JPS60170377A - Automatic black level controlling circuit - Google Patents

Abstract

PURPOSE:To change rapidly the brightness of a picture plane following the change of level even when an image level differs remarkably and to always project stably the pictures by integrating the change in level of video signals. CONSTITUTION:A digital luminance signal YD separated by a Y/C separator of a digital TV receiving set is added to a clamping circuit 21 of an automatic black level controlling circuit 20, and pedestal level is clamped at a specified position. The level clamped output is added to a black level detecting circuit 40 having the function of integration, and the lowest black level LB in the signal YD is detected and added to a switching device 21 together with reference level LR, and the device 21 is controlled by the output of a comparator 22. Outputs selected by the device 21 and the signal YD are added 24, and the black level is shifted to ''0'' level, and an output signal YD' is added to a picture control circuit 25. Brightness of the picture is changed rapidly following the level change even when an image level differs remarkably in the circuit 25, and a stable picture is always projected.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention is an automatic black level control circuit suitable for application to a so-called digital color television receiver in which a luminance signal and a color signal are processed purely digitally. Regarding.

BACKGROUND TECHNOLOGY AND PROBLEMS In television receivers, an automatic black system detects the darkest part below a predetermined level in a video signal and automatically shifts the level to a predetermined black level, such as a pedestal level. Some devices are equipped with a level control circuit, and further detect the brightest portion of the level-shifted video signal and automatically adjust the picture so as to maintain it at a predetermined level, for example, the white level. By providing such an automatic black level control circuit, black parts become less black and white parts become more white, improving contrast and making it possible to reproduce clearer images.

FIG. 1 shows an example of such a black level control operation. As shown in FIG. When the black level is the darkest level LBt - the black level, only the video information section of the video signal Sv is level-shifted so that this level LB becomes a new black level, for example, the pedestal level (FIG. 1B) .

Second, peak level L of video signal Sv after level shift
W is detected, and the level is controlled so that this level becomes a predetermined level, for example, the white level (C in the same figure).

As a result, even if there is a dark part in the video signal,
A clearer image with higher contrast is reproduced (hereinafter referred to as "level shift" and peak level control is referred to as "dynamic picture control").

By the way, in the automatic black level control circuit that performs dynamic picture control like this, the predetermined level L
It detects whether there is a dark part below B and controls the gain of the video output circuit so that the peak level Lw (B in Figure 1) is at a predetermined level. When an image containing only bright parts is displayed immediately after the image is displayed, the gain of the video output circuit is momentarily adjusted to be smaller, contrary to the above-described operation.

Therefore, in such a case, the brightness of the screen changes instantaneously, causing an eyesore. Contrary to the above, even if an image containing dark parts is displayed immediately after an image containing only bright parts, the brightness of the screen will similarly change rapidly.

Purpose of the Invention Therefore, the present invention proposes an automatic black level control circuit that follows the level change and prevents the screen brightness from changing suddenly even when the video level changes instantaneously and greatly. It is something to do.

SUMMARY OF THE INVENTION Therefore, in this invention, instead of directly supplying the video signal Sv to the detection system that detects the level LB, the video signal Sv is first supplied to an integrator in the field direction using a field memory, and its integrated output is It is designed to be supplied to a level detection system.

Since level changes are thereby integrated, control that would cause sudden changes in screen brightness is not performed.

Embodiment Next, an example of the present invention applied to a digital color television receiver will be described in detail with reference to FIG. 2 and subsequent figures.

FIG. 2 is a system diagram showing an example of a digital color television receiver, in which a television signal supplied to a terminal (1) is passed through a tuner (2) and a video intermediate frequency amplifier (3) to an A/D converter. The signal is supplied to a converter (4), where the television signal is converted into a digital signal while remaining in a composite state in this example. The digitized television signal is supplied to a Y/C separator (5) and is separated into a Neifushi digital luminance signal YD and a digital carrier color signal CD;
The digital carrier color signal CD is supplied to a digital color demodulator (7) via an ACC circuit (6), and a pair of digital color difference signals (RY)D and (B-Y)D are demodulated.

The digital luminance signal YD is passed through the automatic black level control circuit 01 according to the present invention to the pair of digital color difference signals (
D/A conversion converter C along with R-Y)D and (B-Y)D
A luminance signal Y and a pair of color difference signals R
-Y, BY is converted into analog.

These are supplied to a matrix circuit (2) to form primary color signals R, G, and B. These primary color signals R2G are supplied to a B company cathode ray tube (not shown) to reproduce a predetermined color image.

The automatic black level control circuit is constructed as shown in FIG.

The pedestal level of the digital luminance signal YD (A1 in FIG. 4, however, all are shown as analog signals in FIG. 4) supplied to the terminal (20a) is clamped to a predetermined level VP by a clamp circuit Qη. From the output, a black level detection circuit o1 having an integration function detects the lowest level signal (referred to as black level LB) among the 7J digital luminance signal Y integrated after pedestal clamping. This black level LB is the lowest level of the digital luminance signal YD of a predetermined field (one field in this example).

Black side/-Z Le LB is gray level, e.g. Ha10~20
It is supplied to the switching means together with the reference level LR (FIG. 4A) set at the IRE, and as described later, L is set depending on the magnitude of the black side level LB with respect to the reference level LR.
Either B or LR is selected.

Therefore, the black level LB is compared with the reference level LR in a digital comparator), and when LB<LR, the black level LB itself is output, and when LB>LR, the reference level L8 is output. , to the switching means) is controlled by its data comparison output.

The reason why the reference level LR is used instead of the bad example level LB is to absorb differences in the set-up levels of television signals transmitted from each broadcasting station and to equalize the black level.

The bad example level LB or reference level LR selected by the switching means Qv is added to the digital luminance signal YD in an adder (c). Since the adder (C) performs two's complement calculation, the digital luminance signal Y is calculated in this adder (C).
For example, the bell LB or the reference level LR is subtracted from D.

For example, when LB<LR (Fig. 4A), YD-LB
A subtraction process is performed, and the level is shifted so that the bad example level LB becomes a digital 0 level (data that is all "0#") (FIG. B). Therefore, the bad example level L
A higher level than B is positive digital data, and a lower level than bad example level LB is negative digital data.

The addition output Y'D is supplied to the picture control circuit (c). Picture control circuit (c) is CR
This is for controlling the peak level of the brightness signal according to the beam current transport of T, and the beam current transport supplied to the terminal (c) is supplied to the beam current detection circuit gradient, and the beam current transport is detected. , this is the A/D converter (c)
The digital signal is converted into a digital signal at , and the digital signal is supplied as a picture control signal to a multiplier (c) where it is subjected to arithmetic processing based on the addition output Y.

For example, in the case of the digital luminance signal YD shown in FIG. 4A,
By shifting the bad example level LB to the digital 0 level, the peak level Lw becomes much closer to the black level side (Fig. 4B), so the picture control circuit (c) shifts the peak level Lw to a predetermined level. , for example, the white level (data of all "1") is controlled (FIG. 4C).

In addition, in order to disable picture control during the horizontal blanking period H-BLK, the picture control circuit (c) is provided with an AND circuit 0. The control system is further provided with a multiplier 0, so that the peak level Lw can be controlled by an external picture control signal (digital data) M-FIX.

The picture-controlled addition output Y% is again supplied to the adder 0, and the pedestal data PD is added.
The digital 0 level is data-shifted to the black level of a normal video signal, for example, the vedestal level scanner, that is, the cutoff level of a CRT. As a result, the image is reproduced so that the bad example level LB becomes the original black level.

In this way, a digital luminance signal YDo whose level is controlled so that the bad example level LB becomes the original black level is obtained at the output terminal (20b).

Contrary to the above, when the bad example level LB is larger than the reference level LR (FIG. 4D), the switching means Q1 selects the reference level LR, so at this time the line reference level L
R is subtracted from the digital luminance signal YD, and the reference level LR is level-shifted to a digital 0 level (FIG. 4, Jp). As a result, the output terminal (20
In b), a digital luminance signal YDo with this digital 0 level as the pedestal level is obtained.

In this way, if the level is controlled so that the reference level LR becomes the pedestal level when LB>LR, even if the black level value differs depending on the broadcasting station, the unevenness of black reproducibility due to the difference in set-up level can be avoided. It is possible to completely eliminate the black level, and even in the case of a video signal with a high setup level, it is possible to reproduce the original black level.

FIG. 5 shows an example of the black level detection circuit θ0, in which the digital luminance signal YD supplied to the terminal 61) is supplied to the field direction integrator (6) and integrated in the field direction.

The integrator) is a case where a recursive digital low/'P filter is used, and it has one field of memory, and the digital luminance signal YD is sent to the adder via the first attenuator). while the output of the adder (to) is supplied to the memo V
It is supplied to the adder 0Q via the OS and the second attenuator (g). As is well known, when the attenuation constant of the first attenuator (the number of time steps of the ronos and filters) is K, the attenuation constant of the second attenuator is selected as (1-1/K). Ru.

By using this integrator θ group, the adder (c)
Since the output digital luminance signal, that is, the digital integrated output YDI, is obtained by integrating in the field direction, even if there is a sudden change in the field direction, it is converted into a slow change.

The digital integral specific power YDI is supplied to the level detector shaft, and in this example, the worst example level LB, which is the worst example of the digital luminance signal YD of one field, is detected. Therefore, the digital integrated output YDI is first supplied to the lowest level detector 6υ to detect the lowest bad example level LBH within one horizontal period, and this is supplied to the first latch circuit 6 composed of D flip frogs. At the same time, the latch output "'BH" and the bad example level "BH" are supplied to the comparator ■, and the latch output "'BH" which is the previous bad example level is outputted to 4.
l and the bad example level LBH on the current horizontal line are compared, and when the bad example level LBH obtained from the current horizontal line is smaller than the latch output "BH", the outputs PP and N of the comparator 6 cause the first The latch circuit of is set to enable mode, and the bad example level LB obtained from the current horizontal line is
H is latched.

Since such a comparison operation is performed continuously during one field, after one field has elapsed, the worst case level in that field is obtained as the latch output "BH". Therefore, this latch output "BH" This results in the above-mentioned bad example level LB.

This bad example level LB is latched by the second latch circuit (A) composed of a D flip-flop and output terminal (A).
For each chef yield, the worst case level LB, which is the worst case in each field, is output. Therefore,
The first latch circuit 6 is supplied with a horizontal pulse PH having a line period as a clock, a vertical pulse Pv having a field period as a pulse, and a second latch circuit 62.
The vertical/4' pulse Pv is supplied as a clock to the latch circuit.

If the bad example level LB is to be detected not in units of one field but in units of n fields, it is sufficient to use vertical/yellow pulse Pv with an n-field period.

When configuring the black level detection circuit θ1 in this way, the digital integral output YD1 integrated in the field direction is
Since the bad example level LIl is detected based on the contrast of the screen, even if the image suddenly changes in the field direction, the bad example level LB in the suddenly changed image will not be detected. 7) j It is possible to eliminate defects that change rapidly.

When the bad example level LB is detected in units of n fields, the rate of change (width of change) in the contrast of the screen can be made gentler than in the case described above.

Of course, even when detecting the signal LBf in units of one field, the same effect as in the case of n-field detection can be obtained by increasing the constant K of the integrator 0.

Constants may be configured in variable forms. That is, in the case of moving images, it is more effective to change the constant K depending on whether the motion is fast or slow than when the constant is fixed.

In that case, an inter-field motion detector Q71'& is added to the above-mentioned integrator θ group as shown in FIG. The first and second attenuators 12 are connected to the output of the detector θ.
All you have to do is control the wheel. Constant K; for example, 2m
When it is expressed as , the exponent part m may be controlled depending on the magnitude of the detected output. Such control can be easily achieved by installing a microcontroller.

In this way, by controlling the constant K for moving images that are close to still images and moving images with rapid movement, even when noise that cannot be compensated for by the integral effect in black level detection occurs, , the rate of change in screen contrast is reduced, and a stable image can be displayed.

As described in detail, according to this invention, even if the video level changes drastically in an instant, the brightness of the screen does not change suddenly, so a stable image can be displayed at all times. .

[Brief explanation of the drawing]

FIG. 1 is a waveform diagram for explaining the operation of this invention, FIG. 2 is a system diagram showing an example of a digital color television receiver to which this invention is applied, and FIG. 3 is an automatic black level control circuit according to this invention. FIG. 4 is a system diagram showing an example of the integrator. FIG. 4 is a waveform diagram for explaining its operation. FIG. 5 is a system diagram showing an example of the black level detection circuit. FIG. 6 is a system diagram showing another example of the integrator. . (e) is an automatic black level control circuit, 06 is a black level detection circuit, Q is a comparator, (c) is a picture control circuit, @→ is an integrator in the field direction, and sail is a level detector.

Claims (1)

[Claims] The video output is performed through an integrator in the field direction using field memory, and the blackest level lower than a predetermined level is detected in a predetermined field unit, and the detected level is controlled to become a new black level. Automatic black level control circuit.