JPS62183695A - Signal processing method for frequency separation system color video camera - Google Patents

Abstract

PURPOSE:To prevent the deterioration in the color reproducibility of a reproduced picture by forming the gain control signal of a chrominance signal gain control circuit from at least the low frequency component of a luminance signal and the driving voltage of an iris, and reducing the difference in the gamma characteristic of the luminance signal and the chrominance signal. CONSTITUTION:In a low illumination part L, the level of the iris driving voltage Va becomes high, thereby, the tracking correction signal obtained from a tracking correction circuit 16 is the first component corresponding to the luminance signal and the second component corresponding to the iris driving voltage Va. The first component controls the level of a red signal R and a blue signal B to the direction in which the gamma characteristic curve is over-lapped on the gamma characteristic curve of the luminance signal Y outputted from an LPF 5. The second component controls the level of these chrominance signals to the direction in which the difference in the gamma characteristic between the luminance signal Y and the red signal R and the blue signal B is eliminated so as to make the gamma characteristic curve (a) of th luminance signal substantially coincide with the gamma characteristic curve (b) of the chrominance signal.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a frequency separation type color video camera, and particularly to a signal processing method for improving color reproducibility.

[Conventional technology]

In a frequency separation type color video camera, the output signal of the image sensor consists of a baseband component of luminance (No. 8) and a color signal that is a high frequency modulation signal component. After these components are separated and processed, they are combined. The levels of the luminance signal and color signal obtained from this image sensor vary depending on the amount of incident light, but the level characteristics of these luminance signals and color signals with respect to the amount of incident light (i.e. , gamma characteristics) are different from each other.

That is, as shown in Fig. 4, when the gamma characteristic of the luminance signal is shown by a solid line a, the gamma characteristic of the color signal is shown by a broken line ab. In part H, the level of the chromaticity signal is lower than the level of the luminance signal. Note that the gamma characteristic curves of the luminance signal and color signal do not overlap because their overall slopes are different, but they are low.

In order to clarify that there is a difference in gamma of these signals in a high-illuminance area, the characteristic curves a and l) are shown to overlap for convenience.

When a difference appears in the gamma characteristics between the luminance signal and the color signal, the balance between the luminance signal and the color signal shifts, and proper color reproduction cannot be obtained.

Incidentally, the smaller the ratio between the luminance signal and the color signal, the greater the influence of such a difference in gamma characteristics.

For this reason, the influence of the difference in gamma characteristics is greater in the low-illuminance portion where the amount of incident light is small than in the high-illuminance portion H where the amount of incident light is large.

From the above, the deterioration of the color reproducibility of the reproduced image appears particularly in the low-illuminance section I.

In order to solve this problem, a tracking correction circuit is conventionally provided, as disclosed in, for example, Japanese Unexamined Patent Publication No. 56-123187. This is because the level of the brightness signal of the baseband component output from the image sensor is proportional to the amount of light incident on the image sensor, so a correction signal is formed from this brightness signal, and this is used to control the level of the color signal. The difference in gamma characteristics between the luminance signal and the color signal is created (that is, the characteristic curves a and b in FIG. 3 are made to overlap).

[Problem that the invention seeks to solve]

Incidentally, in order to prevent deterioration of the color reproducibility of reproduced images, it is necessary to measure the level difference between the luminance signal and the color signal with high precision, especially in the low-illuminance area.

However, the conventional tracking correction circuit has a correction signal forming means for each of the low illuminance part, the medium illuminance part N, and the high illuminance part H, and forms a correction signal from the luminance signal for each of them to determine the gamma characteristics of the color signal. I am trying to correct it. That is, in FIG. 4, the luminance signal Y representing gray scale will be explained. Of this luminance signal Y, the signal component in the level range L' corresponding to the low illuminance part and the medium illuminance part N
A -γ1 correction signal is formed by signal components in a certain range of the level range N' corresponding to From the signal component with the corresponding level range H', Da Tt
A correction signal is formed, and -γ
, a correction signal is formed, and the level range corresponding to the low illuminance part of the color signal is set as the Do-Tt correction signal, the level range corresponding to the medium illuminance part N is set as -γ, +*positive signal, and further high The level range corresponding to the illuminance part H is corrected using the D and -T3 correction signals, respectively.

In this way, by a single correction forming means.

-γ, when a correction signal is formed and the level range corresponding to the low illuminance part of the color signal is corrected, the gamma characteristic of the color signal is adjusted to the gamma characteristic of the luminance signal for the low illuminance part (
That is, it is very difficult to match the characteristic curves a and t) in FIG. Therefore, it is conceivable to divide the low-illuminance area into a large number of regions and form a Dor correction signal for each region, but this would complicate the structure of the tracking correction circuit and increase its size, which is not preferable.

In addition, in conventional color video cameras, the video signal processing circuit that processes the output signal of the image sensor has an AGC that increases the signal level in low-light areas so that the reproduced image is displayed brightly even in low-light areas. A circuit is provided to increase the level of the luminance signal and color signal, but for this purpose,
On the contrary, the difference in gamma characteristics between the luminance signal and the color signal in the low-illuminance area becomes even larger, further deteriorating the color reproducibility of the reproduced image.

An object of the present invention is to provide a frequency separation method color video that solves the above problems and improves color reproducibility by reducing the difference in gamma characteristics between a luminance signal and a color difference signal in a low-illuminance area by a simple means. This invention relates to a camera signal processing method.

[Means for solving problems]

In order to achieve the above object, the present invention supplies an iris drive voltage together with a luminance signal to a tracking correction circuit, and forms a gamma correction signal of a color signal using these signals.

[For production]

Iris becomes free in low light areas! I! child,
Driven. In this way, the driving voltage required to drive the iris is not as large as the value shown on the left in high and medium illuminance areas, but in low illuminance areas, the value increases rapidly. The output gamma correction signal consists of a first component based on the luminance signal and a second component based on the iris drive voltage, but in high and medium illuminance areas, it only consists of the first component based on the luminance signal, and in low In the illuminance section,
It consists of a first component based on a luminance signal and a second component based on an iris drive voltage. Therefore, in the high- and medium-illuminance areas, the gamma characteristics of the color signal are corrected according to the luminance signal, similar to the conventional technology described above, and in the low-illuminance areas, the gamma characteristics of the color signal are corrected using the luminance signal and the iris drive voltage. However, in this case, in the low illuminance area, the second component due to the iris drive voltage reduces the level difference between the luminance signal and the color signal generated by the AGC circuit, and the ti degree multiplied signal color signal and To eliminate the difference in gamma characteristics.

〔Example〕

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

FIG. 1 is a block diagram showing a specific example of a signal processing method for a frequency separation type color video camera according to the present invention, in which 1 is an iris, 2 is an image sensor, 3 is a preamplifier, 4 is a video signal processing circuit, and 5 is LPF (low pass filter),
6 is a luminance signal processing circuit, 7 is an encoder, 8 is an LPF,
9 is a BPF (band pass filter), 10 is a color separation circuit, 11 is a red signal gain control circuit, 12 is a blue signal gain control circuit, 13 and 14 are subtraction circuits, 15 is a white balance circuit, 16 is a tracking correction circuit, 17 is an auto-iris circuit, 18 is an iris drive circuit, 20 is a drive coil, and 21 is a brake coil.

In the figure, optical information incident through a lens (not shown) and an iris 1 is converted into a minute electrical signal by the photoelectric conversion action of a moat image element 2.

This electric signal is amplified by a preamplifier 3 to a level suitable for signal processing, and then supplied to a video signal processing circuit 4 where it is subjected to a process of ν, and then supplied to an LPF 5.8 and a BPF 9.

The output signal of the video signal processing circuit 4 is multiplexed with a luminance signal Y, which is a baseband component, and a color signal C, which is a high frequency modulation signal. A low frequency component (hereinafter referred to as a low frequency luminance signal) YL and a color signal C are separated from the BPF 9.The luminance signal Y is subjected to predetermined processing in a luminance signal processing circuit 6 and is supplied to an encoder 7. Note that the video signal processing circuit 4 is provided with an AGC circuit, and the iris 1
The level of the input electrical signal is increased in low-light areas where the light is released. As a result, the reproduced image is displayed brightly even in low illuminance areas.

On the other hand, the color signal C separated by the BPF 9 consists of a red signal and a blue signal that have been high-frequency modulated, and is separated into the red signal and blue signal by the color separation circuit 10 and demodulated. Therefore, from the color separation circuit 10, the base pan]・
A red signal R and a blue signal B are obtained. These red signal R1 and blue signal B are connected to the red signal gain control circuit 11.
After being gain controlled by the blue signal gain control circuit 12, it is supplied to subtraction circuits 13 and 14. These subtraction circuits 13.1
4 is also supplied with a low-band luminance signal YL having a frequency band approximately equal to the allowable frequency band of the red signal R, fy color signal B, and as a result, the subtraction circuit 13 outputs a color difference signal (R-
YL) is obtained from the subtraction circuit 14, and color difference signals (B-YL) are obtained from the subtraction circuit 14, respectively.

These color difference signals (R-YL) and (B Yt) are supplied to the encoder 7, where they are balanced-modulated, and mixed with the luminance signal Y supplied from the luminance signal processing circuit 6 to produce a reference color video signal, for example, N'I ' An SC color video signal can be obtained.

Here, the red signal gain control circuit 11 and the blue signal gain control circuit 12 are gain controlled by a white balance correction signal from the white balance circuit 15 and a tracking correction signal from the tracking correction circuit 16, and the white balance is controlled by the former. The gamma characteristics of the red signal R2 and the blue signal B are corrected by the latter.

The output signal of the video signal processing circuit 4 is also supplied to an auto-iris circuit 17, and an iris control signal corresponding to the amount of light incident on the image sensor 2 is formed. This iris control signal is supplied to the iris drive circuit 18, where an iris drive voltage (1) is formed and supplied to the drive coil 20, and an iris braking voltage is formed and supplied to the braking coil 21. In this case, in high and medium illuminance areas, the aperture amount of iris 1 changes according to the amount of incident light, so that the luminance signal Y always remains at a constant level, and in low illuminance areas, iris l
The auto iris circuit 17 is set so that the
The iris drive circuit 18 is controlled by the iris control signal from
The iris drive voltage ■ obtained by controlling the iris drive circuit 18 in this way is:
The level is low in the medium illuminance part N and the high illuminance part H, but the level is relatively high in the low illuminance part.

The tracking correction circuit 16 is supplied with a luminance signal in a frequency band comparable to that of the low-range luminance signal YL from the video signal processing circuit 4 and an iris drive voltage (2) from the iris drive circuit 18, and is supplied with the iris drive voltage (2) from the iris drive circuit 18.

A tracking correction signal is formed for each medium illuminance part N and high illuminance part H, and the red color ε gain control circuit 1 is used as a gain control signal.
1 and the blue signal gain control circuit 12.

Therefore, since the level of the iris drive voltage ■□ is low in the medium illumination area N and the high illuminance area H, the tracking correction circuit 16
The tracking correction signal output from the video signal processing circuit 4 changes according to the level of the luminance signal supplied from the video signal processing circuit 4,
As a result, as shown in FIG. 2, the gamma characteristic curves of the red signal R and the blue signal B almost overlap with the gamma characteristic curve a of the luminance signal Y.

On the other hand, in low-light areas, the iris drive voltage ■.

As a result, the tracking correction signal obtained from the tracking correction circuit 16 consists of a first component corresponding to the luminance signal and a second component corresponding to the iris drive voltage (2).

This first component is applied to the red signal R1 in the direction in which the gamma characteristic curve of the color signal overlaps the gamma characteristic curve of the luminance signal Y output from the LPF 5 (represented by '4IAa' in FIG. 3), as in the prior art described above. The level of the blue signal B is controlled, whereas the second component is controlled by the luminance signal Y and the red signal R generated by the AGC circuit in the video signal processing circuit 4.
9. The levels of these color signals are controlled in a direction that eliminates the difference in gamma characteristics from blue signal B, and as shown in FIG. 2, the gamma characteristic curve a of the luminance signal and the gamma characteristic curve of the color signal almost match. I'll do what I do.

In this case, in the low illuminance part, in the correction using the first component, as in the prior art described above, a red signal R2 a blue signal is added to the characteristic curve a (Fig. 3) of the luminance signal Y output from the L P It cannot be combined with the characteristic &1b of B (Fig. 3). However, by this correction, this characteristic curve is brought closer to the characteristic vAa, and furthermore, due to the component @2, the characteristic curves a and b in FIG.
As shown in , it is possible to correct the color signal so that the difference in gamma characteristics between the luminance signal and the color signal is sufficiently small. As a result, the color reproducibility of the reproduced image becomes appropriate. moreover,
Even with custom-made gamma correction of the color signal using such a tracking correction signal, even if there is a slight difference in gamma characteristics between the luminance signal and the chrominance signal, the level of both the luminance signal and the chrominance signal remains the same. These level ratios are thicker. This also improves the color reproducibility of the reproduced image.

In the above embodiment, the input signal of the auto iris circuit 17 may be the luminance signal Y output from the LPF 5, and the tracking correction circuit 16 may be provided with the LPF 5.
A low-band luminance signal output from B may be supplied.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to significantly reduce the difference in gamma characteristics between the luminance signal and the color signal in a low-illuminance area by using the simple method of using the iris drive voltage. The excellent effect of significantly improving color reproducibility can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the signal processing method for a frequency separation type color video camera according to the present invention, and FIG. 2 compares the gamma characteristics of the luminance signal and color signal in the embodiment shown in FIG. 1. 3 is a graph showing a comparison of the gamma characteristics of the luminance signal and color signal output from the image sensor, and FIG. 4 is a tracking correction circuit in a conventional frequency separation type color video camera. FIG. DESCRIPTION OF SYMBOLS 1... Iris, 2... Image sensor, 5... Low pass filter, 6... Brightness signal processing circuit, 7... Encoder, 8... Low pass filter, 9... Band pass filter, 10... Color separation circuit, 11... Red signal gain control circuit, 12 Blue signal gain control circuit, 13.1
4... Subtraction circuit, I6... Tracking correction circuit,
18...Iris drive circuit. Flood 3 diagram fan 4 diagram

Claims (1)

[Claims] 1. Converts the light information that enters through the iris, which opens during low illumination, into an electrical signal consisting of a luminance signal and a color signal,
A frequency separation type color video camera in which the electric signal is processed by a signal processing circuit including an automatic gain control circuit, separated into a luminance signal and a color signal, and the color signal is processed by a color signal gain control circuit. A gain control signal for the chrominance signal gain control circuit is formed from at least a low-frequency component of the luminance signal and a drive voltage of the iris, thereby making it possible to reduce the difference in gamma characteristics between the luminance signal and the chrominance signal. A signal processing method characterized by: