JPH04236593A - Level correction circuit for video signal - Google Patents

Abstract

PURPOSE:To improve contrast and decoloring even when a flair or the like takes place, even when an object has a low contrast or in the case of AGC operation. CONSTITUTION:A clamp circuit 37 clamps a luminance signal Y so that a peak level Vb1 of a black level pulse BP mixed by a pulse mixer 36 is fixed to a pedestal level. The level Vb1 of the black level pulse BP is made corresponding to a minimum level (black floating level) Vb2 of the luminance signal detected by a black peak detection circuit 33 to adjust the luminance signal level in response to the black floating level. Moreover, the pedestal level of each of R, G, B signals, or the amplitude of each of the R, G, B signals or the amplitude of color difference signals such as R-Y, B-Y is corrected in response to the black floating level to reduce decoloring due to flair thereby improving the color reproducibility.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a level correction circuit for video signals, and is intended to improve contrast and color fading with a simple circuit even when flare or the like occurs.

0002

2. Description of the Related Art In a video camera such as a camera-integrated VTR, an optical system forms a subject image on the imaging surface of an image sensor, and a video signal representing the subject image is extracted from the image sensor. By the way, flare occurs when sunlight directly enters the lens of an optical system or when diffused reflection occurs on the optical system or the imaging surface. When flare occurs, the black level of the video signal increases and approaches the white side, reducing the contrast of the image. Therefore, even if a black subject is photographed, the subject will appear slightly whitish in the reproduced image. Such flare becomes more noticeable as the average brightness of the entire image increases.

On the other hand, when photographing a subject through a cloudy or dirty window glass, or when photographing with a telephoto lens on a hazy day, the black level of the video signal becomes high. Image contrast decreases.

A flare compensation circuit is a conventional technique for compensating for an increase in the black level (this is sometimes referred to as a "floating black level"). In conventional flare compensation circuits, the average video level (APL: Av
image level) is detected,
The rise in black level was corrected according to the APL value.

A conventional flare compensation circuit will now be described with reference to FIGS. 6 and 7. Figure 6 shows a camera-integrated VTR.
FIG. 2 is a block diagram showing a recording system of FIG. As shown in the figure,
A subject image obtained by the optical system 1 is incident on an imaging surface of an imaging section 2 including a CCD or the like. The video signal output from the photographing unit 2 is controlled by automatic gain control (AG).
C) After the gain is adjusted in the circuit 3, the signal is separated into a luminance signal Y and a color signal C in a color separation circuit 4.

Of these, the luminance signal Y is sent to the flare compensation circuit 20.
is input to compensate for the rise in black level. Although the details of the flare compensation circuit 20 will be described later, the flare-compensated luminance signal Y is subjected to γ correction in the γ correction circuit 5, and also has the video signal, noise, etc. during the blanking period removed in the blanking circuit 6. The signal is then sent to the recorder section 9 via the encoder 8.

On the other hand, the color signal C undergoes necessary color signal processing in a color signal processing circuit 7 and is sent to a recorder section 9 via an encoder 8.

The recorder section 9 records the video output from the encoder 8 onto a video tape using a head 10.

[0009] Here, compensation for rising black level in the flare compensation circuit 20 will be explained. The integrator 21 of the flare compensation circuit 20 integrates the luminance signal Y (see FIG. 7(a)), and the integrated value is equal to the average video level (A
PL). Level control circuit 22
outputs a value according to the APL, and the switch 23
is input when the horizontal synchronizing pulse HD is at high level and is released when the horizontal synchronizing pulse HD is at low level, so that the black level pulse BP is output in synchronization with the horizontal synchronizing pulse HD. The level of this black level pulse BP corresponds one-to-one to the value of APL. The pulse mixer 24 adds the black level pulse BP to the luminance signal Y, and from this pulse mixer 24, the luminance signal Y to which the black level pulse BP is added during the blanking period (FIG. 7(b))
is output. The clamp circuit 25 clamps the peak level of the black level pulse BP so as to fix it at the pedestal level.

Therefore, when the APL is high, the black level rises significantly due to an increase in the amount of flare, but at this time, the black level pulse BP increases depending on the APL,
The clamp circuit 25 can lower the black level of the luminance signal by the level of the black level pulse BP.

As described above, since the black level is automatically adjusted by the flare compensation circuit 20, an image with high contrast can be obtained even if a flare image occurs.

0012

However, if the subject has a low contrast due to flare or the like, it is not possible to sufficiently compensate for the rise in the black level. In other words, since flare compensation is performed by detecting APL, Fig. 8(a),
Even if the contrast is significantly different as in (b), the AP
When L is the same, the amount of correction for the black level is the same, so in the case of high contrast in FIG. 8(a), the black level becomes the pedestal level, but in the case of low contrast in FIG. 8(b), the black level cannot be lowered completely. Further, the conventional flare compensation circuit 20 cannot exhibit the effect of black level rise compensation when the AGC circuit 3 is operated. The reason is as follows. Normally, in the AGC circuit 3, as shown in FIG. 6, the luminance signal Y output from the color separation circuit 4 is integrated by an integrator 11, and the output voltage of the integrator 11 is compared with a reference potential by a comparator 12 to perform feedback control. I do. Therefore, the potential of the output (point A) of the integrator 11 is always constant. On the other hand, in the flare compensation circuit 20, the luminance signal Y output from the color separation circuit 4 is integrated by an integrator 21, and the black level is corrected according to the output voltage of the integrator 21. However, if the potential at point A becomes constant due to the operation of the AGC circuit 3, the potential at the output (point B) of the integrator 21 of the flare compensation circuit 20 (APL
) also becomes constant, and as a result, the black level correction amount always remains constant and is no longer effective. This is particularly problematic since AGC circuits 3 are commonly used in home video cameras.

[0013] Also, each color signal of R, G, B or R-
Since the Y and B-Y color difference signals were not corrected, the color saturation decreased due to black floating in the color signals, resulting in "fading" and impairing color reproducibility.

SUMMARY OF THE INVENTION In view of the above prior art, it is an object of the present invention to provide a circuit that can accurately correct the level of a video signal so as to prevent color fading.

[0015]

[Means for Solving the Problems] The configuration of the video signal level correction circuit according to the present invention basically consists of a black peak detection circuit that detects the black peak level of a luminance signal in a video period excluding a blanking period. , a black level pulse generation circuit that generates a black level pulse having a level corresponding to the level detected by the black peak detection circuit;
A pulse mixer mixes the generated black level pulse into the blanking period of the luminance signal, and a luminance signal into which the black level pulse is mixed is input, and the peak level of the black level pulse is fixed at a preset level. a clamp circuit that clamps and outputs a luminance signal, and further includes at least one of a pedestal level of a chrominance signal, an amplitude of a chrominance signal, and an amplitude of a chrominance signal, according to the level detected by the black peak detection circuit. The invention is characterized in that it includes a circuit for controlling.

[0016]

[Example] First, the recording system of a camera-integrated VTR incorporating a black level correction circuit developed at the same time as the present invention is shown in Figure 4 (
This will be explained with reference to FIG. 5 (block diagram) and FIG. 5 (signal waveform diagram).

In FIG. 4, a video signal representing a subject image obtained by an optical system 1 is output from an imaging section 2, passes through an auto gain control circuit (AGC amplifier) 3, and is converted into a luminance signal Y and a color separation circuit 4. It is separated into color signal C.

The luminance signal Y undergoes black level correction (details will be described later) in a black level correction circuit 30, and is sent to an encoder 8 via a γ correction circuit 5 and a blanking circuit 6. The color signal C is sent to the encoder 8 via the color signal processing circuit 7. The encoder 8 performs necessary processing on the luminance signal Y and the color signal C to output the video, and the recorder section 9 outputs the video to the head 10.
recorded on tape.

The black level correction circuit 30 will now be explained with reference to FIGS. 4 and 5. In this example, for ease of understanding, the operation will be described when a rectangular waveform luminance signal as shown in FIG. 5(a) is input and a black floating signal Vb1 occurs.

The adder 31 adds a wide blanking pulse WBL to the luminance signal Y during the blanking period. As shown in FIG. 5(b), the wide blanking pulse WBL has a pulse width longer than the blanking period and a level higher than the luminance signal Y. The luminance signal Y to which the wide blanking pulse WBL has been added is amplified by the amplifier 32 (the amplified Y+WBL is shown in FIG. 5(c)).

The black peak detection circuit 33 detects the lowest level of the luminance signal Y to which the wide blanking pulse WBL is added. The detected lowest level value (black peak) is shown as Vb2 in FIG. 5(c). Then, the black peak detection circuit 33 outputs a black peak signal VB whose signal level is Vb2 (FIG. 5(d)). Note that since the blanking period is filled with the wide blanking pulse WBL, the black peak detection circuit 33 detects the lowest level of the luminance signal in the video period excluding the blanking period.

The level control circuit 34 and switch 35 constitute a black level pulse generation circuit. The level control circuit 34 outputs a black peak signal V
The level Vb2 of the signal B is multiplied by the reciprocal of the amplification degree of the amplifier 32 to obtain the level Vb1. Further, the switch 35 is turned on when the horizontal synchronizing pulse HD (FIG. 5(e)) is at a high level, and opened when it is at a low level. Therefore, the black level pulse BP whose level is Vb1 (Fig. 5 (
f) ) will be generated. The level Vb1 of this black level pulse BP is in a proportional relationship with the black peak level Vb2 of the luminance signal.

The pulse mixer 36 mixes the black level pulse BP with the luminance signal Y sent from the color separation circuit 4. The mixing timing is the blanking period, and the luminance signal Y mixed with the black level pulse BP is shown in FIG. 5(g).

The clamp circuit 37 clamps the luminance signal Y so that the peak level of the black level pulse BP is fixed at the pedestal level. The clamped luminance signal Y is shown in FIG. 5(h). As can be seen by comparing FIG. 5(g) and FIG. 5(h) using the pedestal level as a reference, the luminance signal level, which was blackened by level Vb1, is lowered. Therefore, contrast can be improved. Also, APL
Since the black level pulse BP is created and corrected based on the black level Vb2 of the luminance signal instead of the AGC
Even when the circuit 3 is in operation, the black level can be corrected accurately.

The present inventor has developed R, G, and B color signals by using the output (black level) of the black peak detection circuit 33 or the level control circuit 34 included in the above-mentioned black level correction circuit 30 shown in FIG. Alternatively, it has been realized that by correcting color difference signals such as R-Y and B-Y, it is possible to prevent a decrease in color saturation due to black floating, that is, to prevent color fading.

FIG. 1 shows a level correction circuit 130 according to an embodiment of the present invention, which converts the output (level Vb1) of the level control circuit 34 in the black level correction circuit 30 shown in FIG. A pedestal level control circuit (chroma pedestal level control) 41 for R, G, and B color signals is connected through a level control circuit (hereinafter referred to as a color level control circuit) 38, and a color signal amplitude control circuit (
chroma amplifier) 42. These circuits 41 and 42 are well known.

Now, to explain the color signal system, the R, G, and B color signals separated by the color separation circuit 4 are subjected to white balance processing by the white balance circuit 40, and the color signals R, G, and B processed here are processed by the white balance circuit 40. The pedestal level is corrected by the pedestal level control circuit 41, and each color signal R after correction,
The amplitudes of G and B are corrected by an amplitude control circuit 42. Then,
The γ correction circuit 43 performs γ correction on each color signal to obtain R,
Outputs G and B color signals. Also, R, G, B after γ correction
From each color signal, the color difference matrix circuit 44 outputs R-Y and B.
-Y color difference signals are created, amplified by a color difference gain circuit 45, and supplied to the encoder 8.

The color level control circuit 38 sends control signals 38A and 38B having a level proportional to the black peak detected by the black peak detection circuit 33 to the level control circuit 3.
4 and is applied to the pedestal level control circuit 41 and the amplitude control circuit 42.

The larger the control signal 38A is, the more the pedestal level control circuit 41 lowers the black level of the R, G, and B color signals to match the pedestal level. In other words,
Even if the color signal at point A in FIG. 1 output by the white balance circuit 40 is black from the pedestal level as shown in FIG. 2(a), the color signal at point B in FIG. In the color signal, the pedestal level and the black floating level match as shown in FIG. 2(b). However, the amplitude of the color signal from the pedestal level is reduced by the amount that black floating is eliminated.

The amplitude control circuit 42 corrects this amplitude decrease, and amplifies the R, G, and B color signals with a larger gain as the control signal 38B becomes larger. This amplitude control circuit 4
2 outputs the amplified color signal at point C in FIG.
c) Shown below. FIG. 2(d) shows the color signal at point D in FIG. 1 after γ correction.

As described above, even if there is flare or the like, fading can be prevented and color reproducibility can be improved. Note that both the pedestal level control circuit 41 and the amplitude control circuit 42 are not necessarily required, and if at least one is provided, fading will be reduced more than in the past.

FIG. 3 shows a level correction circuit 140 of another embodiment.
The level control circuit 38 creates a control signal 38A having a level proportional to the black peak detected by the black peak detection circuit 33 and supplies it to the pedestal level control circuit 41, and also generates a control signal 38A having a level proportional to the black peak. 38C is created from the output of the level control circuit 34 and given to the color difference gain circuit (amplitude control circuit) 45.

The pedestal level control circuit 41 is the same as the embodiment shown in FIG. 1, and lowers the black floating level of the R, G, and B color signals to match the pedestal level.

The color difference gain circuit 45 amplifies each of the R-Y and B-Y color difference signals with a larger gain as the control signal 38C increases.

As described above, even if there is flare or the like, fading can be prevented and color reproducibility can be improved. Moreover, it is not necessary that the pedestal level control circuit 41 and the color difference gain circuit 45
Both of these are not necessary, and if at least one is used, fading will be reduced compared to the conventional method.

[0036]

Effects of the Invention As specifically explained above in conjunction with the embodiments, in the present invention, the level of the black level pulse is made to correspond to the black peak level (black floating level) of the luminance signal. The black level can be reliably corrected depending on the amount, and even if flare or the like occurs, good contrast improvement can be achieved even if the subject has low contrast.

Furthermore, depending on the black peak level, R, G, B
By correcting the pedestal level of each color signal, the amplitude of each R, G, B color signal, or the amplitude of color difference signals such as RY, B-Y, etc., color fading due to flare etc. is reduced and color reproducibility is improved. It can be improved.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing signal waveforms in the case of color signal correction in the present invention.

FIG. 3 is a block diagram showing another embodiment of the present invention.

FIG. 4 is a block diagram showing a black level correction circuit developed at the same time as the present invention.

FIG. 5 is a diagram showing signal waveforms of a black level correction circuit developed at the same time as the present invention.

FIG. 6 is a block diagram showing a prior art.

FIG. 7 is a diagram showing signal waveforms in the prior art.

FIG. 8 is a diagram illustrating problems with low contrast in the prior art.

[Explanation of symbols]

30 Black level correction circuit 31 Adder 32 Amplifier 33 Black peak detection circuit 34 Level control circuit 35 Switch 36 Pulse mixer 37 Clamp circuit 38 Color level control circuit 40 White balance circuit 41 Pedestal level control circuit 42 Amplitude control circuit (chroma gain circuit) 43 Chroma γ correction circuit 44 Color difference matrix circuit 45 Amplitude control circuit (color difference gain circuit) 130 Level correction circuit 140 Level correction circuit

Claims (5)

[Claims] Claim 1: A black peak detection circuit that detects a black peak level of a luminance signal in a video period excluding a blanking period, and generates a black level pulse whose level corresponds to the level detected by the black peak detection circuit. a black level pulse generation circuit that mixes the generated black level pulse into the blanking period of the luminance signal; and a pulse mixer that mixes the generated black level pulse into the blanking period of the luminance signal; The present invention includes a clamp circuit that clamps and outputs the luminance signal so as to fix it at a set level, and a pedestal level control circuit that controls the pedestal level of the color signal according to the level detected by the black peak detection circuit. Features: Video signal level correction circuit. 2. The video signal level correction circuit according to claim 1, further comprising an amplitude control circuit that controls the amplitude of the color signal according to the level detected by the black peak detection circuit. Claim 3: Instead of the pedestal level control circuit,
2. The video signal level correction circuit according to claim 1, further comprising an amplitude control circuit that controls the amplitude of the color signal according to the level detected by the black peak detection circuit. 4. The video signal level correction circuit according to claim 1, further comprising an amplitude control circuit that controls the amplitude of the color difference signal according to the level detected by the black peak detection circuit. Claim 5: Instead of the pedestal level control circuit,
2. The video signal level correction circuit according to claim 1, further comprising an amplitude control circuit that controls the amplitude of the color difference signal according to the level detected by the black peak detection circuit.