JPH11313271A - Luminance correction circuit for video signal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the simplified luminance correction circuit at a low cost consisting of conventional operational amplifiers, transistors(TRs) and variable gain amplifiers with a simple circuit configuration where stability and a characteristic similar to digital processing are obtained even from analog signal processing. SOLUTION: The correction circuit consists of a pedestal clamp circuit 3 that clamps a black level of a video signal to be a constant DC voltage, an impedance conversion circuit 2 that converts the impedance of the video signal through which the video signal passes into a low impedance to prevent distortion in an original signal produced in the case of slicing and fluctuation of a pedestal level before the input of the video signal to a slice circuit 5, the slice circuit 5 that slices the video signal at a prescribed luminance level, an amplifier circuit 7 whose gain is variable that varies a gain for a range of a level of the video signal, a low pass filter 15 that decreases a high frequency component of an emphasized signal level and an adder 10 that add the processed signal to the original signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a television receiver,
The present invention relates to a luminance correction circuit for a video signal applicable to all consumer video devices that handle video signals such as TRs, DVD players, laser disk players, video movie devices, etc., and is displayed on a monitor screen in a consumer video device. This has the effect of improving the quality of the displayed image.

[0002]

2. Description of the Related Art When a video signal of a general television broadcast, video, laser disk, DVD, or the like is projected on a television screen for viewing, depending on the video content, the gradation of a dark portion or a black portion is lost, and the image is not displayed on a monitor. Therefore, it often happens that each hair cannot be visually recognized. As a means to prevent such a phenomenon, that is, so-called black under-exposure, as a means of preventing such a phenomenon, a gain of about 30% of the luminance signal is increased by a name such as luminance level correction or γ correction, so that the gradation of a dark portion or a black portion of the video signal is increased. A circuit that has the following is practically used.

Conventionally, this circuit generally uses digital signal processing, and has disadvantages such as requiring a dedicated digital IC, requiring a development period, and high cost. Although some analog signal processing has been put to practical use, there are disadvantages such as poor stability and inability to freely set characteristics.

In a technique that has been put to practical use in analog signal processing, gamma correction has been implemented in analog signal processing as a means for switching a gain curve of an image in accordance with luminance. This is a function for correcting on the signal sending side and the receiver side since the luminance of the CRT does not originally have a characteristic of linearly responding to the incoming input signal. Hereinafter, a case where a ramp waveform is input will be described as an example.

In the case of this conventional analog signal processing technology,
A pedestal clamp process is performed at the input stage, and only a portion of the video signal that is desired to be emphasized is cut out. To switch the brightness level depending on the difference in the brightness of the image,
Slice the video signal at the level where you want to change the brightness.
Non-linear characteristics are obtained by adding the respective sliced signals by resistors, and finally, setting is performed so that the signal level becomes the same as that of the input signal.

In the above-described conventional method, since the impedance of the signal to be modified in the signal slicing portion does not have a sufficiently low value, the temperature and time-dependent changes cannot be avoided. There was some problem. Further, there is a disadvantage that the clamp varies due to slicing of the video signal. In addition, it was impossible to achieve fine matching of characteristics such that the level was changed only in a dark portion and a black portion. Therefore, applications have been limited to the purpose of canceling out nonlinear characteristics of a cathode ray tube or a liquid crystal display.

[0007]

In the above-described conventional technique, there are problems such as distortion of the waveform of the original luminance signal during slicing, fluctuation of the pedestal level, and the effect of temperature change of the slice circuit. There was a problem with stability because of its susceptibility. Further, in an application for preventing the gradation of a dark portion or a black portion from being lost, the luminance 30
%, It is necessary to emphasize characteristics up to about 50% and to return to the original characteristic after 50%.

In addition, since a process for increasing the gain in a portion where the luminance signal level is low such as a dark portion or a black portion is performed, there is a disadvantage that the S / N feeling of the screen is deteriorated.

The present invention is a circuit devised so as to solve the above-mentioned drawbacks of the prior art. The same stability and characteristics as those of digital processing can be obtained by analog signal processing, and the circuit configuration is a general-purpose operational amplifier. An object of the present invention is to provide a simple and inexpensive luminance correction circuit for a video signal, which is composed of a transistor and a variable gain amplifier.

[0010]

A luminance correction circuit for a video signal according to the present invention includes a pedestal clamp circuit for fixing a black level of a video signal to a constant DC voltage, and a video signal before being input to a slice circuit. In order to prevent distortion of the original signal and fluctuation of the pedestal level that occur when slicing,
An impedance conversion circuit for lowering the impedance of the video signal, a slice circuit for slicing the video signal at a certain luminance level, and a variable gain amplifier circuit for changing the gain of a video signal in a certain level range are emphasized. It comprises a low-pass filter that drops high-frequency components of the signal level, and an adder for adding the processed signal to the original signal.

In the video signal luminance correction circuit,
An impedance conversion circuit is added before the input to the slice circuit to prevent a change in the distortion pedestal level of the original signal generated at the time of slicing. This enables a stable operation even with a sudden change in luminance or temperature.

In the case of emphasizing a dark part, since the operation of increasing the gain of a small signal portion is performed, there is a disadvantage that the noise level increases and the S / N feeling of the screen deteriorates. The S / N feeling is improved by passing the signal to be added and emphasized through a low-pass filter. Normally, passing a luminance signal through a low-pass filter causes a drop in resolution because the high-frequency component of the video signal is lost.
In the above circuit, a low-pass filter is applied only to a circuit for adding and correcting the luminance level, so that the S / N feeling can be improved with almost no deterioration in resolution.

In the above-mentioned application, it is necessary to set the luminance peak around 30% and to set the luminance so that it does not change at 50% or more. In this case, an inverse correction signal is created and added so that the inverse correction is performed. This makes it possible to satisfy desired specifications.

[0014]

Embodiments of the present invention will be described below with reference to the accompanying drawings. First, an outline of an example of signal processing in a video signal luminance correction circuit according to the present invention will be described.

The circuit according to the present invention operates on the luminance signal after Y / C separation of the video signal. In this circuit, the luminance signal is first subjected to a pedestal clamp process in an input stage to fix the pedestal level to a certain voltage value. The pedestal clamped signal is subjected to impedance conversion using a broadband operational amplifier while being DC-coupled.

The impedance-converted luminance signal is sliced by a voltage set to each luminance level, and is sliced.
Different types of waveforms are generated. By selecting and adding resistance values so that desired characteristics can be obtained, luminance characteristics can be set freely. Further, by using a variable gain amplifier (amplifying circuit) to change the gain of the waveform to be emphasized, the level of luminance emphasis can be freely changed.

Utilizing this, an adaptive operation in which a circuit is configured to detect an image signal, obtain an average luminance of the image signal, and set a luminance enhancement level corresponding to the luminance level is also possible. If the adaptive operation processing is performed, the brightness enhancement level is weakened in a bright scene and the brightness enhancement level is increased in a dark scene automatically. It was made possible.

Next, an example of a circuit according to an embodiment of the present invention will be described with reference to FIG. In FIG. 1, reference numeral 1 denotes a synchronization signal separation circuit, which performs processing for extracting a synchronization signal from a luminance signal. Reference numeral 2 denotes a first impedance conversion circuit, which lowers the input original signal in impedance. Reference numeral 3 denotes a pedestal clamp circuit, which creates a signal for pedestal clamping to an appropriate voltage before impedance conversion of a luminance signal in each circuit.

Reference numeral 4 denotes a first pedestal clamp, which pedestal clamps a luminance signal as an input original signal. Reference numeral 5 denotes a first slice circuit, which extracts a luminance level of a portion to be emphasized. Reference numeral 6 denotes a third slice circuit that performs inverse correction at a luminance level of 50% or more and generates a signal that is restored so as not to be corrected at a luminance level of 50% or more.

Reference numeral 7 denotes a low-luminance partial amplifier circuit, which is an amplifier circuit for generating a signal for emphasis. 8 is the second
This is an impedance conversion circuit that lowers the impedance of the emphasized signal. Reference numeral 9 denotes a second slice circuit, which clamps both sides of the sync and the luminance so that the level of the entire signal does not change when the gain is varied by the low luminance partial amplifier circuit 7.

Reference numeral 10 denotes an adder for adding the respective signals. Reference numeral 11 denotes a second pedestal clamp, which pedestal clamps a signal of a low luminance portion to be emphasized. 1
Reference numeral 2 denotes a third impedance conversion circuit for reducing the impedance of the signal to be inversely corrected. Reference numeral 13 denotes a third pedestal clamp, which performs impedance conversion on a signal after passing through the second slice circuit 6.

Reference numeral 14 denotes a video adjustment circuit which finally adjusts the level of the video signal to a specified value. 1
Reference numeral 5 denotes a low-pass filter, which operates to reduce a high-frequency component of a signal level to be emphasized. Reference numeral 16 denotes a luminance level detection circuit which detects the luminance level by detecting the luminance level.

Next, signal processing in the above circuit will be described. First, when a luminance video signal is input, the pedestal level is first fixed at a constant voltage by the first pedestal clamp 4. The pedestal clamp is performed by a clamp pulse generated by the pedestal clamp circuit 3 based on the synchronization signal generated by the sync separation circuit 1. The fixed luminance video signal is reduced in impedance by the first impedance conversion circuit 2 (voltage follower). The low-impedance luminance video signal is supplied to the first slice circuit 5
Is supplied to the third slice circuit 6.

The signal passing through the first slicing circuit 5 is sliced at a luminance of about 30%. The signal passing through the first slice circuit 5 passes through the low-luminance partial amplifier circuit 7 and is amplified. By using a variable gain amplifier as the low-luminance partial amplifier circuit 7, it is possible to switch the intensity of the luminance correction amount by increasing or decreasing the gain. This allows the user to set the optimal luminance correction according to the scene. A luminance level detecting circuit 16 for detecting a luminance level by detecting a luminance signal.
A luminance level correction intensity adaptive type can also be realized based on a signal from the CPU.

The signal output from the low-luminance partial amplifier circuit 7 is pedestal-clamped by the second pedestal clamp 11, and the impedance is reduced by the second impedance conversion circuit 8. After that, the second slice circuit 9 slices the synchronizing signal portion and the portion exceeding about 30% of the video signal. By slicing the signal by the second slicing circuit 9, even when the luminance level correction is changed by changing the gain in the low luminance partial amplifier circuit 7, it is possible to prevent the entire signal level from changing.

On the other hand, the luminance signal passing through the third slice circuit 6 is sliced from a signal of about 50% or less. The impedance of the sliced signal is reduced by the third impedance conversion circuit 12. The signals output from the third impedance conversion circuit 12 and the second slice circuit 9 in this manner are combined with the original signal by the adder 10. Finally, a luminance video signal subjected to luminance correction is obtained through the video level adjustment circuit 14 in order to match the prescribed video signal level. As a result, as shown in FIG. 2, a signal is obtained in which a signal having a luminance of 50% or less with respect to the linear original signal is emphasized.

As described above, in the circuit described above, before slicing at the set video signal level, the signal after pedestal clamping is subjected to impedance conversion,
The signal is sliced so as not to affect the original signal, and drift factors such as a temperature, a change with time, and a sudden change in the luminance signal level are eliminated, thereby improving the stability.

In addition, in order to raise only the black level and not to change the level of the luminance signal by 50% or more, a process of adding a correction signal therefor is also performed.

[0029]

As described above, according to the present invention,
It is possible to emphasize only the level of a specific luminance signal, which could not be realized by conventional digital signal processing. This makes it possible to reproduce, for example, the gradation of a person's face or the like without underexposure when photographing under backlight. Further, it is possible to prevent a phenomenon such as blackout of a picture in a dark room in a movie or the like.

Advantages of obtaining the same effect as digital by analog signal processing include that it can be configured with an inexpensive general-purpose IC without the need for a dedicated LSI, and that quantum signals are not required for analog signal processing. That is, the distortion due to the conversion error does not occur, and unnecessary radiation that adversely affects the signals processed by itself and other circuits does not occur.

[Brief description of the drawings]

FIG. 1 is a block diagram of a luminance correction circuit for a video signal according to the present invention.

FIG. 2 is a characteristic diagram of a luminance correction circuit for a video signal according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Synchronization separation circuit 2 1st impedance conversion circuit 3 Pedestal clamp circuit 4 1st pedestal clamp 5 1st slice circuit 6 3rd slice circuit 7 Low-luminance partial amplifier circuit 8 Impedance conversion circuit 9 2nd slice circuit 10 adder 11 2nd Pedestal clamp 12 Third impedance conversion circuit 13 Third pedestal clamp 14 Video level adjustment circuit 15 Low-pass filter 16 Luminance level detection circuit

Claims (2)

[Claims] 1. A pedestal clamp circuit for fixing a black level of a video signal to a constant DC voltage, and a distortion of an original signal and a fluctuation of a pedestal level generated when slicing before inputting the video signal to a slice circuit. To prevent this, an impedance conversion circuit for lowering the impedance of the video signal, a slice circuit for slicing the video signal at a certain luminance level, and a variable gain for varying the gain in a certain level range of the video signal. A luminance correction circuit for a video signal, comprising: an amplification circuit; a low-pass filter for reducing a high-frequency component of a signal level to be emphasized; and an adder for adding a processed signal to an original signal. 2. A process according to claim 1, wherein a level of only the black level is raised, and a correction signal for the level is added so as to prevent a level of 50% or more of the luminance signal from changing. Video signal brightness correction circuit.