JP2878851B2 - Video signal black level correction circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for correcting a black level of a video signal, which is intended to improve the contrast with a simple circuit even if flare or the like occurs.

[0002]

2. Description of the Related Art In a video camera, an image of a subject is formed on an imaging surface of an imaging device by an optical system, and a video signal indicating the subject is extracted from the imaging device. By the way, when sunlight is directly incident on the lens of the optical system, or irregular reflection occurs on the optical system or the imaging surface, flare occurs. When the flare occurs, the black level of the video signal increases and the contrast of the image decreases. Therefore, even if a black subject is photographed, the subject becomes slightly whitish and appears in the reproduced image. Such flare becomes more remarkable as the average brightness of the entire image increases.

On the other hand, when a subject is photographed through a white turbid or lightly contaminated window glass, or when photographed in telephoto on a fogged day, the contrast of the image is small and the black of the video signal is low. The level will be higher.

As a conventional technique for compensating for an increase in black level (this may be referred to as "floating black level"), there is a flare compensation circuit. In the conventional flare compensation circuit, the average video level (average pi
(cture level: APL), and the floating of the black level is corrected according to the value of the APL.

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

The luminance signal Y is input to the flare compensating circuit 20 to compensate for the rise of the black level. Although the details of the flare compensation circuit 20 will be described later, the flare-corrected luminance signal Y is γ-corrected by the γ correction circuit 5 and the blanking circuit 6 removes the video signal and noise during the blanking period. Is sent to the recorder section 9 via the encoder 8.

On the other hand, the color signal C is subjected to required color signal processing in a color signal processing circuit 7 and passes through an encoder 8 to a recorder section 9.
Sent to

The recorder section 9 records the video out of the encoder on a video tape by the head 10.

Here, the compensation of the floating of the black level in the flare compensation circuit 20 will be described. Since the integrator 21 of the flare compensation circuit 20 integrates the luminance signal Y (see FIG. 7A), the integrated value indicates the average video level (APL). The level control circuit 22 outputs a value corresponding to the APL, and the switch 23 is turned on when the horizontal synchronizing pulse HD is at a high level and opened when the horizontal synchronizing pulse HD is at a low level. A pulse BP is output. The level of the black level pulse BP corresponds one-to-one with the value of APL. Since the pulse mixer 24 mixes the black level pulse BP with the luminance signal Y, the pulse mixer 24 outputs a luminance signal Y (FIG. 7B) to which the black level pulse BP is added during the blanking period. You. The clamp circuit 25 clamps the luminance signal Y so that the peak level of the black level pulse BP is fixed at the pedestal level.

Therefore, when the APL is high, the black level rises greatly due to an increase in the amount of flare. At this time, the black level pulse BP becomes large depending on the APL. The black level of the luminance signal can be reduced by the level.

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

[0012]

However, the conventional flare compensation circuit 20 cannot sufficiently compensate for the rise of the black level when the subject has low contrast regardless of flare or the like. That is, since the APL is detected and the flare compensation is performed, even if the contrast is largely different as shown in FIGS. 8A and 8B, the correction amount of the black level becomes the same when the APL is the same. In FIG. 8A of the contrast, the black level becomes the pedestal level, but in FIG. 8B of the low contrast, the black level does not decrease completely.

The conventional flare compensating circuit 20 is composed of AG
In a state where the C circuit 3 is operated, the effect of black level floating compensation cannot be exhibited. The reason is as follows.
In the normal 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 for 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 the integrator 21, and the black level is corrected according to the output voltage of the integrator 21. However, if the potential of the point A becomes constant by the operation of the AGC circuit 3, the potential (APL) of the output (point B) of the integrator 21 of the flare compensation circuit 20 also becomes constant, and as a result, the correction amount of the black level always becomes constant. It becomes constant and loses its effect. This is particularly problematic in home video cameras since the AGC circuit 3 is generally used.

In view of the above prior art, the present invention can improve not only the contrast reduction due to flare but also the contrast of a low-contrast image not caused by flare or the like. It is an object of the present invention to provide a circuit that can accurately perform black level correction.

[0015]

According to an embodiment of the present invention, there is provided a black peak detecting circuit for detecting a lowest level of a luminance signal level in an image period excluding a blanking period, and a black peak detecting circuit. A black level pulse generation circuit for generating a black level pulse corresponding to the level detected by the detection circuit, a pulse mixer for mixing the generated black level pulse with a luminance signal during a blanking period, and a black level And a clamp circuit for receiving the luminance signal mixed with the pulse, and clamping and outputting the luminance signal so as to fix the peak level of the black level pulse to a preset level. In addition, the high frequency component in the luminance signal is suppressed by the frequency control circuit, and the lowest level in the luminance signal in which the high frequency component is suppressed is defined as:
It is also characterized by detection by a black peak detection circuit.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a camera-integrated VTR to which the present invention is applied.
FIG. 2 is a block diagram showing a recording system of FIG. In FIG. 1, a video signal indicating a subject image obtained by an optical system 1 is output from an imaging unit 2, passes through an auto gain control circuit 3, and is separated into a luminance signal Y and a color signal C by a color separation circuit 4. .

The luminance signal Y is subjected to black level correction (details will be described later) by 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 chrominance signal C to produce a video-out signal.
To record on the tape.

Here, the black level correction circuit 30 will be described with reference to FIGS. In this example, for ease of understanding, an operation when a rectangular wave-like luminance signal as shown in FIG. 2A is input and a black floating Vb1 is generated will be described.

The adder 31 applies a wide blanking pulse WBL to the luminance signal Y during a blanking period. As shown in FIG. 2B, the wide blanking pulse W
BL has a pulse width longer than the blanking period and a higher level than the luminance signal Y. Wide blanking pulse W
The luminance signal Y to which BL is added is amplified by the amplifier 32 (the amplified Y + WBL is shown in FIG. 2C).

The black peak detection circuit 33 detects the lowest level of the luminance signal Y to which the wide blanking pulse WBL has been added. The detected lowest level value (black peak) is indicated by Vb2 in FIG. Then, the black peak detection circuit 33 outputs a black peak signal VB whose signal level is Vb2 (FIG. 2 (d)). 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 the switch 35 constitute a black level pulse generation circuit. The level control circuit 34 outputs the black peak signal V
The level Vb2 of B is multiplied by the reciprocal of the degree of amplification of the amplifier 32 to obtain a level Vb1. The switch 35 is turned on when the horizontal synchronizing pulse HD (FIG. 2 (e)) is at a high level and is opened when it is at a low level. Therefore, the level is V
The black level pulse BP at b1 (FIG. 2 (f)
) Will be generated. The level Vb1 of the black level pulse BP is equal to the black peak level V of the luminance signal.
It is proportional to b2.

The pulse mixer 36 mixes the luminance signal Y sent from the color separation circuit 4 with a black level pulse BP. The mixing timing is a blanking period, and the luminance signal Y mixed with the black level pulse BP is mixed.
Is shown in FIG. 2 (g).

The clamp circuit 37 clamps the luminance signal Y so that the peak level Vb1 of the black level pulse BP is fixed at the pedestal level. FIG. 2 (h) shows the clamped luminance signal Y. As can be seen by comparing FIG. 2 (g) and FIG. 2 (h) with reference to the pedestal level, the luminance signal level which has been blackened by the level Vb1 is lowered. Therefore, the contrast can be improved.

Also, instead of APL, the black peak level Vb
2, the black level pulse BP is generated and corrected, so that the black level can be accurately corrected even when the AGC circuit 3 operates.

Since the level of the luminance signal floating in black can be reduced, the dynamic range of the video signal is slightly expanded.

FIG. 3 shows a second embodiment of the present invention. The black level correction circuit 130 in this embodiment has a configuration in which a frequency control circuit 38 is added to the black level correction circuit 30 shown in FIG. 1, and the other parts have the same configuration as that shown in FIG. I have. In this embodiment, appropriate correction can be performed even when a thin black linear object (wire net, electric wire, etc.) is included in the screen.

The frequency control circuit 38 is composed of a low-pass filter and suppresses high-frequency components of the luminance signal Y. Therefore, for example, a luminance signal Y as shown in FIG.
Is input to the frequency control circuit 38, the frequency control circuit 38 outputs a filtered luminance signal Y as shown in FIG. That is, in the output luminance signal Y, the black peak level is low in a signal portion having a wide pulse width (low frequency), but the black peak increases in a signal portion having a narrow pulse width (high frequency). This means that the black peak levels V0, V1, V2, V3 shown in FIG.
It is obvious if you look at it. Of course, the frequency control circuit 3
8 by changing the filtering characteristics
It is also possible to adjust the rate at which the black peak level changes according to the frequency.

Here, the main operation of the embodiment shown in FIG. 3 will be described with reference to FIG. When a subject (this is referred to as a main subject) is photographed through a wire mesh, the luminance signal Y includes a low-level and high-frequency video signal K as shown in FIG. When the luminance signal Y including the video signal K passes through the frequency control circuit 38, the frequency control circuit 38 outputs the luminance signal Y from which the video signal K has been removed as shown in FIG.
Is output. Therefore, the black peak detection circuit 38 detects the black peak Vb3 based on the luminance signal Y shown in FIG. 5B, and the level control circuit 34 and the switch 35 cooperate to set the black level pulse Vb3. BP
Is generated. Therefore, from the pulse mixer 36, FIG.
As shown in (c), a luminance signal Y (including the video signal K) mixed with the black level pulse BP whose level is Vb3 is output. The clamp circuit 37 outputs the black level pulse B as shown in FIG.
In order to clamp the peak of P, the black level of the main subject can be clamped to the pedestal level from the clamp circuit 37 as shown in FIG.

By the way, when the frequency adjusting circuit 38 is not provided, the black peak level becomes vb4 and the level of the black level pulse BP also becomes small as Vb4, so that the correction for floating black is insufficient.

[0030]

According to 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. Reliable black level correction is possible,
Even if flare or the like occurs, good contrast can be improved.

Since the average video level (APL) is not used, a complicated adjustment circuit for adjusting the nonlinear characteristic is not required, the circuit configuration is simplified, and the cost is reduced.

[Brief description of the drawings]

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

FIG. 2 is a waveform chart showing signal waveforms in the first embodiment.

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

FIG. 4 is a characteristic diagram illustrating characteristics of a frequency control circuit.

FIG. 5 is a waveform chart showing a signal waveform in the second embodiment.

FIG. 6 is a block diagram showing a conventional technique.

FIG. 7 is a waveform diagram showing a signal waveform in the related art.

FIG. 8 is an explanatory diagram showing a correction state by APL.

[Explanation of symbols]

 Reference Signs List 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 frequency control circuit

Claims (2)

(57) [Claims] 1. A black peak detecting circuit for detecting a lowest level of a luminance signal level in a video period excluding a blanking period, and a black level corresponding to the level detected by the black peak detecting circuit. A black level pulse generation circuit for generating a level pulse, a pulse mixer for mixing the generated black level pulse with a luminance signal in a blanking period, and a luminance signal in which the black level pulse is mixed, and a black level pulse A black level correction circuit for a video signal, comprising: a clamp circuit that clamps and outputs a luminance signal so that a peak level is fixed to a preset level. 2. A frequency control circuit for suppressing a high-frequency component of the luminance signal and passing the luminance signal, and a video signal having the highest level among the luminance signals passing through the frequency control circuit in a video period excluding a blanking period. A black peak detection circuit for detecting a low level, a black level pulse generation circuit for generating a black level pulse corresponding to the level detected by the black peak detection circuit, and a blanking period for the generated black level pulse And a clamp circuit for receiving a luminance signal mixed with a black level pulse and clamping and outputting the luminance signal so that the peak level of the black level pulse is fixed at a preset level. And a black level correction circuit for a video signal.