JP2625823B2 - Color image quality automatic adjustment circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image display circuit that performs A / D conversion of a video signal and displays an image, and more particularly to a circuit that automatically adjusts image quality.

[Conventional technology]

2. Description of the Related Art In the field of image devices such as televisions, devices using a liquid crystal or the like as a display are increasing. For example, in a liquid crystal television, as a method of transferring a video signal to a liquid crystal display unit, there is a method of converting a video signal that is an analog value into a digital value by an A / D converter and transferring the digital signal. Conventionally, video signals 37 to 39 are directly input to A / D converters 31 to 33 as shown in FIG.
The upper reference voltage 40 for setting the / D conversion voltage range is set to the video signal peak level at the time of maximum modulation, and the lower reference voltage 41 is set to the pedestal level.

[Problems to be solved by the invention]

However, the degree of modulation of the video signal differs depending on the content of the program and the broadcasting station. When the degree of modulation is low, the brightness and contrast are significantly reduced, and the image is difficult to see.

Therefore, the present invention solves such a problem.
It is an object of the present invention to maintain a constant luminance and contrast even when the degree of modulation of a video signal is reduced, and to provide a clear color image.

[Means for solving the problem]

An automatic color image quality adjustment circuit according to the present invention is an automatic color image quality adjustment circuit for an image display circuit for performing A / D conversion of a video signal and displaying an image. A level clamp circuit; an A / D converter for converting the video signals of the three primary colors clamped by the pedestal level clamp circuit into digital values; and a video signal for each color converted to a digital value by the A / D converter. And a peak detection circuit that detects a highest peak value from among the pedestal clamped three primary color video signals and supplies the highest peak value as an upper limit reference voltage of the A / D converter. The detection circuit is characterized in that a peak value holding time constant after detecting the peak value is set to several hundred ms.

〔Example〕

Hereinafter, the present invention will be described in detail based on examples.

FIG. 1 is a circuit diagram of a color image quality automatic adjustment circuit of the present invention,
FIG. 2 is a timing chart showing the operation of FIG.

The video signal is decomposed into three primary colors, and input to the A / D converters 3, 4, and 5 through capacitors as an R primary color video signal 15, a G primary color video signal 16, and a B primary color video signal 17, respectively. In the video signal, the back porch of the synchronization pulse indicates the pedestal level, and the luminance becomes higher as the voltage is higher based on the pedestal level.

The video signals 15 to 17 include a DC component, and the pedestal level is higher than the ground potential, that is, 0 [V]. However, after passing through the capacitor, the clamp signal 18 becomes H level at the timing of the back porch, and The switching transistor in the clamp circuit 2 is turned on.
Therefore, the pedestal level of the video signals 21 to 23 is 0 [V].
Therefore, the conversion lower-limit reference voltages 20 of the A / D converters 3 to 5 are also set to 0 [V].

Next, the video signals 21 to 23 after the pedestal clamp are input to the peak detection circuit 1 through the diodes 9 to 11, and the highest voltage among the video signals 21 to 23 is supplied to the capacitor 24.
Is charged and held. Since the voltage of the capacitor 24 is lower than the peak voltage of the video signals 21 to 23 by the forward voltage of the diodes 9 to 11, the voltage of the video signal 21
Raised to a voltage equal to ~ 23 peak voltage. Furthermore,
The impedance is lowered by the buffer transistors 13 and 14 and is given to the A / D converters 3 to 5 as the upper limit reference voltage 19.

First, the peak voltage of the regions 1, B primary color video signals 23 and becomes the highest V _1, the upper limit reference voltage 19 is set to V _1. Next, the peak voltage of the region 2, G primary color video signals 22 but is highest V _2, the upper limit reference voltage 19 lower than V ₁ was shifted gradually V ₂ from V ₁ by the discharge resistor 25, V _Two
And stabilized. Further, the peak voltage of the region 3, R primary color video signals 21 but is highest V _3, is higher than V _2, immediately above the upper limit reference voltage 19 rises to V _3. Thus, even if the modulation degree of the video signal changes and the peak voltage changes, the upper limit reference voltage 19 always follows the peak voltage by the operation of the peak detection circuit 1.

Here, when the degree of modulation decreases, the upper-limit reference voltage 19 decreases, and it takes a while to stabilize. However, if the discharge resistor 25 is appropriately set and the time constant is set to several hundred ms, there is no practical problem at all. And a smooth response is obtained.

Next, the A / D converters 3 to 5 output gray scale signals based on digital values.
, And an image is displayed on the liquid crystal color image display 8 through the image drive circuit 7.

〔The invention's effect〕

As described above, according to the present invention, the upper limit reference voltage of the A / D converter always follows the peak voltage even when the modulation degree of the video signal changes and the peak voltage changes. The pedestal level of the video signal is always A / D
Matches the lower reference voltage of the converter. Therefore, not only is the initial adjustment of the reference voltage of the A / D converter unnecessary, but also the best contrast and brightness are always obtained, and a clear image can be enjoyed.

Also, when a signal indicating the tuning frequency is superimposed on the screen, the peak voltage of this signal is extremely high, so the A / D
The conversion voltage range becomes wide, and the original video signal portion becomes dark. That is, since the background becomes darker, the frequency display becomes clearer and it is easy to read. Further, in the tuning system of the continuous frequency variable system, the background noise during tuning is dark, easy to see, and eyes are not tired.

Also, when a whitish character is broadcast, the background becomes dark and easy to read, which is very practical.

[Brief description of the drawings]

 FIG. 1 is a circuit diagram of a color image quality automatic adjustment circuit according to the present invention. FIG. 2 is a timing chart showing the operation of FIG. FIG. 3 is a circuit diagram of a conventional color image display circuit. DESCRIPTION OF SYMBOLS 1 ... Peak detection circuit 2 ... Pedestal clamp circuit 3-5 ... A / D converter 6 ... Digital multiplexer 7 ... Image drive circuit 8 ... Liquid crystal color image display 9-12 ... Diode 13, 14 Transistor 24 Capacitor 25 Discharge resistance

Claims (1)

(57) [Claims] An automatic color image quality adjustment circuit for an image display circuit for A / D converting a video signal and displaying an image, wherein a pedestal level clamp circuit for setting a pedestal level of the video signal of three primary colors to a predetermined value; An A / D converter for converting the video signals of the three primary colors clamped by the pedestal level clamp circuit into digital values; and a multiplexer for multiplexing each color video signal converted to a digital value by the A / D converter. A peak detection circuit that detects the highest peak value among the three primary color video signals clamped by the pedestal and supplies the highest peak value as an upper reference voltage of the A / D converter. An automatic color image quality adjustment circuit, characterized in that a peak value holding time constant after detecting a value is set to several hundred ms.