JPS61114660A - Circuit for correcting black level of video signal - Google Patents

Abstract

PURPOSE:To reproduce always a stable black level by providing a comparator circuit for the set black level and the black level of a video signal and resetting the set black level according to the result of comparison so as to absorb the fluctuation of the black level due to the fluctuation of a set up level. CONSTITUTION:An operation circuit 4 makes the black level of the output digital video signal 10 coincident with the setting black level given by a signal 13 independently of the black level of the video signal is input. The comparator circuit 5 compares the video signal subject to pedestal clamp with the setting black level signal 13 outputted from a level adjusting circuit 8 and when the minimum level (black level) of the video signal is smaller than the level of the setting black level signal 13, a detection signal 14 is outputted. A counter circuit 6 counts the number of generation of the detection signals 14 and when the count reaches a prescribed count number with a prescribed time, a count- down signal 15 is outputted. When the prescribed count number is small, the sensitivity of black level detection is increased, and when larger, the sensitivity is lower.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a black level correction circuit for a video signal that can be used in a digital television receiver or the like that digitizes a video signal and processes the signal.

[Background of the invention]

In ordinary television receivers that process video signals as analog signals, black level correction for video signals is used to prevent the black level of the video signal from fluctuating for some reason and the brightness of the image becoming unstable. The circuit is known from the prior art.

For example, as a method for correcting variations in black level due to variations in the set-up level of video signals between stations (channels), as shown in Japanese Patent Application Laid-Open No. 56-28570, detects and holds a level that is at the level of and is on the black side from a predetermined level,
It is generally known that a video signal is processed so that the detected and held level becomes the black level of the video signal.

However, with this method, the blackest detected level is held and signal processing is performed so that this level becomes the black level of the video signal5. When the black level detection sensitivity is increased, the black level of the image changes sharply, and the whitish background suddenly becomes brighter, giving an unnatural impression.

In order to prevent such sharp black level fluctuations, it is necessary to constantly lower the black level detection sensitivity, but this creates the problem of not being able to adequately compensate for variations in black level. .

[Purpose of the invention]

The present invention has been made to overcome the disadvantages of the prior art as described above, and therefore, the purpose of the present invention is to:
If a dark area suddenly appears on an entirely whitish screen, even if you increase the black level detection sensitivity, the black level of the image will not change sharply, and therefore it will appear unnatural to the viewer. It is an object of the present invention to provide a black level correction circuit for a video signal which does not give a black level.

[Summary of the invention]

The present invention provides a black level correction circuit for a video signal, which compares and calculates the signal level of an input digital video signal with a certain set black level, and outputs the black level of the video signal so as to match the set black level. , a comparison circuit is provided for comparing the set black level and the black level of the video signal, and as a result of the comparison, if a level blacker than the set black level is detected during the non-blanking period of the video signal, the detection frequency is Accordingly, the set black level is further shifted to a level on the black side and set again, and according to the length of the period in which a level on the black side than the set black level was not detected during the non-blanking period of the video signal. The set black level is now moved to the white level and the setting is made again.

[Embodiments of the invention]

The present invention will now be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention. In the figure, 1 is an input digital video signal, 2 is a pedestal clamp circuit, 3 is a blanking signal created from a human video signal, 4 is an arithmetic circuit, 5 is a comparison circuit, and 6 is a comparison circuit 5 a counting circuit that counts the number of outputs from the comparator circuit 5; 7 a measurement circuit that measures the length of the period between outputs from the comparison circuit 5; 8 a level adjustment circuit for the set black level; 9 a level limit circuit; 10 is an output digital video signal, 11 is a reference signal, 13 is a set black level signal, 14 is a detection signal, 15 is a countdown signal, and 16 is a countup signal.

In addition, the circuit portion S surrounded by the broken line is the set black level signal 13.
This circuit compares the level of the set black level signal 16 with the black level of the input digital video signal 1 whose pedestal level is clamped in the frang circuit 2, and adjusts the level of the set black level signal 16 itself to an appropriate level based on the comparison result. be.

Details of the arithmetic circuit 4 in FIG. 1 are shown in FIG. 2. In the figure, 13 is a set black level signal, 17 is a subtraction circuit, 18 is a switching circuit, 3 is a blanking signal created from the input video signal, and ■ is the waveform of the input digital video signal with the pedestal level clamped. Indicates @
indicates the black level set by the signal 13, and θ indicates the waveform of the output digital video signal 1o corrected for the black level.

In FIG. 2, the subtraction circuit 17 is a circuit that performs subtraction by using the video signal with the pedestal level clamped as the minuend input and the set black level signal 13 as the subtrahend input. 18 is input.

The switching circuit 18 outputs the digital value "D" during the blanking period according to the blanking signal 3, and outputs the input from the subtraction circuit 17 as is during the non-blanking period. According to this configuration, the switching circuit 18 outputs an output digital video signal 10 in which the black level set by the signal 13 is exactly the black level of the video signal.

That is, regardless of the black level that the input video signal ■ had at the time of input, the set black level @ given by the signal 13 is set as its new black level, that is, output as the output video signal θ. That's why.

Referring again to FIG. As already explained with reference to FIG. 2, the function of the arithmetic circuit 4 is to adjust the black level of the output digital video signal 10 to the set black level given by the signal 13, regardless of the black level at the time of input of the video signal. to match the level (this itself corresponds to black level correction operation)
It is in.

In this case, unless the level of the set black level signal 13 supplied from the circuit section S to the arithmetic circuit 4 is constantly adjusted to an appropriate level, appropriate black level correction cannot be performed.

The level adjustment operation of the set black level signal 130 in the circuit portion S will be described below.

For ease of explanation here, the digital video signal is
Assume that as the digital value decreases, the reproduced image tends to become darker.

The comparison circuit 5 compares the levels of the pedestal clamped video signal and the set black level signal 15 output from the level adjustment circuit 8, and if the lowest level (black level) of the video signal is lower than the set black level signal 130 level. ,
In other words, when the black level of the video signal is further black than that of the signal 13, the detection signal 14
Output. The comparison circuit 5 does not perform the comparison operation during the video blanking period due to the blanking signal B, and therefore does not output the detection signal 14 during this period, and performs the comparison operation only during the non-blanking period.

The detection signal 14 output from the comparator circuit 5 is input to the counting circuit 6 and the measuring circuit 7. The counting circuit 6 receives the detection signal 14
counts the number of occurrences of , and when it reaches a predetermined count within a certain period of time, a countdown signal 15 is sent.
Output.

Here, when the predetermined count number is small, the black level detection sensitivity becomes high, and when it is large, it becomes low. In other words, the magnitude of the predetermined count number determines the black level tracking speed of the image. be.

The measurement circuit 7 is a circuit that measures the period from when the detection signal 14 is first generated until when the detection signal 14 is generated next.

That is, if this period is long, it indicates that the level of the input video signal is shifted toward the white side as a whole, and conversely, when this period is short, it indicates that the level of the input video signal is shifted toward the black side as a whole.

This measuring circuit 7 constantly counts clock pulses, and can be constituted by a counting circuit that is reset and restarted from zero when the detection signal 14 is input. In other words, the count value at the time of reset indicates the interval of the detection signal 14.

When the period measured by the measuring circuit 7 reaches a certain predetermined period length, the measuring circuit 7 outputs a count-up signal 16. Here, a black level of 13 is set for the video signal for a predetermined period.
For example, when a video signal with a different set-up level is received, the level of the set black level signal 13 is shifted to the white side when a black level lower than 160 is not detected. We are making adjustments accordingly.

The level adjustment circuit 8 is also constituted by a counting circuit.

That is, the countdown signal 15 and the countup signal 16 are input to a level adjustment circuit 8 as a counting circuit that outputs the set black level signal 16 as a count value, and a certain level adjustment circuit 8 from the counting circuit outputs the countdown signal 16. 15, the count value is counted down to correct (adjust) the set black level signal 13 to a smaller value, that is, to a level on the black side, and the count up signal 16 is used to count up the count 11 to increase the set black level signal 13. In other words, the level is corrected (adjusted) to the white side.

The set black level signal 13 corrected to an appropriate level in this manner is input to the comparator circuit 5 and the arithmetic circuit 4.

Next, the limiting circuit 9 will be explained. This limiting circuit 9 is
No matter what kind of adjustment is made in the level adjustment circuit 8, the purpose is to limit the set black level signal 13 to be smaller than a certain reference signal 11, that is, to be a signal on the black side.

This reference signal 11 is used for the black level correction operation by the level adjustment circuit 8 in the case of a relatively bright image where there is no black level lower than the level of the set black level signal 13 in the video signal. Therefore, this signal is used to limit the gradual darkening of the image, and uses a signal that is slightly closer to the white level than the black level in a normal video signal.

As described in step 5 above, the black level of the input video signal is appropriately corrected, and the output digital video signal 10 is obtained in which steep black level fluctuations are prevented.

FIG. 6 is a block diagram showing another embodiment of the present invention.

That is, this is an embodiment in which a single switching circuit 12 is added in place of the limiting circuit 9 in the embodiment of FIG. 1 to achieve the same effect.

In FIG. 3, the operations of the arithmetic circuit 4 and circuit portion S are exactly the same as those in FIG.

In the switching circuit 12, the pedestal clamped video signal, the reference signal 11 and the blanking signal 3
is input, and the output signal of the switching circuit 12, that is, the input signal to the comparison circuit 5, becomes the reference signal 11 during the video blanking period due to the blanking signal 3, and becomes the pedestal clamped video signal during the non-blanking period. Become.

1 Comparison circuit 5 compares the above signal with the set black level signal 16 output from the level adjustment circuit 8 over the entire period, regardless of blanking period or non-blanking period, and compares the above signal with the set black level signal 16 from the set black level signal 13. When the signal is small, that is, when it is a black signal, a detection signal 14 is output.

According to this embodiment, it is clear that the same effect can be obtained without using the limiting circuit 9.

Note that in the above embodiment, the black level correction is performed after pedestal clamping the digital video signal, but the black level correction may be performed by digitizing the analog video signal after pedestal clamping or peak clamping the analog video signal. . Furthermore, in order to prevent the comparison circuit from erroneously detecting noise in the video signal, the video signal may be band-limited and then input to the black level correction circuit. Furthermore, the arithmetic circuit 4 for black level correction in the embodiment
is a delay circuit that compensates for the signal delay caused by black level detection, and a digital signal underflow that occurs as a result of calculation.
It may also include a circuit for correcting overflow.

〔Effect of the invention〕

According to the present invention, when receiving a video signal, it is possible to absorb fluctuations in the black level due to fluctuations in the set-up bell, and always reproduce a stable black level. It is also possible to prevent sudden fluctuations in the black level due to sudden changes in the image, and it is possible to stabilize the quality of the reproduced image.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention, FIG. 2 is a block diagram showing details of the arithmetic circuit 4 in FIG. 1, and FIG. 3 is a block diagram showing another embodiment of the invention. be. Code explanation 1... Input digital video signal, 2...
Pedestal clamp circuit, 3...Blanking signal, 4... Arithmetic circuit, 5... Comparison circuit, 6... Counting circuit, 7... ...Measuring circuit, 8... Level adjustment circuit, 9... Level limiting circuit, 10... Output digital video signal,
11...Reference signal, 12...Switching circuit, 13...Setting black level signal, 14...
・Detection signal, 15... Countdown signal, 1
6... Count up signal, 17...
Subtraction circuit, 18...Switching circuit agent Patent attorney
Akira Namiki Futomi Figure 1 Figure 2

Claims (1)

[Scope of Claims] 1) The black level of a video signal is calculated by comparing the signal level of an input digital video signal with a certain set black level, and outputs the black level of the video signal so as to match the set black level. The level correction circuit includes a comparison circuit that compares the signal level of the input digital video signal with the set black level excluding the blanking period, and as a result of the comparison, the black level of the video signal becomes lower than the set black level (and The result of the same comparison is the counting circuit that counts the frequency (on the black side).
a measurement circuit that measures a period from a certain point in time when the black level in the video signal becomes lower than the set black level to the next point in time when the black level becomes lower than the set black level; and a set level of the set black level according to the frequency counted by the counting circuit. A black level correction circuit for a video signal, comprising: a level adjustment circuit that lowers the set level and raises the set level according to the length of the period measured by the measuring circuit. 2) In the black level correction circuit according to claim 1, the set level of the set black level is limited so as not to rise above at least a certain limit level, regardless of the operation of the level adjustment circuit. A black level correction circuit characterized in that it is provided with means for. 3) In the black level correction circuit according to claim 1, the comparison circuit compares the level of a certain standard signal with the set black level during the blanking period. Signal black level correction circuit.