JPH07143509A - Chromanoise suppressing method for video camera - Google Patents

Abstract

PURPOSE:To suppress the generation of chromanoise when performing photographing at low illuminance. CONSTITUTION:An image pickup signal S corresponding to an optical image formed by a lens 1 passing through an iris 2 is outputted from a CCD 3, and at the time of low illuminance, the image pickup signal S is amplified by an AGC circuit 5. A Y/C separating circuit 6 performs filtering to the image pickup signal S and outputs a luminance signal Y and color synthesizing signals Cr and Cb. At low-illuminance photographing, a chrominance signal processing circuit 20 respectively adds offset alpha to respective primary color signals R, G and B provided by matrix arithmetic at a matrix arithmetic circuit 21, by using an offset adding circuit 22. Thus, a gamma correcting circuit 23 performs correction to primary color signals R+alpha, G+alpha and B+alpha with offset, and color difference signals R-Y and B-Y are formed by a chrominance signal processing circuit 24.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for suppressing chroma noise in a video camera, which suppresses chroma noise even when photographing in low illuminance and enables good photographing.

[0002]

2. Description of the Related Art In video cameras, automatic exposure (A
E: automatic exposure) circuit is provided.

In an AE circuit mounted on a video camera, a luminance signal of an image pickup signal is integrated to obtain a luminance signal integrated value, and a photometric value (AE photometric value) is obtained based on the luminance signal integrated value.
Further, the deviation between this AE photometric value and the target (this value is set in advance, and the optimum exposure is obtained when the AE photometric value becomes equal to the target value) is obtained. And the magnitude and direction of the deviation (= "target value"-"photometric value") from the neutral point (positive,
Negative) to obtain the iris control value and send the iris control value to the iris meter. In the iris meter, rotate the ring at a speed according to the iris control value (rotate in the direction to close the iris if the iris control value is large, and rotate in the direction to open the iris if the iris control value is small) to open or close the iris. , The exposure is controlled. If the exposure is insufficient even when the iris is fully opened (during low-illuminance shooting), the gain of the image pickup signal is increased by the AGC circuit to compensate for the shortage.

As will be described later in detail, in the color signal processing circuit of the video circuit, red, green, and blue primary color signals R, G, and B are formed by matrix calculation, and the primary color signals R, G, and B are formed.
White balance control calculation and gamma (γ) correction are performed on G and B to form color difference signals RY and BY.

The gamma correction is a correction for processing so as to obtain a good image in accordance with the characteristics of a cathode ray tube of a television, and the amplification factor is large at a low signal level (amplitude) and becomes high as the signal level becomes high. Amplification processing is performed by a circuit having a non-linear amplification characteristic that reduces

Therefore, when the color signal shown in FIG. 3A is subjected to γ correction processing, it becomes a signal as shown in FIG. 3B, and the signal in the low level region is greatly raised (amplified).

[0007]

By the way, during low-illuminance shooting, the gain is increased by the AGC circuit, and the low-level signal is greatly amplified by the γ correction circuit, so that the color signal (chroma signal) is greatly amplified. At the same time, noise is greatly amplified. As a result, the S / N ratio is greatly reduced especially in the low level region of the color signal. This chroma noise disturbs the color of the reproduced image.

In view of the above-mentioned prior art, it is an object of the present invention to provide a chroma noise suppressing method for a video camera, which can suppress the chroma noise even when photographing is performed at low illuminance.

[0009]

SUMMARY OF THE INVENTION The structure of the present invention for solving the above problems determines the illuminance of a subject on the basis of a value obtained by integrating a luminance signal, adjusts the opening of the iris, and opens the iris fully. Even if it is determined that the exposure is insufficient,
Raise the amplification factor of the automatic gain adjustment circuit that amplifies the image pickup signal output from the image pickup device, and increase the amplification factor of the automatic gain adjustment circuit in a video camera that performs gamma correction on the color signal in the color signal processing circuit. In this case, an offset is added to the color signal before gamma correction.

[0010]

According to the present invention, an offset is added to the primary color signal before gamma correction at the time of low-illuminance shooting, and the gamma correction is performed on the primary color signal to which the offset is added.
Equivalently lowers the gain in the low signal level area to suppress chroma noise.

[0011]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows an image pickup system of a video camera according to an embodiment of the present invention. As shown in the figure, the optical image formed by the lens 1 and passing through the iris 2 is formed on the light receiving surface of the CCD 3. Complementary color filter (Ye, Mg, Cy, G)
The image pickup signal S is output from the CCD 3 provided with. The image pickup signal S is sampled and held by the sample and hold circuit 4, and the gain is adjusted by the AGC (automatic gain adjustment) circuit 5 as necessary. Furthermore, the image pickup signal S is Y / C
The separation circuit (filter) 6 separates the luminance signal Y into the color synthesis signals C _r and C _b .

The brightness signal Y is input to the brightness signal processing circuit 10, and the brightness signal Y and the color composite signals C _r and C _b are input to the color signal processing circuit 20.

In the brightness signal processing circuit 10, the brightness signal Y is gamma-corrected by the gamma correction circuit 11, and the brightness signal processing unit 12 performs processing such as horizontal contour compensation and addition of a synchronization signal on the gamma-corrected brightness signal and outputs the result. To do.

The integrating circuit 7 integrates the brightness signal Y to obtain a brightness integrated value IY. The brightness integrated value IY corresponds to the exposure amount. The camera microcomputer 8 uses the integrated luminance value I
A photometric value is obtained based on Y, and an iris control value E _C according to the deviation between the photometric value and the target value is obtained and output. Iris 2
Changes the iris opening according to the iris control value E _c . The iris opening is the iris sensor (hall element).
The iris opening degree E _o detected by 9 is sent to the camera microcomputer 8. The camera microcomputer 8 sends a gain-up command G _A to the AGC circuit 5 when it is determined that the iris 2 will be fully exposed even if the iris 2 is fully opened. AG
The C circuit 5 has a gain corresponding to the gain-up command G _A ,
The image pickup signal 5 is amplified (gain up).

On the other hand, in the color signal processing circuit 20, the matrix circuit 21 causes the luminance signal Y and the color composite signals C _r and C _b.
Matrix calculation is performed to form a red signal R, a green signal G, and a blue signal B. The offset adding circuit 22, which is a feature of the present invention, adds an offset α to each primary color signal R, G, B. The value of the offset α is controlled by the camera microcomputer 8 and corresponds to the value of the gain-up command G _A.
That is, when the gain of the AGC circuit 5 is 1, the value of the offset α is set to zero, and the value of the offset α is increased in the positive direction as the AGC gain increases (in the low illuminance shooting). The maximum value of the offset α is about 10% of the dynamic range. Since the offset α is added in this way, for example, as shown in FIG.
The primary color signals R, G, B shown in (a) become offset-added primary color signals R + α, G + α, B + α as shown in FIG. 2 (b).

The gamma correction circuit 23 is an amplification circuit having a non-linear amplification characteristic in which the amplification factor is large when the signal level is low and decreases as the signal level becomes high, and the offset primary color signals R + α, B + α and G + α are supplied. Gamma correct. In this case, since the value of the offset α becomes large during low-illuminance shooting, the primary color signal with offset R +
The values of α, B + α, and G + α become relatively large with respect to noise, and the gamma-corrected signals R + α, G + α,
The gain of the low signal level portion of B + α is equivalently decreased, and the S / N ratio is increased particularly at a low signal level. Therefore, it is possible to suppress the chroma noise even when the image is captured with low illuminance.

The color signal processing section 24 performs signal processing on the gamma-corrected primary color signals with offset R + α, G + α, B + α to form and output color difference signals RY, BY.

[0019]

As described in detail with the embodiments, the present invention adds a positive value offset to each primary color signal before gamma correction when shooting at low illuminance. The gain of a portion of the subsequent signal having a low signal level is equivalently reduced, and the chroma noise generated by the gain increase (AGC and gamma correction) is suppressed, and good photographing can be performed.

[Brief description of drawings]

FIG. 1 is a block diagram showing a video camera imaging system according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a state where an offset is added.

FIG. 3 is an explanatory diagram showing a state of gamma correction.

[Explanation of symbols]

1 lens 2 iris 3 CCD 4 sample hold circuit 5 AGC (automatic gain adjustment) circuit 6 Y / C separation circuit 7 integrating circuit 8 camera microcomputer 9 iris sensor 10 luminance signal processing circuit 11 gamma (γ) correction circuit 12 luminance signal processing unit 20 color signal processing circuit 21 matrix circuit 22 offset adding circuit 23 gamma correction circuit 24 color signal processing section S imaging signal Y luminance signal C _r , C _b color composite signal IY luminance integrated value E _c iris control value E _o iris opening G _A Gain up command α Offset R, G, B primary color signal RY, BY color difference signal

Claims (2)

[Claims] 1. When the illuminance of a subject is determined based on a value obtained by integrating a luminance signal to adjust the opening of the iris, and when it is determined that the exposure is insufficient even when the iris is fully opened, the image sensor outputs it. In a video camera that increases the amplification factor of the automatic gain adjustment circuit that amplifies the image pickup signal, and in the color signal processing circuit that performs gamma correction on the color signal, increase the amplification factor of the automatic gain adjustment circuit. A method for suppressing chroma noise in a video camera, which comprises adding an offset to the previous color signal. 2. The chroma noise suppression method for a video camera according to claim 1, wherein the value of the offset is changed according to the value of the amplification factor of the automatic gain adjustment circuit.