JPH066638A - White peak clip circuit - Google Patents

Abstract

PURPOSE:To suppress a white peak without causing a defect such as brightness change by compressing only a signal for a portion to be a clip level or below when a white peak of a video signal is a reference clip level or over in a video processing circuit for a television receiver or the like. CONSTITUTION:A video signal 21 is given to a clamp circuit 1, in which DC recovery is implemented and the result is inputted to a white peak detection circuit 2 or the like. The white peak detection circuit 2 detects a white peak in a signal and its white peak is compared with a reference clip level 22 by a limiter 3. As a result, when the white peak is the reference clip level or over, the level is outputted and when the white peak is less than the reference clip level, the reference clip level is outputted. With respect to the white peak over the reference clip level, only the portion is compressed while decreasing a multiple factor of a multiplier 9. The multiple factor is obtained through negative addition of the result of arithmetic operation by a comparison arithmetic operation circuit 5 (control correction signal 23) in response to the white peak and a reference control signal 26 at an adder 8. A comparator 6 turns on a switch circuit 7 at a compression start timing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an analog video processing circuit such as a receiver or monitor device in the field of television technology, and when the white peak of a video signal is higher than a reference clip level, only the signal of that part is output. The present invention relates to a white peak clipping circuit that compresses below the clip level.

[0002]

2. Description of the Related Art In a cathode ray tube (CRT) monitor,
In the bright part of the image, the electron beam becomes thick and the image quality deteriorates. In order to prevent the deterioration of image quality, a method is generally used in which a so-called white peak clip circuit is provided and the bright portion of the video signal is suppressed below a certain level. Figure 3
Shows the input / output characteristics of the conventional white peak clip circuit. FIG. 7A shows the characteristics of the circuit in which the limiter circuit prevents the signal from increasing above a certain level. In this case, since the current of the white peak is limited, it is possible to prevent the electron beam from becoming thick, but white crushing occurs and the image quality deteriorates. On the other hand, (B) of the figure shows the characteristics of the circuit in which the white crushing in (A) is avoided by providing the gamma characteristic which makes the inclination in the bright portion gentle. However, in this case, the white crushing can be avoided, but the gradation in the always bright portion is gentle, so that the image quality is deteriorated.

On the other hand, FIGS. 3C and 3D show the characteristics of the ABL and ACL circuits used for suppressing the excess beam current. In the case of a bright image in (C), the characteristics move from a to b, and in the case of a bright image in (D),
The slope of the characteristic (straight line) is changed from “c” to “d” to avoid white crush. However, in (C), black crushing may occur, and in (D), the brightness of the entire image changes extremely, resulting in an unnatural image.

[0004]

DISCLOSURE OF THE INVENTION The present invention is designed to suppress a white peak without causing any adverse effects such as a white crush, a black crush, a gradation expression deterioration, or a luminance change which the various white peak clipping methods have. It is an object to provide a white peak clip circuit.

[0005]

SUMMARY OF THE INVENTION The present invention provides a clamp circuit for direct current reproduction of an input video signal, a white peak detection circuit for detecting a white peak for each field from the video signal from the clamp circuit, and the white peak detection circuit. A limiter circuit that compares the white peak from the peak detection circuit with a reference clip level, extracts the white peak when the reference clip level is higher than the reference clip level, and outputs the reference clip level when the reference clip level is lower than the reference clip level, and the limiter circuit. A sample and hold circuit that maintains the output of the above for one field period, a comparison calculation circuit that compares the signal maintained by the sample and hold circuit with the reference clip level, and outputs a control correction signal based on the difference between the two, and the clamp. The video signal from the circuit is compared with a predetermined compression start level, and the video signal is below the compression start level. In the case of, a comparator that outputs a signal for turning on the following switch circuit, and a switch circuit that is turned on by an ON signal from the comparator and transmits the control correction signal from the comparison operation circuit to the adder below. An adder that adds the control correction signal as a negative value to a reference control signal, and a multiplier that multiplies the video signal from the clamp circuit by a predetermined level based on the control signal from the adder, A white peak clipping circuit for compressing a white peak equal to or higher than a reference clip level to a level equal to or lower than the clip level.

[0006]

The video signal is reproduced by direct current to facilitate the white peak detection. The DC reproduced signal is branched and input to the multiplier, the white peak detection circuit and the comparator. The white peak detection circuit detects a white peak from the DC reproduced signal, and the detected white peak is compared with a reference clip level by a limiter. As a result of the comparison, when the white peak is equal to or higher than the reference clip level, the level is output, and when it is equal to or lower than the reference, the reference clip level is output. For white peaks above the reference clip level, the multiplier of the multiplier (amplifier) is lowered only for that part to compress. The multiplier is set based on the result calculated by the comparison calculation circuit according to the white peak. This setting is obtained by subtracting the control correction signal and the reference control signal from the comparison operation circuit via the switch circuit for turning on the comparator. The comparator compares the video signal with the compression start level, and turns on the switch circuit at the timing of the start of compression.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A white peak clipping circuit according to the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of essential parts showing an embodiment of a white peak clip circuit according to the present invention.
In the figure, 1 is a clamp circuit for DC reproduction of an input video signal 21, 2 is a white peak detection circuit for detecting a white peak for each field from a DC reproduced video signal, 3 is the white peak detection circuit 2 The white peak is compared with the reference clip level 22, the white peak is extracted when the reference clip level is higher than the reference clip level, and the reference clip level is output when the white peak is lower than the reference clip level. The limiter circuit 4 outputs the output of the limiter circuit 3. The sample and hold circuit 5 for maintaining one field period is provided with the signal held by the sample and hold circuit 4 and the reference clip level 22.
And a comparison operation circuit for outputting a control correction signal 23 based on the difference between the two, and 6 compares the video signal from the clamp circuit 1 with a predetermined compression start level 24, and the video signal is the compression start level. In the above case, a comparator that outputs a signal 25 for turning on the following switch circuit 7 is turned on by an ON signal 25 from the comparator 6, and the control correction signal 23 from the comparison operation circuit 5 is added as follows. A switch circuit for transmitting to the adder 8, an adder 8 for adding the control correction signal 23 as a negative to a reference control signal 26, and a reference numeral 9 for the clamp circuit 1 based on a control signal 27 from the adder 8. It is a multiplier that multiplies a video signal by a predetermined level.

Next, the operation of the present invention will be described. The reason that the clamp circuit 1 performs the direct current reproduction is to facilitate the detection of the white peak level. The white peak detection circuit 2
A white peak for each field is detected from the DC reproduced video signal, and reset for each field. The white peak detection by the white peak detection circuit 2 is the detection of the white peak in the field, and the magnitude of the level does not matter. The size of the level is divided by the limiter circuit 3. The input / output characteristics of the limiter circuit 3 are shown in FIG. That is, the limiter circuit 3 compares the white peaks from the white peak detection circuit 2 with the clip level set as the reference value as the limiter level, and outputs the level for an input above the clip level, and the input is below the clip level. In the case of, a constant clip level is output. In this way, the sample and hold circuit 4 holds the signals from the limiter circuit, which are successively output in this manner, in units of one field for the period until the next field. Here, the reason for holding is to perform the calculation in the comparison calculation circuit 5, which is the next step. A clip level is set in the comparison operation circuit 5 similarly to the limiter circuit 3, and this clip level is compared with the signal during the hold period in the sample hold circuit 4. In this comparison,
When a signal above the clip level is included,
The comparison operation circuit 5 calculates the control correction signal 23 required to be a multiplier (control signal 27) for compressing the corresponding white peak level of the output of the multiplier 9 to the clip level. The control correction signal 23 based on this calculation increases as the white peak level exceeding the clip level increases, and conversely is zero for white levels below the clip level. The reason why it becomes zero is that the output of the limiter circuit 3 for the white peak below the clip level is the clip level (constant) as described above, and the comparison operation circuit 5 compares this level with the clip level.

A value for the white peak above the clip level or a control correction signal 23 of zero is a switch circuit (described later).
And is input to the adder 8. The adder 8 sets the multiplier of the multiplier 9 by using the control signal 27, and makes the video signal from the clamp circuit 1 a video output signal 28 of a predetermined level. Therefore, the adder 8 outputs a video output signal of a predetermined level.
The reference control signal 26 required to obtain 28 is input as one. On the other hand, the control correction signal 23 is input through the switch circuit 7. Here, the adder 8 adds the control correction signal 23 as a negative value and adds it to the reference control signal 26 (as a result, subtraction). As a result, the multiplier as the adder output (control signal 27) is the reference control signal when the control correction signal 23 is zero.
When the white peak is the same value as 26 and is equal to or higher than the clip level, the value of the reference control signal 26 is decreased according to the white peak. This reduction means reducing the multiplier of the multiplier 9. That is, the video amplitude is compressed. If the multiplier has the same value as the reference control signal 26, there is no compression.

This compression is performed only when the white peak is at the clip level or higher. The comparator 6 and the switch circuit 7 are responsible for this timing. Hereinafter, both operations will be described with reference to FIGS. The video signal after the direct current reproduction from the clamp circuit 1 is input to the comparator 6, while the compression start level 24 is set. The compression start level 24 is set to a level lower than the clip level 22 (FIG. 2 (B)). The reason why the compression start level 24 is set lower than the clip level 22 is to eliminate the incompleteness of the clip operation due to time loss (delay). Comparator 6
Compares the video signal from the clamp circuit 1 with the compression start level 24, and when it is below the compression start level, the switch circuit 7 is turned off, but when it is above the compression start level, it switches only during that period. Signal 25 to turn on circuit 7
Is output (OFF again at compression start level 24 or lower). In this way, the comparator 6 compares the video signal with the compression start level 24 and turns on / off the switch circuit 7 accordingly. As a result, since the control correction signal 23 from the comparison operation circuit 5 is input to the adder 8 only when the switch circuit 7 is turned on, the compression of the image is performed only during that period.

Next, the state of video compression and the input / output characteristics in the present invention will be described. In FIG. 2 (B),
(A) shows the case where the video signal is below the clip level, and (B) shows the case where the video signal has a component above the clip level. In the case of FIG. 2 (B) (a), compression (clip)
Since it is not necessary, it is output as it is as shown in FIG. However, since the clip level is partially exceeded in the case of FIGS. 2B and 2B, in this case, FIG.
As shown in (b), the compression is performed up to the clip level only in the exceeding portion. As described above, the white peak clip according to the present invention has a great feature in compressing the original waveform without crushing the original waveform, and has an advantage that image deterioration such as white crushing does not occur. The input / output characteristics in the block diagram of FIG. 1 are as shown in FIG. When the input level is above the clip level, the slope becomes gentle (a to b). Compression (clip) is performed in this gentle portion. Although this figure and FIG. 3 (B) seem to have the same characteristics, in the present invention, only the portion at the clip level or higher is compressed, whereas in the case of FIG. 3 (B), some portions do not require compression. It is fundamentally different in that it compresses.

[0012]

As described above, according to the present invention, when the image quality is deteriorated due to the electron beam thickening at the white peak, only the white peak of the clip level or more set as the reference is compressed while maintaining the original waveform. It is possible to limit the white peak without causing the adverse effects such as the black crushing or the gradation expression deterioration as in the conventional white peak clipping circuit, which contributes to the improvement of the image quality.

[Brief description of drawings]

FIG. 1 is a block diagram of a main part of an embodiment of a white peak clip circuit according to the present invention.

FIG. 2A is an input / output characteristic of the limiter circuit 3, and FIG.
And (C) are relationship diagrams between the input signal and the output signal related to the white peak clip, and (D) is the input / output characteristic of FIG.

FIG. 3 shows input / output characteristics of various conventional white peak clip circuits.

[Explanation of symbols]

 1 Clamp circuit 2 White peak detection circuit 3 Limiter circuit 4 Sample and hold circuit 5 Comparative arithmetic circuit 6 Comparator 7 Switch circuit 8 Adder 9 Multiplier 21 Input video signal 22 Clip level 23 Control correction signal 24 Compression start level 26 Reference control signal 27 Control signal 28 Output video signal

Claims (1)

[Claims] 1. A clamp circuit for direct-current reproduction of an input video signal, a white peak detection circuit for detecting a white peak for each field from a video signal from the clamp circuit, and the white peak from the white peak detection circuit. With a reference clip level, and a white peak is extracted when the reference clip level is higher than the reference clip level, and the reference clip level is output when the white peak is lower than the reference clip level.
A sample hold circuit for maintaining the output of the limiter circuit for one field period, and a comparison operation circuit for comparing the signal maintained by the sample hold circuit with the reference clip level and outputting a control correction signal based on the difference between the two. ,
A comparator for comparing the video signal from the clamp circuit with a predetermined compression start level and outputting a signal for turning on the following switch circuit when the video signal is equal to or higher than the compression start level, and an on-state from the comparator. A switch circuit that is turned on by a signal and transmits the control correction signal from the comparison operation circuit to the adder below, an adder that adds the control correction signal as a negative to a reference control signal, and an adder from the adder. And a multiplier that multiplies the video signal from the clamp circuit by a predetermined level based on a control signal, and compresses white peaks above a reference clip level to below the clip level. White peak clip circuit.