JPH0738779A - Gamma correcting circuit - Google Patents

Abstract

PURPOSE:To prevent the flicker of a picture, to obtain a corrected output with gradation, and to prevent the generation of black painting-out or white painting- out by detecting and correcting the mean luminance level of a video signal input, switching a gamma correcting characteristic according to the corrected mean luminance level, and correcting the video signal input. CONSTITUTION:The mean luminance level of the video signal input in a prescribed period is detected by an APL detecting circuit 3, a detected signal from the APL detecting circuit 3 is converted into APL data by an A/D converter 4, and the APL data are corrected by a stabilizing circuit 7 so that the switching of the gamma correcting characteristic can be stabilized. The signal and a signal obtained by converting the video signal input into a digital signal by an A/D converter 2 are inputted to a ROM 5 for gamma correction as address signals. Then, the plural gamma correcting characteristics preliminarily stored in the ROM 5 for gamma correction are switched by the corrected APL data, and the video signal input is gamma-corrected and outputted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gamma correction circuit, and more particularly to a gamma correction circuit used for correcting a video signal according to the gamma characteristic of the display to be displayed when the video signal is displayed on the display.

[0002]

2. Description of the Related Art A conventional gamma correction circuit is shown in FIG.
As shown in FIG. 3, the video signal input is converted into an n-bit digital signal by the A / D converter 2, and the n-bit signal is input as an address signal to the gamma correction ROM 5 and stored in the gamma correction ROM 5 in advance. The gamma correction is performed on the video signal input converted into the digital signal by the conversion table of the gamma correction characteristic having the input / output characteristic as shown in FIG. 4 (A) and is output from the output terminal 6 as an n-bit signal. Was there.

[0003]

However, FIG. 4 (A)
When gamma correction is performed with the input / output characteristics as shown in, the slope of the characteristics rises in the middle region of the input voltage.
Although a video signal with gradation can be obtained as the output, the gradation of the output video signal cannot be obtained because the slope is gentle at the black level part where the input level is low or the white level part where the input level is high. However, there was a problem such as blackout or whiteout. The present invention detects the average brightness level of a video signal input, stabilizes the switching of the gamma correction characteristics by correcting the detected average brightness level, and switches the gamma correction characteristics according to the corrected average brightness level, By correcting and outputting the video signal input, it is possible to prevent the flicker that occurs in the image and to obtain a corrected output with gradation so that black or white shadows do not occur. And

[0004]

SUMMARY OF THE INVENTION A gamma correction circuit of the present invention is a gamma correction circuit for detecting an average luminance level of a video signal input for a predetermined period and correcting and outputting a detection signal from the detection means. It comprises a stabilizing means for switching characteristics, a switching means for switching a plurality of gamma correction characteristics by an output signal from the stabilizing means, and a means for gamma correcting an image signal input by the switched gamma correction characteristics. is there.

[0005]

According to the present invention having the above-mentioned structure, the detecting means detects the average luminance level of the video signal input, and the stabilizing means corrects the detected average luminance level so that the switching of the gamma correction characteristics is stable, and the correction is performed. A plurality of gamma correction characteristics are switched according to the averaged brightness level. By stabilizing the switching of the gamma correction characteristics, flicker that occurs in the image is prevented, and normally, as shown in FIG. When the gamma correction characteristic is used and the average brightness level is low, a portion with a low input level, as shown in FIG.
When the average luminance level is high, the gamma correction characteristic in which the portion a is raised is used, and the gamma correction characteristic in which the portion b is raised as shown in FIG. 4C is used. As a result, it is possible to obtain a corrected output with gradation even in the intermediate region, the low luminance portion, or the high luminance portion, and it is possible to prevent black crushing or white crushing from occurring.

[0006]

1 is a block diagram showing an embodiment of a gamma correction circuit of the present invention. An analog video signal input through the input terminal 1 is converted into an n-bit digital signal by the A / D converter 2, and the n-bit digital signal is input to the gamma correction ROM 5 as an address signal. The average brightness level (hereinafter abbreviated as APL) detection circuit 3 detects, for example, an APL for a predetermined period of video signal input using an integration circuit, converts the APL into a DC voltage corresponding to the APL, and inputs the DC voltage to the A / D converter 4. is doing. In the A / D converter 4,
The output from the APL detection circuit 3 is converted into an m-bit digital signal by using the same sampling clock signal of the A / D converter 2 or the sampling clock signal which is phase-synchronized, and is input to the stabilization circuit 7. There is.

2A to 2D are explanatory views showing an embodiment of the stabilizing circuit 7 shown in FIG. 1, and FIG. 2A shows a block diagram of the stabilizing circuit 7. 2 (B) to (D)
Indicates the input / output of each part. Although the DC voltage from the APL detection circuit 3 is converted into a digital signal by the A / D converter 4, when the DC voltage level becomes a value near the threshold level of the A / D converter 4, A / D converter 4 The output of the D converter 4 becomes a digital signal which fluctuates between two values, and when the gamma correction characteristic is switched by the signal, the switching is not stable and appears as a flickering image on the display screen. Therefore, the stabilizing circuit 7 is provided to correct the APL data output from the A / D converter 4 to stabilize the switching of the gamma correction characteristics.

The stabilizing circuit 7 latches the m-bit APL input data input through the input terminal 10 for one data and outputs the latched signal, and the latch circuit 11.
Comparing circuit 12 for comparing the output of C. and input data, control circuit 13 for outputting a control signal based on the comparison result of comparing circuit 12, and arithmetic circuit 15 for adding or subtracting 1 to / from the input data to output. It is composed of input data input to the input side and a switch 16 which selects and outputs the data from the arithmetic circuit 15 by the control signal. The same clock signal as the sampling clock signal used in the A / D converter 4 is input as a latch signal to the latch circuit 11 via the input terminal 14, and is input via the input terminal 10 by the latch signal. 1 bit of m-bit input data
Latch only data and output.

FIG. 2B is an explanatory diagram showing the input / output of the comparison circuit 12. If the input data is Dm, the data output from the latch circuit 11 will be D (m-1), and the comparison circuit will be described. At 12, the magnitudes of Dm and D (m-1) are compared, and a and b are output as output signals based on the comparison result. When Dm> D (m-1), a = H, b = L, Dm <
When D (m-1), a = L, b = H, Dm = D (m
In the case of -1), a = b = L. FIG. 2C is an explanatory diagram showing the input / output of the control circuit 13. The output signals a and b are input to the control circuit 13, and the control signal S is output according to the input. When a = H and b = L, S = L, a = L, b = H,
When, S = H, and when a = b = L, S = hold (same output is maintained).

If, for example, an adder circuit is used as the arithmetic circuit 15, the output from the arithmetic circuit 15 is Dm + 1. The switcher 16 uses, for example, a multiplexer and receives Dm + 1 input from the arithmetic circuit 15 and Dm as input data. When the control signal S is L as shown in FIG. 2D, Dm is selected. Then, when the control signal S is H, Dm + 1 is selected and output from the output terminal 17. Therefore, for the previous APL input data, D (m-1), the current APL input data Dm
When the value rises, Dm is output, and when the value falls, 1 is added to Dm and the value is output. Hysteresis can be generated when the APL data rises and when it falls. , APL data can be stabilized so as not to fluctuate between two adjacent levels for each sampling period of the A / D converter 4.

FIG. 3 is an explanatory diagram showing the operation of the stabilizing circuit 7 shown in FIG. For example, it is assumed that APL data is quantized into 5 bits and a vertical synchronization signal is used as a sampling clock signal of the A / D converter 4. The dotted line shows the APL data input to the stabilizing circuit 7, the thick line shows the APL data output from the stabilizing circuit 7,
For example, it is possible to eliminate the vertical movement of 1 bit of the input APL data in the e section and output the stabilized APL data. For example, even if a subtraction circuit that subtracts 1 from the input APL data Dm is used as the arithmetic circuit 15, similarly stabilized APL data can be output. The thin solid line represents the APL data input to the stabilizing circuit 7 in this case, and the thick line represents the APL data output from the stabilizing circuit 7.

As the gamma correction ROM 5, a ROM in which a plurality of gamma correction characteristic conversion tables are stored in advance is used.
4A to 4C are explanatory diagrams of the gamma correction characteristic. FIG. 4A shows a normal gamma correction characteristic, in which the intermediate area of the input level is raised, and a corrected output with gradation is obtained in the intermediate area. In the characteristic, the low level area of the input level, a part is raised, so that a corrected output with gradation is obtained in the low frequency area. With the gamma correction characteristic of FIG. 4C, the high input level is obtained. The high frequency region and the portion b are raised so that a corrected output with gradation can be obtained in the high frequency region.

As the gamma correction ROM 5, n × (2
(N power of n) × (m power of 2) bits is used, and a plurality of conversion tables of gamma correction characteristics as shown in FIGS. By assigning the m-bit digital signal input from the / D converter 4 to the upper address of the gamma correction ROM 5, the conversion table of the gamma correction characteristics can be switched.
The video signal input converted into the n-bit digital signal input from the D converter 2 is gamma-corrected and output. A
Since the sampling clocks of the A / D converter 2 and the A / D converter 4 are the same or the sampling clocks which are phase-synchronized with each other are used, the time constant of the integrating circuit used in the APL detection circuit 3 is selected. It is possible to detect the APL of the video signal input for each line, for each field, or for each frame, switch the conversion table of the gamma correction characteristics, and gamma-correct the video signal input and output it.

Therefore, the AP output from the stabilizing circuit 7
The conversion table of the gamma correction characteristics of the gamma correction ROM 5 is switched according to the L data. When the APL data varies, the conversion table switching also varies, and the gamma correction output of the video signal input fluctuates, causing a change in the display screen. The image flicker appears, but the flicker of the image can be prevented because the stabilizing circuit 7 corrects and stabilizes the APL data. Further, even when the video signal input has a normal APL, a low APL, or a high APL, correction outputs having gradations are obtained so that black crushing or white crushing does not occur. be able to.

[0015]

As described above, according to the present invention,
The average luminance level of the video signal input is detected, the stabilizing means corrects the detected average luminance level so that the switching of the gamma correction characteristics is stable, and a plurality of gamma correction characteristics are switched according to the corrected average luminance level. By stabilizing the switching of the gamma correction characteristics, the flicker that occurs in the image is prevented, and the video signal input is corrected and output with the gamma correction characteristics selected by the switching. It is possible to provide a gamma correction circuit capable of obtaining a corrected output with gradation and capable of preventing black crushing or white crushing from occurring.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a gamma correction circuit of the present invention.

2A to 2D are explanatory diagrams showing an embodiment of the stabilizing circuit shown in FIG.

FIG. 3 is an explanatory diagram showing the operation of the stabilizing circuit shown in FIG. 1.

4A to 4C are explanatory diagrams of gamma correction characteristics.

FIG. 5 is a block diagram showing a conventional gamma correction circuit.

[Explanation of symbols]

 1 Input Terminal 2 A / D Converter 3 APL Detection Circuit 4 A / D Converter 5 ROM 6 Output Terminal 7 Switcher 10 Input Terminal 11 Latch Circuit 12 Comparison Circuit 13 Control Circuit 14 Input Terminal 15 Arithmetic Circuit 16 Switcher 17 Output Terminal

Claims (3)

[Claims] 1. A detecting means for detecting an average luminance level of a video signal input for a predetermined period, a stabilizing means for switching gamma correction characteristics for correcting and outputting a detection signal from the detecting means, and a stabilizing means. A gamma correction circuit comprising switching means for switching a plurality of gamma correction characteristics according to an output signal from the means, and means for gamma-correcting a video signal input according to the switched gamma correction characteristics. 2. An APL detection circuit for detecting an average luminance level of a video signal input for a predetermined period, a first A / D converter for converting an output from the APL detection circuit into a digital signal, and the first A / D converter. A gamma correction characteristic switching stabilization circuit for correcting and outputting a digital signal from the A / D converter, and a second A for converting a video signal input into a digital signal.
A / D converter and a ROM that stores a conversion table of a plurality of gamma correction characteristics. The digital signal from the stabilizing circuit and the digital signal from the second A / D converter are stored in the address of the ROM. The conversion table of the gamma correction characteristic of the ROM is switched by inputting an address from the stabilizing circuit, and the video signal input from the second A / D converter is gamma-corrected and output. Gamma correction circuit. 3. The latch circuit, wherein the stabilizing circuit latches and outputs input data for one data, a comparison circuit for comparing the output of the latch circuit with the input data, and a comparison result of the comparison circuit. A control circuit that outputs a control signal, an arithmetic circuit that adds or subtracts 1 to / from input data and outputs the input data, and the input data input to the input side and the data from the arithmetic circuit are selected by the control signal. The gamma correction circuit according to claim 2, comprising a switch for outputting.