significantly reduced, and the color reproducibility is improved. It will improve.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an embodiment of the present invention, Fig. 2 is a characteristic diagram of a detection circuit, Fig. 3 is a signal waveform diagram for explaining the operation of the embodiment of Fig. 1, and Fig. 4 is a diagram of a conventional example. FIG. 5 is a block diagram showing a subject, and FIG. 6 is a signal waveform diagram for explaining the operation of the conventional example. 6...Red signal gain control circuit, 7...Blue signal gain control circuit, 8,9゛...1st. second mixing circuit, 12,
13...1st. 2nd average value circuit, 14.15...1st. second error amplification circuit, 16.17...first. 2nd gate circuit, 18.19... 1st. second detection circuit, 20
...Order circuit.

Claims (1)

[Claims] (1) a red signal gain control circuit, a blue signal gain control circuit, and a first mixing circuit that mixes the red signal and luminance signal from the red signal gain control circuit to output an R-Y color difference signal; a second mixing circuit that mixes the blue signal and the luminance signal from the blue signal gain control circuit and outputs a B-Y color difference signal; and a first mixing circuit that outputs an average value of each of the color difference signals.
Each of the gain control circuits includes a second average value circuit, and first and second error amplification circuits for error amplifying the output of each of the average value circuits, and corresponds to the output of each of the error amplification circuits as a gain control signal. In the auto white balance control circuit that controls the white balance by applying signals to each of the mixing circuits, the first and second circuits each output a detection signal when the amplitude of each color difference signal output from each of the mixing circuits exceeds a predetermined amplitude. 2 detection circuits are respectively provided between the first mixing circuit and the first average value circuit and between the second mixing circuit and the second average value circuit, and each An auto white balance control circuit comprising first and second gate circuits that respectively cut off the output of the mixing circuit.