LCD display flashes appear to resolve the causes and
Comparison of two types of CRT and LCD displays, one of the most common to see the issue of difference is blinking. Tend to think that there are flickering CRT screen, and the LCD is not, found two types of displays have a certain degree of blinking, the mechanism of the differences and correct way to affect the success rate. This article appears on the LCD screen flashes the reasons for, and proposed methods to avoid flicker.
LCD display Technology
Liquid crystal display (LCD) in 1973 in the calculator, the LCD is embedded in the first paragraph calligraphy are so figures can be displayed. The next generation of LCD is available in 1980, are dot-matrix (dot-matrix) show that, in addition to the figures show, there are characters and graphics, such as a simple monochrome computer monitors or the popular "electronic pet" his mother Brother pool. Start with these design matrix array of rows and columns of pixels as a control for each pixel to be replaced by a separate control line. To the late 1980s, the successful color filter embedded inside the LCD design. From the beginning, the successive products are focused on the screen size, display the weight, energy efficiency, such as to enhance the perspective.
Figure 1. Through the imposition of voltage-controlled liquid crystal array of vertical elements, the light can pass through along the molecule.
Although continuous improvement, but the basic operation of the LCD display remains unchanged. LCD display pixel array to be back at the end of continuous light shine, long CRT light eliminates the flicker found in a class of (P-point with the refresh cycle for the pulse of each flash)). In contrast, LCD pixels are in the next two vertical pit pattern with each other between the glass, as shown in Figure 1. These pit Patterns at the liquid crystal to form the channel, to the back past the end of the screen in front of Optical Communication. The amount of light transmission depending on the orientation of liquid crystal. With the imposed voltage proportional to.
Both upper and lower polarizing plates are perpendicular to each other's. Voltage between the two boards to adjust liquid crystal mode in the distorted to suit each of the polarizing plate. Background light from the light through the liquid crystal has been switched. In contrast, when the liquid crystal has not been tuned, the light was blocked, to adjust the weight in proportion with the applied voltage, and brightness control for the play.
Table 1. Listed a variety of video formats the pixel array size
Plate is outer filters (RGB), red, green and blue areas (each referred to as sub-pixels) included in each pixel, the color associated with each color separately addressable areas, it can display full-color and luminosity. The number of pixels determines the clarity of the screen. Table 1 lists a variety of different video formats the pixel array size. Array size should be noted that the number of pixels rather than directly to aspect ratio, because the pixels are not square was typical.
The reasons for the flashing LCD
Figure 2. LCD display flashes of visual examples.
(A) (left) LCD flickering optimized. (B) (right) LCD excessive blinking
In the flickering LCD screen is different from the CRT, LCD flicker is itself a fade, but it is certainly not the pulse of light. Figure 2a and 2b as shown in Figure 2a by the LCD display has been to reduce the flicker adjustment, and Figure 2b is the LCD had a strong flashes, this is because the LCD refresh rate up to 300Hz caused.
Figure 3. A single LCD, like the circuit diagram
Figure 3 shows a single LCD pixel drive circuit. Charge for a gate voltage switches are generally enlarged to become the-5V to 20V, the general video source voltage ranging from 0V to 10V, the provision appears in the pixel's brightness information. Pixels below the screen are connected to the backplane, in this node's voltage Vcom.
Although such a layout the way to play a role, but to reduce the life of the screen, it is assumed that in the Vcom voltage. Pixels on the voltage change from 0V to 10V, the assumption that an average of 5V, which have a significant DC voltage at the two ends of each pixel, this DC voltage caused by charge storage. In the long term, due to the pixel electrode on the plating
There are a result of impurity ion pixels to deteriorate, this is the reason for residual image is commonly found in the old panel TFT-LCD color screen.
LCD screen of the structure is symmetrical (Figure 1), any positive and negative pressure can be used to adjust the liquid crystal, which can take full advantage of this aspect of the public to the video signal voltage of the mid-point (5V) Now up and down the video signal voltage swing in public (Vcom), the result in pixels to create a "net zero effect" (net-zero effect). This occurred in the liquid crystal net zero effect on the elimination of the residual problem of aging and images. Definition of this technology to make co-ordination, since the video signal to full brightness 5V to walk instead of 10V.
Figure 4. Alternate frames for the reverse mode of the three-phase distribution of the LCD pixels: frame inversion, line inversion, inversion point.
To screen a net zero effect, the entire LCD screen can be used on a variety of inversion mode (Figure 4). The simplest type is inverted frame (Frame Inversion), in such a drive method, each screen pixel in each subsequent frame in the reverse. Inversion in the frames to create a pixel compared to a net zero effect of the time. The other two methods are incorporated into each frame of the inversion, line inversion (line inversion) at every level to change the phase alternating line. Inversion line of the turn of the way in a public phase on the horizon (rather than a single line), known as the line of inversion (line-paired inversion). Inversion point (dot inversion) is the inversion of adjacent pixels in each phase of the chess board as if the West. Three methods to create in pixels in relation to time on the net zero effect. Inversion models to choose from the factory, and embedded in the drive circuit. In all cases, the frames are displayed alternately in each of the RP.
Vcom voltage to be accurately placed in the mid-point of the video signal in order to avoid flicker. To explain why the display when the flashes, it is assumed that the relationship between manufacturing screen. Vcom set at 5.5V. If the video signal swing in between 0V and 10V, full-scale voltage will vary for each Field in a full-scale field map voltage is 4.5V, while in another map of the full-scale field voltage is 5.5V in full-scale voltage of the difference in brightness will be translated into poor, so there blinking.
Figure 5. The use of light sensors screen flicker measurement EL7900
Figure 5 show that there is with the screen flickering light difference, light-colored, more DC waveform level, recorded in the screen does not flicker, the measurement using a photosensitive sensor EL7900. Photosensitive sensor light into current, the greater the current in Oscilloscope produce greater voltage deflection.
In order for you to understand these results, first of all need to know that there are two LCD screen: "white" screen with the "black" screen. White screen in a state of relaxation (no voltage applied to the LCD on) to the light through the LCD, while the black screen in the barrier to ease the state of all light. Restrictions on the LCD when the voltage increases, rotating LCD. This block more light (as in the case of white screen) or to allow more light through (as in the case of black screen). The screen is a white screen test, so imposition of the liquid crystal voltage the bigger the screen the more dark. If accurate Vcom voltage setting in the middle (no flicker), then, the average AC voltage is zero, the screen will highlight in the most. If not in the middle of Vcom voltage. In that case, the results of AC voltage will be higher, so the screen brightness will be dark.
Figure 2b is due to the fading Vcom voltage imbalance led to the liquid crystal caused by an error voltage is not the question of the overall light intensity.
Method to eliminate the flashing LCD
Figure 6. (A) (left) the use of mechanical potentiometers adjust VCOM. (B) (right) regulating the use of digital potentiometers VCOM.
Each LCD display as a result of both changes in the structure, the best value in the Vcom voltage between the LCD and the LCD will be the difference, therefore, the original equipment manufacturers must adjust the screen each factory, to eliminate this flicker of on the small screen, the visualization of a low-impedance backplane, the way of a potentiometer can be added as a voltage regulator in public, in general, the use of mechanical potentiometers and the need for additional working hours. For the small screen in terms of
This is acceptable, even in regard to the big screen. Less accurate and can easily be assembled during the destruction, the need to replace entire components. More than 19 inches of screen, floor can no longer be regarded as a single low-impedance node, takes a different location on the screen to make multiple amendments, may be more than partial compensation to the five networks, four in the corner, one in the middle, in this case Next: NC Potentiometers (Digitally Controlled Potentiometer, DCP) can be processed automatically to the manufacturer of the processing, for the big screen and is unable to carry out manual adjustment is necessary. Figure 6a and Figure 6b showing the mechanical potentiometers and NC solutions.
Figure 7. DCP application of software-programmable VCOM circuit
From the mechanical potentiometer switch to DCP methods and systems found is very simple to implement, as shown in Figure 7 for the DCP software programming applications Vcom drive circuit, ISL45042 for the current output type, non-volatile DCP, operate up to 20V on AVDD , ISL45042 using two, from top to bottom interface, is extremely accurate 7 bit devices, resolution is 128, Vcom value required can be stored in the on-board EEPROM. Digital circuit by the voltage range of 2.25V to 3.6V. This is so that it can used with many of today's boundary controller. Imposed on the simulation of analog power resistor ladder, the operational in the 4.5V to 20V, normally takes less than 10V for the analog power of the small screen and must be greater than 15V analog power in terms of the big screen this is an important feature. DCP and the output voltage by the EL5111 buffer amplifier (180mA output current) to Vcom bus.
With the popular view is exactly the opposite, LCD display there is a flicker, but a simple potentiometer adjustment is to reduce the impact, as appears in the LCD flicker offset voltage of the public, rather than in the refresh signal.
With the growing popularity of LCD screen sizes and increasing, in a single point on the floor manual adjustment is no longer feasible, the use of ISL45042 DCP and EL5111 Vcom buffer will be able to multi-point backplane Vcom shift on the automatic amendments to effect more visible.
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