JP2878853B2 - 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. 3 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 output of the encoder 8 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. 4A), 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 luminance signal Y with the black level pulse BP, the pulse mixer 24 outputs a luminance signal Y (FIG. 4B) 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, the black level correction amount is the same when the APL is the same even if the contrast is largely different as shown in FIGS. The black level is the pedestal level in FIG. 5 (a) for contrast, but the black level is not completely reduced in FIG. 5 (b) for low contrast.

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. 3, 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 reduction in contrast 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 capable of accurately performing the black level correction.

[0015]

According to the structure of the present invention, which solves the above-mentioned problem, a luminance signal mixed with a black level pulse is input, and the luminance is adjusted so that the peak level of the black level pulse is fixed at a preset level. A clamp circuit for clamping and outputting a signal, a black peak detection circuit for detecting the lowest level of the luminance signal output from the clamp circuit in a video period excluding a blanking period, and a black peak detection circuit A black-level pulse generating circuit for generating a black-level pulse, comprising: a comparator for comparing the level detected by the circuit with a preset black level; A pulse mixer which mixes with a luminance signal and sends it to the clamp circuit at a stage preceding the circuit , Characterized by having a.

[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 mixer 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, and records the data on the tape by the head 10 of the recorder unit 9.

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

A pulse mixer 35 mixes the luminance signal Y with a black level pulse BP (BP will be described later), and a clamp circuit 36 converts the luminance signal so that the peak level of the black level pulse BP is fixed at the pedestal level. Clamp. The clamped luminance signal Y (FIG. 2)
(a)) is sent to the gamma correction circuit 5 and the adder 31.

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
FIG. 2C shows the luminance signal Y to which BL has been added.

The black peak detection circuit 32 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 Vb in FIG. Then, the black peak detection circuit 32 outputs a black peak signal VB whose signal level is Vb (FIG. 2 (d)). Since the blanking period is filled with the wide blanking pulse WBL, the black peak detection circuit 32 detects the lowest level of the luminance signal in the video period excluding the blanking period.

The black level setting circuit 33 and the switch 34 constitute a black level pulse generation circuit.
The black level setting circuit 33 outputs the level V of the black peak signal VB.
b + VS and a preset black level VL (=
VS) and performs automatic control to eliminate the difference.
Therefore, the level of the black level pulse BP is Vb
(FIG. 2 (f)). The switch 34 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, a black level pulse BP (FIG. 2 (f)) whose level is Vb is generated. Further, by adjusting the value of the set black level VL, the level of the black level pulse BP, and thus the black level, can be easily adjusted.

The pulse mixer 35 mixes the black level pulse BP with the luminance signal Y sent from the color separation circuit 4. 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 36 clamps the luminance signal Y so that the peak level 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) based on the pedestal level, the level V
The level of the luminance signal that has floated black by b is reduced. Therefore, the contrast can be improved.

As described above, in the present invention, a feedback circuit (clamp circuit 36 → adder 31 → black peak detecting circuit 32 → black level setting circuit 33 → switch 34 → pulse mixing) is used to set the level of the black level pulse BP. (Path of the container 35), it is hardly affected by variations in parts and environmental fluctuations such as temperature.

[0026]

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.

Further, since the average video level (APL) is not used, an integrating circuit having a non-linear characteristic need not be used, and a complicated adjusting circuit for adjusting the non-linear characteristic is not required. The configuration is simplified, and the cost is reduced.

Further, since the black level correction circuit is constituted by a feedback circuit, a stable operation is obtained which is hardly affected by fluctuations. The black level can be easily set by adjusting the set black level of the black level setting circuit.

[Brief description of the drawings]

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

FIG. 2 is a waveform diagram showing a signal waveform according to the present invention.

FIG. 3 is a block diagram showing a conventional technique.

FIG. 4 is a waveform diagram showing a signal waveform in the related art.

FIG. 5 is an explanatory diagram showing a correction state by APL.

[Explanation of symbols]

 Reference Signs List 30 black level correction circuit 31 adder 32 black peak detection circuit 33 black level setting circuit 34 switch 35 pulse mixer 36 clamp circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H04N 5/14-5/217

Claims (1)

(57) [Claims] 1. A clamp circuit which receives a luminance signal mixed with a black level pulse, clamps and outputs the luminance signal so that the peak level of the black level pulse is fixed at a preset level, and a blanking period. In the excluded video period, a black peak detection circuit that detects the lowest level among the levels of the luminance signal output from the clamp circuit is compared with the level detected by the black peak detection circuit and a preset black level. A black-level pulse generating circuit for generating a black-level pulse, and mixing the generated black-level pulse with a luminance signal in a blanking period and at a preceding stage of the clamp circuit, to the clamp circuit. Black level correction of a video signal, comprising: circuit.