JP2878852B2 - 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, and 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 such as a camera-integrated VTR, a subject image is formed on an imaging surface of an image sensor by an optical system, and a video signal indicating the subject image is extracted from the image sensor. 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 flare occurs, the black level of the video signal increases and approaches the white side, 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 slightly dirty windowpane, or when photographed in telephoto on a fogged day, the black level of the video signal increases. The contrast of the image is reduced.

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 a conventional flare compensation circuit, the average video level (APL: Aver
age Picture Level) was detected, and the floating of the black level was corrected according to the APL value.

Here, a conventional flare compensation circuit will be described with reference to FIGS. 7 and 8. FIG. Figure 7 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 unit 2 including a CCD or the like. Then, the video signal output from the photographing section 2 is subjected to auto gain control (AG
C) After the gain is adjusted in the circuit 3, the color separation circuit 4 separates the luminance signal Y and the color signal C.

[0006] The luminance signal Y is a flare compensation circuit 20
And the floating of the black level is compensated. Although the details of the flare compensation circuit 20 will be described later, the flare-compensated 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 sent to a recorder section 9 via an encoder 8.

The recorder section 9 records the video output of the encoder 8 on a video tape by the head 10.

Here, the floating compensation of the black level in the flare compensation circuit 20 will be described. The integrator 21 of the flare compensation circuit 20 integrates the luminance signal Y (see FIG. 8A), and the integrated value is the average video level (AP
L). 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 is opened when the horizontal synchronizing pulse HD is at a low level. BP is output. The level of the black level pulse BP corresponds one-to-one with the value of APL. The pulse mixer 24 adds a black level pulse BP to the luminance signal Y. The pulse mixer 24 outputs a luminance signal Y (FIG. 8B) to which the black level pulse BP is added during the blanking period. Is output. The clamp circuit 25 detects the black level pulse B
Clamp so that the peak level of P 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,
In the clamp circuit 25, the black level of the luminance signal can be reduced 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, even if a flare image occurs, an image with high contrast can be obtained.

[0012]

However, when the object has a low contrast regardless of flare or the like, the floating of the black level cannot be sufficiently compensated. That is, since the flare compensation is performed by detecting the APL, FIGS. 9 (a) and 9 (b)
As shown in FIG. 9A, when the APL is the same even when the contrast is largely different, the correction amount of the black level becomes the same. Therefore, the black level becomes the pedestal level in FIG. In), the black level cannot be reduced.

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. 7, 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]

The circuit for correcting the black level of a video signal according to the present invention comprises an AGC amplifier for inputting a luminance signal obtained from an imaging section , and a black peak of the luminance signal amplified by the AGC amplifier. A black peak detection circuit for detecting a level in a video period excluding a blanking period;
A black level pulse generating circuit for generating a black level pulse having a level corresponding to the level detected by the black peak detecting circuit, and a generated black level pulse during a blanking period of the luminance signal amplified by the AGC amplifier. A pulse mixer for mixing, a luminance signal mixed with the black level pulse is input, and a clamp circuit for clamping and outputting the luminance signal so as to fix the peak level of the black level pulse to a preset level; An AGC feedback circuit that supplies a control signal corresponding to the level of the luminance signal to the AGC amplifier.

In another configuration of the video signal black level correction circuit according to the present invention, an iris whose aperture for an image pickup unit is variable by a control signal and a black peak level of a luminance signal obtained from the image pickup unit are set to a blanking period. A black peak detection circuit that detects during the excluded video 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, and a black level pulse that generates the black level pulse. A pulse mixer that mixes with the luminance signal during the ranking period and a luminance signal that is mixed with a black level pulse are input, and the luminance signal is clamped and output so that the peak level of the black level pulse is fixed at a preset level. Clamp circuit and control signal according to level of clamped luminance signal It is characterized in that it comprises a feedback circuit for iris give the iris, the.

[0017]

First, a recording system of a camera-integrated VTR incorporating a black level correction circuit developed simultaneously with the present invention is shown in FIG.
(Block diagram) and FIG. 6 (signal waveform diagram).

In FIG. 5, a video signal representing a subject image obtained by the optical system 1 is output from an image pickup unit 2, passes through an auto gain control circuit (AGC amplifier) 3, and is output by a color separation circuit 4 to a luminance signal Y. Color signal C.

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, in order to facilitate understanding, an operation will be described when a rectangular wave-like luminance signal as shown in FIG. 6A is input and a black floating Vb1 occurs.

The adder 31 applies a wide blanking pulse WBL to the luminance signal Y during a blanking period. As shown in FIG. 6B, 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. 6C).

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. 6 (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. 6 (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. 6 (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. 6 (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. 6H shows the clamped luminance signal Y. As can be seen by comparing FIG. 6 (g) and FIG. 6 (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,
Since the black level pulse BP is created and corrected based on the black peak level Vb2 instead of APL, AG
Even when the C circuit 3 operates, the black level can be accurately corrected.

The black level correction circuit 30 shown in FIG. 5 developed simultaneously with the present invention has the following points to be improved. (B) When photographing a bright, low-contrast subject, such as a wall with a whitish surface or a blue sky,
The level of the luminance signal Y increases as a whole as shown in FIG. 3A, and the level Vb2 of the black peak also increases. At the same time, the level Vb1 of the black level pulse BP as shown in FIG.
Will be higher. Therefore, the correction is excessively performed in this state, and the level of the clamped luminance signal Y is greatly reduced as shown in FIG. When this is viewed in a reproduced image, the walls and the like are projected black.

The present inventor has developed an improved black level correction circuit in consideration of the above-mentioned phenomenon (a).

FIG. 1 shows a black level correction circuit 130 according to an embodiment of the present invention, which has the black level correction circuit 30 shown in FIG. 5 provided in an AGC (automatic gain control) loop. It has become. Therefore, a feedback circuit 40 for providing the control signal 40A to the AGC amplifier 3 is added to the circuit 30 in FIG. 5, and the other configuration is the same as that shown in FIG. Eliminate duplicate descriptions.

The feedback circuit 40 comprises an integrating circuit 41 and a differential amplifier 42. The integration circuit 41 inputs and integrates the luminance signal output from the clamp circuit 37 to provide a signal indicating an average level of the luminance signal to the differential amplifier 42. As described above, the output of the clamp circuit 37 clamps the luminance signal obtained by mixing the black level pulse BP corresponding to the black peak Vb2 during the blanking period so that the peak level Vb1 of the black level pulse BP is fixed at the pedestal level. And black level correction has been performed.

The differential amplifier 4 in the feedback circuit 40
2 generates an AGC control signal 40A by comparing the integrated output of the integration circuit 41 with the AGC set voltage, and generates an AGC amplifier 3
Give to. That is, as the integration output (the average level of the luminance signal output from the clamp circuit 37) is smaller, the level of the control signal 40A is higher, and the amplification of the amplifier 3 is higher. 43 is a setting device.

Next, the operation of the black level correction circuit 130 will be described. With reference to FIG. 2, an operation when an object such as a white wall, which has a whitish surface and little contrast, will be described. At this time, the luminance signal (point A in FIG. 1) obtained from the imaging unit 2 is high as shown in FIG. 2A, and the level Vb2 of the output signal of the black peak detection circuit 33 is extremely high. Therefore, when the AGC loop does not operate, the gain of the amplifier 3 is 1, and the luminance signal (point B in FIG. 1) output from the amplifier 3 is as shown in FIG. Therefore, the output waveform (point C in FIG. 1) of the pulse mixer 36 is shown in FIG.
As shown in (b), the level of the black level pulse BP becomes close to the white peak of the luminance signal. As a result, the level of the luminance signal after clamping (point D in FIG. 1) decreases as shown in FIG. 2 (c), resulting in a dark image as a whole.

On the other hand, when the AGC loop is formed according to the present invention, since the level of the luminance signal input to the integration circuit 41 is low, the integrated output is small, and therefore the control signal 40
A increases and the gain of the AGC amplifier 3 increases. As a result, the output of the amplifier 3 becomes larger than the input as shown in FIG. Accordingly, a black level pulse BP is added as shown in FIG. 2 (e), and the level of the luminance signal after clamping is increased as shown in FIG. 2 (f). As a result, the reduction in the level of the white peak is improved as described above.

FIG. 4 shows a black level correction circuit 140 according to another embodiment of the present invention, which has a configuration in which the black level correction circuit 30 shown in FIG. 5 is provided in an iris feedback loop. I have. Therefore, there is no AGC amplifier, and a feedback circuit 40 for providing a control signal 40B to the iris 44 is added to the circuit 30 of FIG. 5, and other configurations are the same as those shown in FIG. To avoid duplication of description.

The feedback circuit 40 comprises an integrating circuit 41 and a differential amplifier 42. The integration circuit 41 inputs and integrates the luminance signal output from the clamp circuit 37 to provide a signal indicating an average level of the luminance signal to the differential amplifier 42. As described above, the output of the clamp circuit 37 clamps the luminance signal obtained by mixing the black level pulse BP corresponding to the black peak Vb2 during the blanking period so that the peak level Vb1 of the black level pulse BP is fixed at the pedestal level. And black level correction has been performed.

The differential amplifier 4 in the feedback circuit 40
2 generates an iris control signal 40B by comparing the integrated output of the integrating circuit 41 with the iris set voltage,
Give to. That is, as the integration output (the average level of the luminance signal output from the clamp circuit 37) is smaller, the level of the control signal 40B is larger, and the opening of the iris 44 is larger. 45 is a setting device.

Next, the operation of the black level correction circuit 140 will be described. In this case as well, an operation when an object such as a white wall, which has almost no whitish contrast and has almost no contrast, will be described. At this time, the luminance signal obtained from the imaging unit 2 is high, and the level Vb2 of the output signal of the black peak detection circuit 33 is extremely high. Therefore, when the iris feedback loop does not operate, the opening of the iris 44 is small, and the level of the black level pulse BP is close to the white peak of the luminance signal. As a result, the level of the luminance signal after clamping is reduced, resulting in a dark image as a whole.

On the other hand, when the iris feedback loop is formed according to the present invention, since the level of the luminance signal input to the integration circuit 41 is low, the integration output is small, and thus the control signal 40B is large and the iris 44
Opens greatly. As a result, the output of the imaging unit 2 increases. Accordingly, the luminance signal becomes sufficiently large, and the level of the luminance signal after clamping becomes large.

In each of the above-described embodiments, the feedback circuit 40 supplies the control signal 40A or 40B corresponding to the average level of the clamped luminance signal to the AGC amplifier 3 or the iris 44, but is limited to the average level. Not something. For example, a control signal corresponding to the white peak of the clamped luminance signal may be provided. In this case, a white peak detection circuit may be used instead of the integration circuit 41.

[0039]

According to the present invention, the level of the black level pulse corresponds to the black peak level (black floating level) of the luminance signal in the present invention. The black level can be reliably corrected in accordance with the amount, and excellent contrast can be improved even if a flare or the like occurs or the subject has a low contrast.
Further, since the black level correction circuit is included in the AGC loop or the iris feedback loop, even when the whole is whitish and has little contrast, the black level correction does not make the image too dark (white level decrease).

[Brief description of the drawings]

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

FIG. 2 is a signal waveform diagram showing an operation when there is almost no contrast in the present invention.

FIG. 3 is a diagram showing a signal waveform when the present invention is not applied.

FIG. 4 is a block diagram showing another embodiment of the present invention.

FIG. 5 is a block diagram showing a black level correction circuit developed simultaneously with the present invention.

FIG. 6 is a diagram showing signal waveforms of a black level correction circuit developed simultaneously with the present invention.

FIG. 7 is a block diagram showing a conventional technique.

FIG. 8 is a diagram showing a signal waveform in the related art.

FIG. 9 is a diagram showing a problem at the time of low contrast in the related art.

[Explanation of symbols]

 3 AGC Amplifier 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 40 Feedback Circuit 41 Integration Circuit 42 Differential Amplifier 43 Setting Device 44 Iris 45 Setting Device 130 black level correction circuit 140 black level correction circuit

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-54-16919 (JP, A) JP-A-64-37174 (JP, A) JP-A-60-239178 (JP, A) 75780 (JP, A) JP-A-2-33276 (JP, A) JP-A-1-198873 (JP, A) JP-A-63-24774 (JP, A) JP-A-59-164360 (JP, U) (58) Field surveyed (Int.Cl. ⁶ , DB name) H04N 5/14-5/217

Claims (2)

(57) [Claims] An AGC amplifier for inputting a luminance signal obtained from an image pickup unit; and a black peak detection circuit for detecting a black peak level of the luminance signal amplified by the AGC amplifier in a video period excluding a blanking period. A black level pulse generation circuit for generating a black level pulse having a level corresponding to the level detected by the black peak detection circuit; and a blanking period of a luminance signal obtained by amplifying the generated black level pulse by an AGC amplifier. And a clamp circuit that receives the luminance signal mixed with the black level pulse and clamps and outputs the luminance signal so that the peak level of the black level pulse is fixed at a preset level. A control signal corresponding to the level of the
A black level correction circuit for a video signal, comprising: an AGC feedback circuit provided to a GC amplifier. 2. An iris whose aperture for an imaging unit is variable by a control signal, a black peak detection circuit for detecting a black peak level of a luminance signal obtained from the imaging unit in a video period excluding a blanking period, and a black peak detection. A black level pulse generation circuit for generating a black level pulse corresponding to a level detected by the circuit; a pulse mixer for mixing the generated black level pulse with a luminance signal during a blanking period; and a black level pulse. And a clamp circuit that receives a luminance signal mixed with the luminance signal and clamps and outputs the luminance signal so that the peak level of the black level pulse is fixed at a preset level, and a control signal corresponding to the level of the clamped luminance signal. And an iris feedback circuit for providing the iris with A black level correction circuit for a video signal.