JPH10191102A - Video signal amplifier circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress unnaturalness of a white peak part depending on an average picture level(APL) of a video input signal in a video signal amplifier circuit that corrects a video input signal through gamma correction or the like and amplifies the corrected signal. SOLUTION: In the case of correcting a video input signal by using a gamma correction amplifier 104, a DC component of the video input signal is eliminated prior to the correction and a fixed bias circuit applies a fixed bias to the gamma correction amplifier 104. Through the above constitution, a white peak part is adaptively controlled corresponding to an APL. For example, when an APL is 50%, an the fixed bias is selected for the video input signal so that a 100IRE is an operation point of the gamma correction, a 75IRE is the gamma correction operating point when the APL approaches to 0%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video signal amplifying circuit used for amplifying a video signal in a video device for displaying a television signal or the like using a CRT or the like.

[0002]

2. Description of the Related Art Generally, this kind of video signal amplifying circuit is
It is used to receive and amplify a composite video input signal and display it on a video device such as a CRT. Here, a television signal supplied to a video device such as a CRT has an average beam current determined by characteristics of the CRT or the like. In this case, when the average luminance level of the video input signal, that is, the average video level (hereinafter, referred to as APL) is high, the automatic luminance limiting circuit (ACL) is operated to lower the contrast, thereby reducing the average beam current in the CRT. Is not exceeded.

When this configuration is adopted, when the APL is low, the automatic luminance limiting circuit (ACL) does not operate.
The contrast becomes the maximum direction, and phenomena such as blooming due to concentration of the beam current on a small area and distortion (doming) of an image due to the beam current not correctly hitting the shadow mask appear.

When the contrast is suppressed to such a level that the problems of blooming and doming do not occur,
For example, when the APL is low, gradation characteristics in a dark part cannot be obtained, resulting in a dark screen.

In consideration of this, when the APL is low,
A gamma correction amplifier having a gamma characteristic for lowering the amplification in the white level direction or lowering a composite video input signal level equal to or higher than a predetermined level is provided in the preceding stage, whereby a gray level in a dark portion when the APL is low is provided. Improvements in characteristics have been made.

Here, with respect to the improvement of the above-mentioned gradation characteristics, Conventional Examples 1, 2, and 3 will be described with reference to FIGS.

The video signal amplifying circuit shown in the prior art 1 shown in FIG. 4 processes an input composite video input signal by a video chroma processing circuit and outputs a color difference signal (R-
Y, BY, GY) and a luminance signal (-Y),
Of these signals, a luminance signal (-Y) reproduced by DC is corrected by the gamma characteristic and then output to a matrix circuit to obtain an RGB signal.

On the other hand, the video signal amplifying circuit shown in FIG. 5 as Conventional Example 2 is a circuit that can be configured by combining conventional techniques. In this predictive circuit configuration, a composite video input signal is converted by a video chroma processing circuit.
After the R, G, and B signals are directly separated and extracted in a matrix, each signal is sent to a gamma correction amplifier to perform correction based on gamma characteristics.

Further, a third conventional example shown in FIG. 6 is another video signal amplifying circuit which can be constructed by using the prior art.
In this example, a gamma correction amplifier including a clamp circuit for clamping a composite video input signal or a luminance video input signal and a gamma correction circuit is provided at a stage preceding a video chroma processing circuit (ACL). In this configuration, the pedestal level of the composite video input signal or the luminance video input signal is clamped by the clamp circuit, and the DC-reproduced composite signal or the luminance signal is corrected by the gamma characteristic.

[0010]

A video signal amplifying circuit for correcting only a DC-reproduced luminance signal (-Y) by a gamma characteristic as in Conventional Example 1 has a large circuit scale and a recent high-speed signal. In order to meet the demand for image quality, a more complicated circuit configuration is required. In consideration of this, the circuit configuration of the conventional example 1 is practically no longer adopted, and a method of directly and individually separating the R, G, and B signals as in the conventional example 2 has become mainstream. I have.

Conventional Example 2 is superior to Conventional Example 1 in terms of high image quality and miniaturization of the circuit. However, when the separated R, G, and B signals are subjected to gamma correction as in Conventional Example 2, gamma correction amplifiers having the same gamma correction characteristics are required. However, in practice, it is difficult to obtain a gamma correction amplifier having the same characteristics. For this reason, it is conceivable to use gamma correction amplifiers having characteristics that do not match each other.However, if gamma correction amplifiers having characteristics different from each other are used, the white balance in the white peak portion changes, or the hue is changed. Will change.

Further, when the video signal amplifying circuit having the configuration of the conventional example 3 is used, a gamma characteristic is applied to a signal having a high APL which can suppress the contrast by the ACL, and black appears in a bright image. The disadvantage that it becomes a complicated screen was found.

An object of the present invention is to provide a video signal amplifying circuit capable of adaptively changing the luminance at a white peak portion in accordance with the APL and reproducing and displaying a video which is not visually unnatural. is there.

Another object of the present invention is to provide a video signal amplifying circuit capable of preventing a phenomenon in which black in a video appears to be in a floating state, blooming, doming, or deterioration of gradation reproduction.

Still another object of the present invention is to provide a video signal amplifier circuit capable of detecting a white peak portion and switching a gamma characteristic with high accuracy and a simple configuration.

[0016]

The principle of the present invention will now be described. First, the video input signal has a pedestal level (0IRE) serving as a black level reference and 100
% White level (100% IRE). If the DC component of such a video input signal is not intentionally reproduced,
When the APL is 100%, the level of the DC component of the video input signal increases to an intermediate level between the pedestal level and the 100% white level, while when the APL is 0%, the level of the DC component approaches the pedestal level. It has been found. In other words, if the DC component of the video input signal is removed, the pedestal level of the video input signal can be adaptively changed according to the change in APL.

In the present invention, utilizing this, a fixed bias is applied to the video signal from which the DC component has been removed, and then applied to a gamma correction amplifier having a constant gamma correction characteristic. In this case, by selecting a fixed bias, for example, when the APL is 50% or more, the gamma correction amplifier is operated linearly from 0 IRE to 100 IRE. As a result, the gamma correction is not performed, while the APL is 50%. When the APL is lower, the gamma correction can be performed only near 100 IRE, and when the APL is near 0%, the gamma correction can be performed at about 75 IRE or more.

According to one embodiment of the present invention, receiving a video input signal, removing a DC component of the video input signal,
DC component removing means for obtaining a first video signal from which a DC component has been removed, and applying a fixed bias to the first video signal to obtain a second video signal.
And a correction means for correcting the second video signal from the bias means and outputting a corrected third video signal. A circuit is obtained.

According to still another embodiment of the present invention, a gamma correction circuit is provided as the correction processing means, and the gamma correction circuit compares the means for applying a reference voltage with the reference voltage and the fixed bias. Then, a video signal amplifying circuit including comparison and amplification means for outputting a signal amplified according to the comparison result and processing means for processing the amplified signal and outputting it as a third video signal is obtained.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIGS. 1 and 2, a schematic configuration and operation of a video signal amplifier circuit according to the present invention will be described. First, the video signal amplifying circuit shown in FIG. 1 receives a video input signal IN and removes a DC component thereof from a capacitor C.
And a video signal from which a DC component has been removed is supplied to a fixed bias circuit. The illustrated fixed bias circuit is connected between a power supply of a power supply voltage VCC and ground, and is configured by a series circuit of a first resistor 102 having a resistance value R1 and a second resistor 103 having a resistance value R2. The capacitor 101 is connected to a common connection point with the first and second resistors 102 and 103, and this common connection point is also connected to a gamma correction amplifier 104 shown by a rectangular block. .

A vertical line of the illustrated gamma correction amplifier 104 indicates a DC voltage corresponding to a video signal supplied from a fixed bias circuit, and VC-VD indicates a dynamic range of the gamma correction amplifier 104. The gamma correction amplifier 104 has an amplification factor A1 of β between VC and VB, and exhibits an amplification factor A2 of α between VB and VD. Here, the amplification factor A1 is larger than the amplification factor A2, and therefore, there is a relationship of β> α. Here, VA is a fixed bias voltage (DC voltage).

The gamma correction amplifier 104 sends the corrected and amplified video signal to the video / chroma processing circuit 105,
Even if this chroma processing circuit 105 is divided into R, G, and B, it is sent to the CRT to display an image.

Referring to FIG. 2A, of the video signals supplied from the fixed bias circuit, APL is 100
%, The video signal in this case changes sharply from the black level pedestal (0 IRE) to the white peak level (100 IRE).
The level of 0IRE is maintained for one horizontal scanning period. At this time, the level of the DC component of the video signal is located near 50IRE, but since this DC component has been removed by the capacitor 101, the potential difference between the leading end of the horizontal synchronization signal and the fixed bias VA is Maximum and 100I
The RE level has not reached the voltage VB at which the amplification rate changes. Therefore, the video signal shown in FIG. 2A is amplified at a constant amplification factor A1 (β), and the amplified video signal is output. This means that APL is 100%
Means that no gamma correction can be applied, and the video signal is uniformly amplified from 0 IRE to 100 IRE. Therefore, as in the past, gamma correction is applied to high-luminance areas,
It is possible to prevent a phenomenon that the intermediate luminance becomes relatively high and the image becomes a white image.

On the other hand, referring to FIG.
A video signal when L is close to 0% is shown. From this video signal, the DC component is located near 0IRE, and since this DC component has been removed by the capacitor 101, Signal pedestal (0IR
E) is at the position of the fixed bias VA, the potential difference between the tip of the horizontal synchronizing signal and the fixed bias VA becomes small, and a voltage of about 75 IRE or more is a voltage V at which the amplification factor changes.
B or more. Therefore, the video signal level 0IRE
From about 75 IRE to about 75 IRE.
The amplification rate α is between RE and 100 IRE. This indicates that when the APL is small, the gamma correction is applied only to the white peak portion, and as a result, the gain of the white peak portion decreases. Therefore, when the APL of the video signal is small, blooming and doming at the white peak portion can be prevented, and at a low luminance portion, amplification is performed at a large amplification factor, so that there is no deterioration in gradation reproduction.

Referring to FIG. 3, the fixed bias circuit 311 and the gamma correction amplifier 310 of the video signal amplifying circuit according to one embodiment of the present invention are more specifically shown. 3, a fixed bias circuit 311 to which a video input signal IN is supplied includes a capacitor 301 having a capacitance C, a first resistor 302 having a resistance value R1, and a
It is constituted by a series circuit of a second resistor 303 having a resistance value R2, and the series circuit is supplied with a power supply voltage VCC. Here, the resistance values R1 of the two resistors 302 and 303,
The common connection point of R2 is set to be a fixed bias voltage VA.

On the other hand, the gamma correction amplifier 310
02, 303 are provided with a PNP transistor 308 having a base connected and a collector grounded, and an emitter of the transistor 308 having a resistance R
3 is connected to the power supply of the power supply voltage VCC via the resistor 304. The emitter of the transistor 308 is connected to the emitter of a reference voltage comparison PNP transistor (hereinafter, referred to as a reference transistor) 307 through resistors 305 and 306 having resistance values R4 and R5 connected in series. The collector of the reference transistor 307 is grounded, and the base thereof is connected to a power supply 309 for applying a voltage VB corresponding to the change point of the amplification factor shown in FIG. Further, the common connection point of the resistors 305 and 306 is connected to the video chroma processing circuit 105 of FIG.
The video output signal shown in FIG. 2 is output.

In the figure, the base bias voltage VA of the transistor 308 is obtained by changing the power supply voltage VCC to the resistance values R1 and R2.
Is determined by voltage division. In this case, the base current is ignored. Here, when the emitter voltage VE of the transistor 308 is lower than the sum of the voltage VB applied to the base of the reference transistor 307 and the base-emitter voltage VBE of the reference transistor 307, that is, (VB + VBE),
7 is cut off, and a voltage equal to the emitter voltage of the transistor 308 is output as a video output signal. This means that the signal is amplified with the amplification factor A1 (β). In this example, A1 = 1.

Conversely, when the emitter voltage of the transistor 308 is higher than (VB + VBE) of the reference transistor 307, the reference transistor 307 becomes active and a signal obtained by dividing the emitter voltage of the transistor 308 as a video output signal. Is output. The voltage division ratio in this case is represented by (R4 / R4 + R5), and the amplification A2 is also equal to this voltage division ratio.

Here, the actual voltage relation will be described. Assuming that the level of the video input signal is 1 VPP, 0IRE to 100IRE becomes 0.714 VPP. When the APL is close to 0%, the level of the DC component becomes almost pedestal (0IRE), and VB is set to VA + 0.
When set to 53V, A1 = 1 for 75IRE or less, and A2 = (R4 / R4
+ R5). Here, if R4 = R5, A2 =
If R4 = 2R5, then A2 = 2
/ 3 can be easily understood.

When the video input signal IN is a signal of the NTSC system, the fixed bias circuit does not reproduce the direct current, and needs to have a cutoff frequency capable of sufficiently passing the minimum frequency of 60 Hz. There is. Therefore, a capacitor 301 having a capacitance C and resistance values R1, R2
The relationship between the first and second resistors 302 and 303 is 1 /
2πC (R1R2 / (R1 + R2)) is 1/60 of 60 Hz.
It is desirable to be 10 or less.

In the embodiment described above, the case where the gamma correction amplifier has two types of amplification has been described. However, three or more types of amplification may be provided. In this case, a plurality of transistors having different base voltages may be connected in parallel as the reference transistor in FIG. Also,
The same effect can be obtained even when the amplification characteristic is a gamma characteristic. At this time, a characteristic is obtained in which the effect of the gamma correction is small at a voltage lower than VB, and the effect of the gamma correction is increased at a voltage higher than VB. As a circuit having such characteristics, a commercially available circuit can be used.

As a video input signal to the video signal amplifier circuits shown in FIGS. 1 to 3, only a luminance video input signal may be used, or a composite video input signal may be input.

[0033]

As described above, in the present invention, since the gamma characteristic or a similar correction can be directly applied to the video input signal, the color density of the white peak portion becomes unnatural. There is an advantage that the brightness of only the white peak portion can be suppressed without changing the hue. In addition, since the amount of correction can be adaptively controlled according to the APL, it is possible to prevent a phenomenon in which a dark portion of a video input signal having a high APL becomes unnaturally bright. Further, in the present invention, the detection of the APL and the control of the correction amount can be smoothly realized with a simple circuit configuration.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of a video signal amplifier circuit according to the present invention.

2A is a diagram illustrating an example of a relationship between a video input signal input to the video signal amplifier circuit illustrated in FIG. 1 and a video signal output. FIG. 2B is a diagram illustrating another example of the relationship between the composite video input signal input to the video signal amplifier circuit illustrated in FIG. 1 and the output video signal.

FIG. 3 is a circuit diagram showing an example of a specific configuration of a video signal amplifier circuit according to one embodiment of the present invention.

FIG. 4 is a block diagram showing an example of a conventional video signal amplifier circuit.

FIG. 5 is a block diagram for explaining an example of a video signal amplifier circuit that can be configured by using a conventional technique.

FIG. 6 is a block diagram illustrating another configuration example of a video signal amplifier circuit that can be configured using the conventional technique.

[Explanation of symbols]

101, 301 capacitor 102, 103, 302, 303 first and second
Resistance 104, 310 gamma correction amplifier 105 video / chroma processing circuit 106 CRT 311 fixed bias circuit

Claims (9)

[Claims] 1. A video signal amplifier for receiving a video input signal and amplifying the video input signal at a plurality of amplification levels with a plurality of DC voltage levels as a boundary, wherein a fixed bias circuit is provided at a signal input section of the video input signal. A video signal amplifier circuit provided. 2. The video signal according to claim 1, further comprising: a circuit for removing the DC component from the video input signal and outputting a video signal from which the DC component has been removed, wherein the fixed bias circuit has the video signal from which the DC component has been removed. And a video signal amplifier circuit. 3. The video signal amplifying circuit according to claim 1, further comprising an amplifying unit for amplifying a video signal supplied via the fixed bias circuit, wherein the amplifying unit has a gamma characteristic. 4. The video input signal according to claim 3, further comprising: a circuit for outputting a video signal from which the DC component has been removed, wherein the fixed bias circuit is supplied with the video signal from which the DC component has been removed. Video signal amplifier circuit. 5. The video signal amplifying circuit according to claim 1, wherein said video input signal is one of a luminance video input signal and a composite video input signal. 6. A DC component removing means for receiving a video input signal, removing a DC component of the video input signal and obtaining a first video signal from which the DC component has been removed, and applying a fixed bias to the first video signal. Bias means for applying and outputting as a second video signal, correction processing means for correcting the second video signal from the bias means and outputting a corrected video output signal. A video signal amplifying circuit. 7. The gamma correction circuit according to claim 6, further comprising a gamma correction circuit as the correction processing means.
A means for applying a reference voltage, a comparison / amplification means for comparing the reference voltage with the fixed bias, and outputting an amplified signal according to the result of the comparison; and processing the amplified signal and outputting it as a video output signal. A video signal amplifying circuit comprising processing means. 8. The video signal amplifying circuit according to claim 7, wherein said comparing and amplifying means includes amplifying means for amplifying at a plurality of mutually different amplification degrees with a plurality of DC voltage levels as boundaries. . 9. The method according to claim 8, wherein the amplifying unit is configured to output a signal when the APL of the composite video input signal is high.
A video signal amplifying circuit, wherein amplification is performed at a high amplification degree among the amplification degrees, and when the APL is low, the amplification is performed even at the lower amplification degree.