JPH03182186A - Video camera - Google Patents

Abstract

PURPOSE:To obtain a screen with good coloring by amplifying the amplitude of a chrominance signal in accordance with the amplification of the amplitude of the part below a prescribed level which corresponds to the dark part of the screen of a luminance signal when compensating back light. CONSTITUTION:When compensating the back light, the amplitude of color signals R-Y, B-Y are amplified at an amplifier 32 in company with the amplification of the amplitude of the part below the prescribed level which corresponds to the dark part of the screen of the luminance signal Y at an amplifier 21. Then, the amplitude of the low level part of a carrier chrominance signal at an encoder 8 becomes large, and the amplitude of the low level part of a video signal SV outputted by the encoder 8 is also amplified at this time. Thus, the colorfulness at the dark part can be sufficiently raised, and the screen with good coloring can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is directed to video cameras, and more particularly to improvements in backlight correction for video cameras.

[Conventional technology] FIG. 10 is an example of a circuit configuration of a video camera.

In the figure, image light from a subject (not shown) is passed through an imaging lens 1 and an iris 2 to an imaging device 3, such as C
The signal is supplied to the CD solid-state image sensor.

The output signal of the image sensor 3 is subjected to automatic gain control in the AGC circuit 4 and then supplied to the process circuit 5, which outputs a luminance signal Y and color difference signals (red difference signal RY, blue difference signal B-Y) is output.

The luminance signal Y output from the process circuit 5 is supplied to the gamma correction circuit 6. In this case, as shown in FIG. 11, if the gamma value γI of the image sensor 3 is 1, the gamma value γ3 of the cathode ray tube is 2°2, so the gamma value γ2 at the gamma correction aperture WII6 is 1/ Correction is performed by setting 2.2=0.45.

As a result, the total gamma Mr is γ1 ・γ2 ・γ3ξl
Then, the human power light to the image sensor 3 and the output light of the cathode ray tube are corrected so that they correspond linearly (see broken line a in Fig. 12). (γ3=2.2>, and the solid line C is the gamma correction characteristic of the video camera (γ2=0.
45).

Gamma correction times i? The luminance signal Y output from 86 is supplied to the encoder 8 via the gain amplifier 7. Also,
Color difference signals R-Y and B output from the process circuit 5
-Y is supplied to the encoder 8.

In the encoder 8, the synchronization signal 1 is applied to the luminance signal Y.
1 addition, etc. are performed, and a burst signal is added, color modulation, etc. are performed between the color difference signals R-Y and B-Y to form a carrier color signal C, and furthermore, these luminance signals Y and the carrier color The color video signal Sv is formed by adding the signal C.

The color video signal Sv formed by the encoder 8 is supplied to an output terminal 9 derived from the encoder 8.

By the way, although not mentioned above, the opening of the iris 2 is automatically controlled according to the light level of the subject. Therefore, when capturing an image of a subject with strong contrast (brightness in the periphery of the dark area is high and the light amount level is large), the contrast in the dark area cannot be taken, resulting in so-called blackout.

Backlight correction is performed to prevent such blackouts. During backlight correction, a control signal S8L is supplied from the terminal 10 to the iris 2 and the AGC circuit 4, and control is performed so that the opening of the iris 2 and the gain of the AGC circuit 4 are increased. As a result, the luminance signal Y
Since the level of is increased, it is possible to obtain sufficient contrast in dark areas, and it is possible to avoid blocked up shadows.

[To be solved by the invention: 1u] However, as mentioned above, the opening of the iris 2 and the AGC
According to the circuit that controls the gain of circuit 4 to increase, not only the low level part of the luminance signal Y corresponding to the dark area but also the high level part increases at the same rate, so that the high level part becomes saturated. As a result, so-called overexposure often occurred.

Therefore, in the present invention, it is possible to perform backlight correction while minimizing the occurrence of overexposure.

[Means for Solving the Problems] The present invention includes a signal processing circuit that forms a luminance signal and a color signal based on an output signal of an image sensor, and extracts a portion below a predetermined level corresponding to a dark part of the screen from the luminance signal. a signal extracting circuit; a first amplifying circuit that amplifies the amplitude of a portion of the luminance signal that is below a predetermined level based on a portion that is below a predetermined level extracted by the signal extracting circuit during backlight correction; The second amplifier circuit amplifies the amplitude of the chrominance signal in response to the amplification of the amplitude of the portion of the luminance signal below a predetermined level by the amplifier circuit.

[Function] During backlight correction, the amplifier circuit amplifies the amplitude of the portion of the luminance signal below a predetermined level that corresponds to the dark area of the screen, so even when imaging a subject with strong contrast, the contrast of the dark area can be improved. This will allow you to take a sufficient amount of water and avoid the situation of crushed black.

In this case, by amplifying the part of the video signal corresponding to the dark part of the screen, the overall S-radiation also increases, but the rate of increase in the high-level part of the video signal is compared to the rate of increase in the low-level part. becomes smaller. Therefore, it is possible to suppress the occurrence of overexposure when backlight correction is performed.

In addition, during backlight correction, as the amplitude of the portion of the luminance signal below a predetermined level corresponding to the dark part of the screen is amplified in the amplification circuit #i21, the amplitude of the color signal is amplified in the amplification circuit 32. , it becomes possible to improve the coloring of dark areas.

[Example] First, the example shown in FIG. 2 will be described. In FIG. 2, parts corresponding to those in FIG. 10 are designated by the same reference numerals.

In the figure, a luminance signal Y output from a process circuit 5 is supplied to a modulation amplifier 21 via a buffer circuit 11, and an output signal of this amplifier 21 is supplied to a gamma correction circuit 6.

Further, this luminance signal Y is amplified by an amplifier 13 and then supplied to a clamp circuit 14. The reason for amplifying with the amplifier 13 is to increase the clipping accuracy in the clip circuit described later.For example, the amplifier I3 has a 20 dB
The maximum g of the luminance signal Y is used.
It is amplified so that the width becomes 2Vρ-p.

A reference voltage VREF is supplied to the clamp circuit 14 from a terminal 15 as a clamp voltage. That is, the clamp circuit 14 clamps the luminance signal Y amplified by the amplifier 13 so that the minimum level becomes equal to the reference voltage V REF.

Clamp circuit] Luminance signal Y output from 4 (Fig. 3A
) is supplied to a peak clipping circuit 16, where a low level portion of the luminance signal corresponding to a dark portion of the screen is extracted. In this case, a resistor 17 and a series circuit are connected between the terminal 15 to which the reference voltage VREF is supplied and the power supply terminal Vcc, and the voltage obtained at the connection point of these resistors 17 and 18 is the clip level VCL. Supplied as. This clip level VCL can be set arbitrarily, but in this example, VCL=VREF +0. 3V1m setting sarel.

The output signal of the peak clipping circuit 16 (shown in FIG. 1B) is supplied to the base clipping circuit 22, where clamp scratches and black level noise are suppressed. In this case, the reference voltage V REF supplied from the terminal 15 is supplied as a clip level.

The output signal of the base clip circuit 22 (shown in C in the figure) is supplied to a gain control amplifier 23. A gain control signal SGC is supplied to this amplifier 23, and its gain is controlled. In this amplifier 23, the level corresponding to the clip level VCL (see A in the same figure) is fixed to OV, and the reference voltage V R is set by the gain control signal SGC.
The level V MOD corresponding to EF is, for example, OV~2V
It is variable up to.

In this case, level V MOD is set to OV when backlight correction is not performed (during normal imaging), and on the other hand, when backlight correction is performed (during backlight correction), it is set to be greater than Ov depending on the degree of correction. be done.

The output signal of the amplifier 23 is supplied to the modulation amplifier 21 as a correction signal SC. The amplifier 21 is supplied with an offset gain control signal SOG, and by applying this gain control signal SOG to l1m, the buffer circuit 11
The gain of the amplifier 21 is controlled so that the maximum fi @ level of the luminance signal Y supplied by the amp becomes a predetermined value, for example, 200 mVp-p (when 5c=ov).

Further, the amplifier 21 uses a level V corresponding to the clip level VCL (see FIG. 3A) based on the correction signal SC.
Amplitude modulation of the luminance signal Y is performed using CL' as a reference. In other words, the gain of the amplifier 21 is such that the correction signal SC is Ov.
When it is, it is OdB (1 times), and as the correction signal SC becomes larger than Ov, it is made larger than OdB, and when the correction signal SC becomes 2V, for example, 6 dB (2 times).
It is said that

In this case, during backlight correction, in the part (uncorrected part) of the brightness signal Y higher than the level VCL', the correction signal SC is always OV and the amplitude level of the brightness signal Y does not change. In a portion (correction section) lower than the level VCL', the lower the level, the larger the correction signal SC becomes, and the amplitude level of the luminance signal Y becomes larger. Note that during normal imaging, the correction signal SC is always OV, and the amplitude level of the luminance signal Y does not change.

FIG. 4 shows the operation of the amplifier 21 in a graph form.

In the figure, Yin is the luminance signal Y supplied to the amplifier 21, Yout is the luminance signal Y output from the amplifier 21, G is the amplification factor of the amplifier 21 based on the correction signal SC from the amplifier 23, and a is the gain. control signal SO
The gain ms,b of the amplifier 21 controlled by G is the amplification factor of the amplifier 23.

When Yin≦0 (correction section), G=-bYin+1, and Yout is expressed as follows.

Yout := (-bYin+1) X (aYin
)=-a(bYin2+YIn)◆・◆(1〉Yin＞
0 (uncorrected part), G=1, and "y' out
is as follows.

YOUt=aYIn...(2) These (1)
As is clear from the equations and (2), the overall amount of increase in Yout is determined by the amplification factor a, and the correction amount is determined by the amplification factor s.

Note that the tangent at the correction start point (0,0) in equation (1) is as shown in equation (3), which matches equation (2). Therefore, no step occurs at the correction start point, that is, at the boundary between the correction section and the non-correction section.

YouUt ′ = a Y 1n ・ ・ a
(3) FIG. 5 shows a specific configuration of the modulation amplifier 21.

In the figure, 201 to 204 are NPN transistors for gain control, respectively. The collectors of transistors 201 and 203 are connected to each other, and the connection point thereof is connected to a power supply terminal 206 via a resistor 205, and an output terminal 207 is led out from the connection point. The collectors of transistors 202 and 204 are connected to each other, and their connection point is connected to power supply terminal 206 via resistor 20B.

The emitters of transistors 201 and 202 are connected to each other, and their connection point is connected to the collector of NPN transistor 209 for gain current control. transistor 2
The emitters of 03 and 204 are connected to each other, and their connection point is connected to the collector of an NPN transistor 210 for gain current control. transistors 209 and 21
The emitters of 0 are connected to each other via resistors 211 and 212, respectively, and the connection point thereof is connected to a power supply terminal 218 via a constant current R213.

The brightness signal Yin from the buffer circuit 11 is supplied from the input terminal 214 to the bases of the transistors 201 and 204. Gain control signal SOG is input terminal 21
5 to the bases of transistors 202 and 203. The correction signal SC from the amplifier 23 is input to the input terminal 21.
6 to the base of transistor 210. Further, an amplifier reference voltage VR (OV in this example) is supplied to the base of the transistor 209 from the input terminal 217.

In the above configuration, the impedance between the emitter of transistor 202 and the collector of transistor 209 and between the emitter of transistor 203 and the collector of transistor 210 are changed by the gain control signal SOG, and the gains of transistors 201 and 204 are changed. It will be done. That is, as shown in FIG. 6, the amplification factor a of the amplifier 21 changes depending on the gain control signal SOG. As a result, as described above, the maximum amplitude level of the luminance signal Yin supplied from the buffer circuit 11 is controlled to a predetermined value, for example, 200 mVp-p.

Also, since il+12=73, ΔB=SC-V
When R changes, the impedance described above changes and the gain of the amplifier 21 changes. In other words, as shown in FIG. 6, as ΔB becomes thicker in the correction section, the amplifier 2
The amplification factor of 1 becomes larger.

In this case, for example, if the amplification factor when ΔB=0 is a, the amplification factor when ΔB=2 is set to 2a.

This example is configured as described above, and during backlight correction, in the portion where the luminance signal Y is lower than the level VCL' (in the correction section), the correction signal SC from the amplifier 23 is
becomes large (as shown in the third diagram), and the tM@ level of the luminance signal Y supplied to the gamma correction circuit 6 becomes large (as shown in the third diagram). Therefore, according to the example in FIG. 2, when photographing a subject with strong contrast, it is possible to obtain sufficient contrast in dark areas, and it is possible to avoid the blackout condition in the same way as with conventional backlight correction.

Also, during backlight correction, the correction signal SC becomes large in a portion lower than the level VCL', and the gamma correction circuit 6
As the amplitude level of the luminance signal Y supplied to It is possible to suppress the occurrence of overexposure when performing backlight correction compared to backlight correction.

FIG. 7 shows the output signal of the gamma correction circuit 6. The dashed line a in the figure shows the case without backlight correction, and the solid line in the figure shows the case with backlight correction. In the figure, in order to simplify the comparison, the maximum amplitude level without backlight correction is normalized to 1 and shown. As is clear from this figure, in the backlight correction of this example, the rate of increase in the high-level portion of the luminance signal Y is smaller than the increase rate in the low-level portion.

Furthermore, during backlight correction, in a portion (correction section) lower than level VCL', the lower the level, the larger the correction signal SC becomes, and the rate of increase in the amplitude level of the luminance signal Y supplied to the gamma correction circuit 6 increases. Since it is made larger, there is no difference in level between the correction section and the non-correction section (as shown in FIG. 3E). Furthermore, since the increase in the amplitude level of the luminance signal Y at the correction start point is the same as in the uncorrected portion, the level of the noise N does not suddenly increase from the correction start point (as shown in FIG. 8). In other words, S/N=a'/
b' does not change significantly at the boundary, so the corrected portion and uncorrected portion are no longer clearly visible on the screen.

In this manner, in the example of FIG. 2, the amplitude of the low level portion corresponding to the dark portion of the luminance signal Y is increased by the modulation amplifier 21 during backlight correction. For example, when the luminance signal Y supplied to the amplifier 21 is as shown in FIG. 9A, the luminance signal Y supplied from the amplifier 21 to the gamma correction circuit 6 is as shown in FIG. The luminance signal Y output from the correction circuit 6 is as shown by the solid line in FIG. The broken line in C in the same figure shows the case where the amplitude of the low level portion of the luminance signal Y is not performed by the amplifier 21 during normal reproduction.

Furthermore, the amplitude of the carrier color signal C formed in the encoder 8 does not change from that during normal reproduction even during backlight correction (shown by a solid line in the figure);
The video signal SV outputted from the video signal SV is as shown by the solid line in FIG.

In other words, when correcting the backlight, it is possible to obtain sufficient contrast in the dark areas of the screen as described above.
Although the blackout condition can be avoided, the amplitude of the color signal in the dark area remains the same as during normal reproduction, resulting in a screen with insufficient chroma.

Therefore, the present invention is designed to eliminate such drawbacks.

An embodiment of the present invention will be described below with reference to FIG. In FIG. 1, parts corresponding to those in FIG. 2 are designated by the same reference numerals, and detailed explanation thereof will be omitted.

In this example, the color difference signals R-Y and B-Y output from the process circuit 5 are supplied to the modulation amplifier 32 via the buffer circuit 31, and the output signal of this amplifier 32 is sent to the gamma correction circuit 25 and the gain amplifier 26. The signal is supplied to the encoder 8 via the encoder 8. The gamma correction circuit 25 is
As will be described later, the color difference signals R-Y and B-Y are amplified by the amplifier 32 during backlight correction, but this is for suppressing the final amplitude level of the high level portion of the color difference signals.

Amplifier 32 is configured similarly to amplifier 21 described above.

A correction signal SC from the amplifier 23 is supplied to the amplifier 32, and the amplification factor is controlled in the same way as the amplifier 21.

This example is configured as described above, and during backlight correction, the amplifier 21 generates a luminance signal Y corresponding to the dark part of the screen.
Since the amplitude of the low level portion of the signal is amplified, the same effect as in the example of FIG. 2 can be obtained.

Furthermore, during backlight correction, as the amplifier 21 amplifies the amplitude of the portion of the luminance signal Y that is below a predetermined level corresponding to the dark area of the screen, the amplifier 32 amplifies the amplitude of the color difference signal R-Y.
, B-Y are amplified. Therefore, encoder 8
The carrier color signal C has a large amplitude in the low level portion as shown by the broken line in Figure 9, and the video signal S2 outputted from the encoder 8 at this time becomes as shown by the broken line in Figure E. Therefore, according to this example, the saturation in dark areas can be sufficiently increased, and a screen with good coloring can be obtained.

In the above embodiment, the color difference signals R-Y, B-Y
However, the carrier color signal C formed by the encoder 8 may also be amplified. However, it becomes impossible to obtain the effect of suppressing the amplitude level by the gamma correction circuit 25.

[Effects of the Invention] As explained above, according to the present invention, during backlight correction, the amplitude of the portion of the luminance signal below a predetermined level corresponding to the dark portion of the screen is amplified by the amplifier circuit, so that Even when capturing an image of a subject, it is possible to obtain sufficient contrast in dark areas, and avoid overshadowing. In this case, by amplifying the part of the video signal corresponding to the dark part of the screen, the overall amplitude also increases, but the rate of increase in the high-level part of the video signal is greater than the rate of increase in the low-level part. This makes it possible to suppress the occurrence of overexposure when performing backlight correction to a lesser extent than with conventional backlight correction. In addition, during backlight correction, the amplitude of the part of the luminance signal that is below a predetermined level corresponding to the dark part of the screen is amplified, and the amplitude of the color signal is also amplified, so the saturation of the dark part is sufficiently adjusted. It is possible to obtain a screen with good coloring.

[Brief Description of the Drawings] Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is a diagram showing an example of the circuit structure of a video camera, and Figs. 3 to 9 are for explanation. 10 are configuration diagrams of a conventional example, and FIGS. 11 and 12 are explanatory diagrams of gamma correction. 2... Iris 3... Image sensor 21° 4 ・ 5 ◆ 25 ・ 8 ・ 9 ・ 11 # 14 # 6- 32 # 2- 23 ・ ・AGC circuit・process circuit・gamma correction circuit e encoder・output terminal・Adder ・Clamp circuit ・Peak clip circuit ・Modulation amplifier ・Base clip circuit ・Gain control amplifier

Claims (1)

[Claims] (1) A signal processing circuit that forms a luminance signal and a color signal based on the output signal of the image sensor; a signal extraction circuit that extracts a portion below a predetermined level corresponding to the dark part of the screen from the luminance signal; , a first amplifier circuit that amplifies the amplitude of a portion of the luminance signal that is below a predetermined level based on the portion that is below a predetermined level extracted by the signal extraction circuit; a second amplification circuit that amplifies the amplitude of the color signal when amplifying the amplitude of the portion below a predetermined level of the color signal.