JPH04236579A - Black level correcting circuit for video signal - Google Patents

Abstract

PURPOSE:To improve the contrast with an inexpensive and simple circuit even when flare or the like takes place. CONSTITUTION:A clamp circuit 37 clamps a luminance signal Y so that a peak level of a black level pulse BP mixed by a pulse mixer 36 is fixed to a pedestal level. A level of the black level pulse BP corresponds to a minimum level (black floating level) of the luminance signal detected by a black peak detection circuit 33. Thus, the luminance signal level is adjusted in response to the black floating level.

Description

[Detailed description of the invention]

0001

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

0002

2. Description of the Related Art In a video camera, an image of a subject is formed on the imaging surface of an image sensor using an optical system, and a video signal representing the subject is extracted from the image sensor. However, 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 the contrast of the image decreases. 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 slightly dirty window glass, or when photographing with a telephoto lens on a hazy day, the contrast of the image is low and the blackness of the video signal is low. The level will become higher.

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 (avera
ge picture level (APL)) and corrected the rise in black level according to the value of APL.

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

[0006] The luminance signal Y is input to a flare compensation circuit 20, and the rise in the black level is compensated for. Although the details of the flare compensation circuit 20 will be described later, the flare-corrected luminance signal Y is subjected to γ correction in the γ correction circuit 5, and the video signal and noise during the blanking period are 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 is subjected to necessary color signal processing in a color signal processing circuit 7, passes through an encoder 8, and is sent to a recorder section 9.
sent to.

[0008] The recorder section 9 records the encoder's video out onto a video tape using the head 10.

[0009] Here, compensation for rising black level in the flare compensation circuit 20 will be explained. Since the integrator 21 of the flare compensation circuit 20 integrates the luminance signal Y (see FIG. 7(a)), the integrated value indicates the average video level (APL). The level control circuit 22 outputs a value according to the APL, and the switch 23 is turned on when the horizontal synchronizing pulse HD is high level and opened when it is low level, so the black level is set in synchronization with the horizontal synchronizing pulse HD. Pulse BP is output. The level of the black level pulse BP corresponds one-to-one to 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 the luminance signal Y (FIG. 7(b)) to which the black level pulse BP is added during the blanking period. Ru. 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 significantly due to an increase in the amount of flare, but at this time, the black level pulse BP increases depending on the APL, and the clamp circuit 25 controls the black level pulse BP. The black level of the luminance signal can be lowered by the same amount.

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, the conventional flare compensation circuit 20 cannot sufficiently compensate for the rise in black level when the subject has low contrast regardless of flare or the like. In other words, since flare compensation is performed by detecting the APL, even if the contrast is greatly different as shown in Figures 8(a) and (b), if the APL is the same, the amount of black level correction will be the same. Figure 8 (a) of contrast
), the black level reaches the pedestal level, but in the low-contrast image shown in FIG. 8(b), the black level cannot be lowered completely.

Further, the conventional flare compensation circuit 20 has an AG
When the C circuit 3 is operated, the effect of black level rise compensation cannot be achieved. 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 (APL) of the output of the integrator 21 of the flare compensation circuit 20 (point B) also becomes constant, and as a result, the amount of black level correction is always constant. It becomes constant and becomes ineffective. This is particularly problematic since AGC circuits 3 are commonly used in home video cameras.

In view of the above-mentioned prior art, the present invention not only improves contrast reduction due to flare, but also improves the contrast of low-contrast images not caused by flare etc., and also improves the contrast of video signals even when the AGC circuit is operating. It is an object of the present invention to provide a circuit that can accurately perform black level correction.

[0015]

[Means for Solving the Problems] The structure of the present invention which solves the above problems includes a black peak detection circuit that detects the lowest level among the levels 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 detection circuit; a pulse mixer that mixes the generated black level pulse with a luminance signal during a blanking period; The present invention is characterized in that it includes a clamp circuit that receives a luminance signal in which pulses are mixed and that clamps and outputs the luminance signal so as to fix the peak level of the black level pulse at a preset level. Another feature is that the frequency control circuit suppresses high frequency components in the luminance signal, and the black peak detection circuit detects the lowest level of the luminance signal with the high frequency components suppressed.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of the present invention will be described below in detail with reference to the drawings. Figure 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 the figure, a video signal representing 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 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.
Record on tape.

The black level correction circuit 30 will now be explained with reference to FIGS. 1 and 2. In this example, to facilitate understanding, the operation will be described when a rectangular waveform luminance signal as shown in FIG. 2(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. 2(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. 2(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. 2(c). Then, the black peak detection circuit 33 outputs a black peak signal VB whose signal level is Vb2 (FIG. 2(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. 2(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. 2 (
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. 2(g).

The clamp circuit 37 clamps the luminance signal Y so as to fix the peak level Vb1 of the black level pulse BP at the pedestal level. The clamped luminance signal Y is shown in Fig. 2(h). As can be seen by comparing FIG. 2(g) and FIG. 2(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.

[0024] Also, the black peak level Vb is not the APL.
Since the black level pulse BP is created and corrected based on 2, the black level can be accurately corrected even when the AGC circuit 3 is operating.

[0025] Furthermore, since the level of the luminance signal, which appears black, can be lowered, the dynamic range of the video signal is slightly expanded.

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

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, when a brightness signal Y as shown in FIG. 4(a) is input to the frequency control circuit 38, the frequency control circuit 38 outputs a filtered brightness signal Y as shown in FIG. 4(b). Output. That is, the output luminance signal Y has a low black peak level in a signal portion with a wide pulse width (low frequency), but the black peak level increases in a signal portion with a narrow pulse width (high frequency). This means that the black peak levels V0 and V1 shown in Figure 4(b)
, V2, and V3. Of course, by changing the filtering characteristics of the frequency control circuit 38, it is also possible to adjust the rate at which the black peak level changes depending on the frequency.

The main operations of the embodiment shown in FIG. 3 will now be explained with reference to FIG. When a subject (referred to as a main subject) is photographed through a wire mesh, the luminance signal Y includes a low-level, high-frequency video signal K, as shown in FIG. 5(a). 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. 5(b). Therefore, the black peak detection circuit 38 detects the black peak Vb3 based on the luminance signal Y shown in FIG.
working together to generate a black level pulse BP whose level is Vb3. Therefore the pulse mixer 36
As shown in FIG. 5(c), a luminance signal Y (which includes the video signal K) mixed with a black level pulse BP whose level is Vb3 is output. Since the clamp circuit 37 clamps the peak of the black level pulse BP as shown in Figure 5(d), the clamp circuit 37 can clamp the black level of the main subject to the pedestal level as shown in Figure 5(e). , the black float can be optimally adjusted.

Incidentally, if the frequency adjustment circuit 38 is not provided, the black peak level will be Vb4, and the level of the black level pulse BP will also be as small as Vb4, resulting in insufficient correction for black floating.

[0030]

[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. Reliable black level correction is possible,
Good contrast can be improved even when flare etc. occur.

Furthermore, since the average video level (APL) is not used, a complicated adjustment circuit for adjusting nonlinear characteristics is not required, the circuit configuration is simplified, and costs are reduced.

[Brief explanation of the drawing]

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

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

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

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

FIG. 5 is a waveform diagram showing signal waveforms in a second embodiment.

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

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

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

[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 Frequency control circuit

Claims (2)

[Claims] Claim 1: A black peak detection circuit that detects the lowest level among the luminance signal levels in a video period excluding a blanking period, and a black whose level corresponds to the level detected by the black peak detection circuit. A black level pulse generation circuit that generates a level pulse, a pulse mixer that mixes the generated black level pulse with a luminance signal during the blanking period, and a luminance signal mixed with the black level pulse is inputted, and the black level pulse is A black level correction circuit for a video signal, comprising: a clamp circuit that clamps and outputs a luminance signal so as to fix the peak level to a preset level. 2. A frequency control circuit that suppresses high frequency components of the luminance signal and passes the luminance signal; A black peak detection circuit that detects a low level, a black level pulse generation circuit that generates a black level pulse whose level corresponds to the level detected by the black peak detection circuit, and a black level pulse generation circuit that generates a black level pulse that has a level corresponding to the level detected by the black peak detection circuit, and a blanking period for the generated black level pulse. A pulse mixer that mixes the luminance signal with the black level pulse, and a clamp circuit that receives the luminance signal mixed with the black level pulse, clamps the luminance signal so as to fix the peak level of the black level pulse at a preset level, and outputs it. A black level correction circuit for a video signal, comprising: