JPH0580865B2 - - Google Patents

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 setup level of video signals between stations (channels), Japanese Patent Laid-Open No. 56-28570
As shown in the publication, a level that is the blackest level within a video period and is blacker than a predetermined level is detected and held, and this detected and held level becomes the black level of the video signal. It is generally known to process video signals.

However, this method retains the blackest detected level and performs signal processing so that this level becomes the black level of the video signal. When a portion of the image appears, 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 feeling.

In order to prevent such steep black level fluctuations, it is necessary to lower the black level detection sensitivity at all times, but this results in the problem that it is not possible to sufficiently correct variations in black level.

[Purpose of the invention]

The present invention has been made in order to overcome the disadvantages of the prior art as described above.Therefore, an object of the present invention is to solve the problem when a dark part suddenly appears on an entirely whitish screen. To provide a black level correction circuit for a video signal that prevents the black level of an image from changing sharply even when the black level detection sensitivity is increased, and therefore does not give an unnatural feeling to the viewer. It's about doing.

[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, the video signal, and the black level, 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 adjusted. 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. This time, the set black level is moved to the white level and the settings are made again.

[Embodiments of the invention]

Next, embodiments of the present invention will be described 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 the input 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.

Further, a circuit portion S surrounded by a broken line compares the level of the set black level signal 13 with the black level of the input digital video signal 1 whose vedestal level is clamped by the clamp circuit 2, and based on the comparison result,
This is a circuit for adjusting the level of the set black level signal 13 itself to an appropriate level.

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 ○A is the input digital video signal whose pedestal level is clamped. The waveforms are shown, ○ and ○ indicate the black level set by the signal 13, and ○ and ○ indicate the waveform of the output digital video signal 10 that has been corrected for the black level.

In FIG. 2, the subtraction circuit 17 is a circuit that performs subtraction by using the video signal clamped at the vedestal level as the minuend input and the set black level signal 13 as the subtrahend input, and the output of the subtraction circuit 17 is switched. The signal is input to the circuit 18.

The switching circuit 18 outputs a digital value "0" 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 the same as the black level of the video signal.

In other words, regardless of the black level that the input video signal ○A had at the time of input, the set black level ○B given by the signal 13 is set as its new black level, that is, the output video signal ○H. This is what is output.

Referring again to FIG. The function of the arithmetic circuit 4 is
As already explained with reference to FIG. 2, making the black level of the output digital video signal 10 match the set black level given by the signal 13 (this This itself corresponds to a black level correction operation).

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.

Below, the setting black level signal 1 in the circuit section S
Level adjustment operation No. 3 will be explained.

For ease of explanation, it is assumed here that as the digital value of a digital video signal becomes smaller, 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 13 output from the level adjustment circuit 8, and determines that the lowest level (black level) of the video signal is lower than the level of the set black level signal 13. In other words, when the black level of the video signal is further black than that of the signal 13, the detection signal 14 is output. The comparison circuit 5 does not perform a comparison operation during the video blanking period due to the blanking signal 3, 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. Counting circuit 6
counts the number of occurrences of the detection signal 14,
When it reaches a certain predetermined count within a certain period of time, a countdown signal 15 is output.

Here, if the predetermined count number is small,
If the black level detection sensitivity is high and large,
In other words, the magnitude of the predetermined count determines the black level tracking speed of the image.

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 constructed of 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, if a black level lower than the set black level 13 is not detected in the video signal for a predetermined period, the level of the set black level signal 13 is shifted further toward the white side, so that, for example, the video signal with a different setup level is When you receive the
The level of the set black level signal 13 is modified accordingly.

The level adjustment circuit 8 is also constituted by a counting circuit. That is, the countdown signal 15 and countup signal 16 are input to a level adjustment circuit 8, which is a counting circuit that outputs the set black level signal 13 as a count value. The counted value is counted down to correct (adjust) the set black level signal 13 to a smaller value, that is, to the black level, and the counted value is counted up using the count up signal 16 to increase the set black level signal 13, that is, to the black level. Correct (adjust) the level 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 for limiting 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, no matter what kind of adjustment is made in the level adjusting circuit 8. .

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

FIG. 3 is a block diagram showing another embodiment of the invention. That is, this is an embodiment in which a 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 arithmetic circuit 4 and the circuit section S
The operation is exactly the same as that in FIG.

In the switching circuit 12, the pedestal clamped video signal, the reference signal 11 and the blanking signal 3 are input, and the output signal of the switching circuit 12, that is, the input signal to the comparator circuit 5 is switched to the video blanking signal by the blanking signal 3. During the ranking period, the reference signal 11 is used, and during the non-blanking period, the video signal is subjected to vedestal clamping.

The comparison circuit 5 compares the above signal with the set black level signal 13 outputted from the level adjustment circuit 8 over the entire period, regardless of whether it is a blanking period or a non-blanking period, and selects the above input signal from the set black level signal 13. is small, that is, the signal is on the black side, the 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.

In the above embodiment, the digital video signal is pedestally clamped and then the black level is corrected. However, the analog video signal is pedestally clamped or peak clamped, then the video signal is digitized and the black level is corrected. Good too. In addition, in order to prevent the comparison circuit from erroneously detecting noise in the video signal, after band-limiting the video signal,
The signal may also be input to a black level correction circuit.
Furthermore, the arithmetic circuit 4 for black level correction in the embodiment includes a delay circuit that compensates for signal delay due to black level detection and a circuit that corrects underflow and overflow of the digital signal that occurs as a result of the arithmetic operation. It's okay.

〔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 setup level, 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 present invention. 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, 16...countup signal, 17...subtraction circuit, 18...switching circuit.

Claims (1)

[Scope of Claims] 1. A black level of a video signal that is calculated by comparing the signal level of an input digital video signal with a certain set black level, and outputting the black level of the video signal so as to match the set black level. The 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; a measuring circuit that measures the 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 as a result of the comparison; A level adjustment circuit that lowers the set level of the set black level according to the frequency counted by the counting circuit, and increases the set level according to the length of the period measured by the measuring circuit. A black level correction circuit for a video signal, characterized in that: 2. In the black level correction circuit according to claim 1, the set level of the set black level is set regardless of the operation of the level adjustment circuit.
A black level correction circuit characterized in that a black level correction circuit is provided with means for restricting the increase so that the level does not rise above at least a certain limit level. 3. The black level correction circuit according to claim 1, wherein the comparison circuit compares the level of a certain standard signal with the set black level during a blanking period. black level correction circuit.