The quality of a LCD depends first on its panel, because the panel directly affects the viewing effect of the screen, and LCD TV panel accounts for more than half of the total cost of the whole machine, which is the main factor affecting the cost of LCD TV, so to choose a good LCD, the first thing is to choose its panel. LCD panel can largely determine the brightness, contrast, color, visual angle and other very important parameters of LCD. LCD panels are developing rapidly from three generations in previous years to four and five generations, then skipping six generations to seven generations, and renewing the eighth generation of panels is also under planning. At present, the main manufacturers of LCD panels are Samsung, LG-Philips, Youda and so on. Due to the difference of technical level of each country, the LCD panels produced can be roughly divided into different types of machines. Common are TN panel, MVA panel and PVA panel, IPS panel and CPA panel.


1, TN panel


TN is called Twisted Nematic (Twisted Nematic) panel. The low cost of production makes TN the most widely used entry-level LCD panel. It is widely used in the mainstream low and medium-end LCD in the current market. At present, most of the TN panels we see are improved TN+film, film is compensation film, which is used to compensate for the deficiency of the visual angle of TN panels. At present, the visual angle of the improved TN panels reaches 160 degrees. Of course, this is the limit value measured by the manufacturer when the contrast is 10:1. In fact, when the contrast is reduced to 100:1, the image has been distorted or even biased.


As a 6Bit panel, TN panel can only display 64 colors of red/green/blue, and only 262.144 kinds of maximum actual colors. Through the "jitter" technology, it can obtain more than 16 million colors, and can only display three primary colors of 0 to 252 gray scales. Therefore, the final color display information is 16.2 M color, not the true color 16.7 M color as we usually call it; plus TN panel. It is difficult to improve contrast. The problems directly exposed are thin color, poor restoring ability and unnatural transition.



The advantage of TN panel is that it has fewer gray class output, fast deflection speed of liquid crystal molecules and easy to improve response time. At present, TN panel is basically used in liquid crystal products below 8ms on the market. In addition, Samsung has developed a B-TN (Best-TN) panel, which is actually an improved type of TN panel. In order to balance the high-speed response of TN panel, the contradiction of quality must be sacrificed. At the same time, the contrast can reach 700:1, which is close to the panel of MVA or early PVA. Many panel manufacturers in Taiwan produce TN panels. TN panels belong to soft screens. Similar water marks will appear when you gently scratch them with your hands. In addition, if you look closely at the screen, it is roughly like this:




2. Class VA Panel


Class VA panels are widely used in high-end liquid crystals nowadays, and belong to wide-view panels. Compared with TN panel, 8 bit panel can provide 16.7M color and large visual angle, which is the capital of positioning high-end panel, but the price is also more expensive than TN panel. VA panel can be divided into MVA panel dominated by Fujitsu and PVA panel developed by Samsung. The latter is the inheritance and improvement of the former. The front (face-to-face) contrast of the VA panel is the highest, but the uniformity of the screen is not good enough, and color drift often occurs. Sharp text is its killer mace, black and white contrast is quite high.



Fujitsu MVA technology (multi-domain Vertical Alignment, multi-quadrant vertical alignment technology) can be said to be the earliest wide-view LCD panel technology. This kind of panel can provide a larger visual angle, usually up to 170 degrees. Through technical authorization, Chimei Electronics (Chimei Optoelectronics), Youda Optoelectronics and other panel enterprises in Taiwan Province of China have adopted this panel technology. The visual angle of the improved P-MVA panel can reach 178 degrees close to the level, and the gray scale response time can reach less than 8 ms.


Samsung Samsung Electronics PVA (Patterned Vertical Alignment) technology also belongs to the category of VA technology, it is the successor and developer of MVA technology. Its comprehensive quality has surpassed the latter in an all-round way, and the improved S-PVA has been able to keep pace with P-MVA to obtain a wide visual angle and faster response time. PVA uses transparent ITO electrodes instead of liquid crystal protrusions in MVA. Transparent electrodes can obtain better opening ratio and minimize the waste of backlight. This mode greatly reduces the possibility of "bright spots" in LCD panels, and its status in the era of LCD TV is equivalent to the "long tube" in the era of CRT TV. Samsung's wide-view technology of PVA mode is widely used by Japanese and American manufacturers because of its strong capacity and stable quality control system. At present, PVA technology is widely used in high-end LCD or LCD TV. The VA panel also belongs to the soft screen. The similar water marks will appear when you gently scratch it with your hands. Look closely at the screen roughly as follows:




3. IPS Panel


IPS (In-Plane Switching) technology is a liquid crystal panel technology launched by Hitachi in 2001, commonly known as "Super TFT". The IPS camp, headed by Hitachi, has gathered a number of manufacturers, such as LG-Philips, Hanyu Caijing, IDTech (Chimei Electronics and Japan's IBM joint venture), but not many models can be seen in the market. IPS panel's biggest characteristic is that its two poles are on the same surface, unlike other liquid crystal mode's electrodes are on the upper and lower sides, three-dimensional arrangement. Because the electrodes are on the same plane, the liquid crystal molecules are always parallel to the screen in any state, which will cause the opening rate.


4. CPA panel (ASV panel)

CPA (Continuous Pinwheel Alignment) mode wide-angle technology (soft screen), CPA mode wide-angle technology strictly belongs to the VA camp, the liquid crystal molecules radiate toward the central electrode in a firework-like arrangement. Because the electric field on the pixel electrodes varies continuously, this wide-angle mode is called "continuous pyrotechnic arrangement" mode. CPA is mainly promoted by Sharp, the father of LCD. What we need to pay attention to here is that the ASV that Sharp has always advocated does not refer to a specific wide-view technology. It refers to the products that have used TN+Film, VA, CPA wide-view technology as ASV. In fact, only CPA mode is Sharp's own Broadview technology. The products of this mode are basically the same as MVA and PVA. That is to say, Sharp brand LCD TV may not use Sharp's own CPA mode LCD panel. It may use Taiwan's VA mode panel or other manufacturers'LCD panel. Sharp's CPA panel color restoration is real, visual angle is excellent, image is delicate, the price is relatively expensive, and Sharp seldom sells CPA panel to other manufacturers. CPA panel also belongs to the soft screen, with the hand gently strokes will appear similar watermarks, carefully look at the screen is roughly like this:


In addition, some other manufacturers also have their own LCD panel technology, such as NEC ExtraView technology, Panasonic OCB technology, modern FFS technology, etc. These technologies are improvements to the old TFT panel, providing visual angle and response time, usually only used in their own brand LCD display or LCD TV. In fact, these panels belong to TFT panels, but now various panels have their own technology and name, so the name TFT is not often used.

A kind of


Black-and-white response time

The so-called black-and-white response time is the speed at which each pixel of the LCD reacts to the input signal, that is, the time required for the pixel to turn dark or light to dark (the principle is to apply voltage in the liquid crystal molecule to make the liquid crystal molecule twist and recover). The commonly used 25ms and 16ms refer to the response time. The shorter the response time, the less the user will feel the tail shadow dragging when looking at the dynamic picture. In general, the black-and-white response time is divided into two parts: Rise time and Fall time, and the representation is based on the sum of the two.


In CRT display, as long as the electron beam strikes the phosphor, it can emit light immediately, and the glow residue time is very short, so the response time of traditional CRT display is only 1-3 Ms. Therefore, response time is generally not mentioned in CRT monitors. Because LCD controls the turn-off of light by twisting liquid crystal molecules, and the twisting of liquid crystal molecules requires a process, the response time of LCD is obviously longer than that of CRT.

From the early 25 ms to the well-known 16 ms to the recent 12 ms or even 8 ms, the response time has been constantly shortened, and the old idea that LCD is not suitable for entertainment is being challenged. A simple conversion can be made: 30 milliseconds = 1/0.030 = 33 frames per second; 25 milliseconds = 1/0.025 = 40 frames per second; 16 milliseconds = 1/0.016 = 63 frames per second; 12 milliseconds = 1/0.012 = 83 frames per second. It can be seen that the birth of 12ms means a great progress in liquid crystal manufacturing.

However, it should be noted that LCD has a scanning frequency limit, especially for the field frequency (also known as refresh rate), many of which are limited to less than 75Hz. In general, 75Hz means 75 frames per second refresh, so that it does not seem to reach 83 frames per second corresponding to 12ms.

In fact, the 12ms response time mentioned above is the time needed for switching between full black and full white pictures. The driving voltage required for switching between full white and full black pictures is relatively high, so the switching speed is relatively fast, which can reach 12ms. In practical applications, most of them are gray-scale pictures switching (in essence, liquid crystal is not completely twisted, not fully transparent). The required driving voltage is relatively low, so the switching speed is relatively slow. Since 2005, many manufacturers have begun to emphasize the importance of gray-scale response time, but gray-scale response time can be improved by special methods, so there is no clear correspondence between black-and-white response time, which is equivalent to a new parameter describing response time.

According to the data, the response time of 30 milliseconds = 1/0.030 = per second display can display 33 frames, which is already able to meet the needs of DVD playback; the response time of 25 milliseconds = 1/0.025 = per second display can display 40 frames of screen, fully meet the needs of DVD playback and most games; and play that kind of intense action games (such as QUAKEIII, UT2003, DOMMIII), extreme chase race. If the game is to achieve no shadow, the required screen display speed should be more than 60 frames per second, that is, the required response time = 1/second display can display 60 frames per second screen = 16.6 milliseconds.

Grey scale response time


When it comes to gray scale response time, let's first look at what gray scale is. We can see that every point on the LCD screen, that is, a pixel, is composed of three sub-pixels: red, green and blue (RGB). In order to change the color of the screen, we must control the brightness of the three sub-pixels of RGB separately, so as to "mix" different colors. The more layers of light and shade in the middle, the more delicate the effect of the picture can be presented. Take the 8 bit panel as an example, it can show 256 brightness levels (the 8th power of 2), which we call 256 gray levels.


Because of the rotation of liquid crystal molecules, each point on the LCD screen transits from the former color to the latter color change, which will have a time process, which is what we usually call response time. Because the conversion process between different gray scales of each pixel is complex and varied in length, it is difficult to express it by an objective scale. Therefore, the traditional definition of liquid crystal response time tries to take the conversion speed of liquid crystal molecules from all black to all white as the response time of liquid crystal panel. Because the conversion speed of liquid crystal molecules from black to white is not exactly the same as that from white to black, in order to show the reaction speed of liquid crystal panel as meaningfully as possible, the traditional definition of response time basically takes the whole response time of "black-white-black" as the standard.


But when we play games or watch movies, the content of the screen can not only switch between the darkest and the whitest, but also the colorful pictures, or the changes in depth and shade, which are the conversion between gray levels. In fact, the conversion speed and torsion angle of liquid crystal molecules are determined by the applied voltage. From all black to all white liquid crystal molecules are facing the biggest twist angle, which requires a higher voltage, and the twist speed of liquid crystal molecules is faster. However, it is difficult to realize the gray scale switching involving different shades. And all the images seen on the display are the result of gray scale change. Therefore, the measurement method of black-and-white response can not correctly express the practical significance. Therefore, the concept of gray scale response time has been adapted.


It should be noted that although the grey scale response is more difficult to control and takes longer time, the actual situation may be quite the opposite. Because manufacturers can greatly improve the gray scale response time through special technology, which in turn is much shorter than the traditional black-and-white response time. For example, using response time acceleration chip, the product with 25 ms black-and-white response time can have 8 ms gray-scale response time. Grey-scale response time differs greatly from the original black-and-white response time in meaning and properties. There is no clear correspondence between them, but both are descriptions of liquid crystal response time.


Grey scale response has been used by many manufacturers since 2005. Generally speaking, these products usually use better response time control methods, such as the response time of each pixel is more stable and uniform. The products with short gray-scale response time have fewer shadows and better picture quality, especially when playing moving images. Therefore, game players or video users can consider this parameter of LCD more.


Horizontal scanning frequency



Also known as horizontal refresh rate, it refers to the number of scanning lines per second in KHz. Line frequency = line number * field frequency, for example, at 800 * 600 resolution, when refresh rate is 85 Hz (usually expressed as 800 * 600 @85Hz), line frequency = 600 * 85Hz = 51Khz.



Vertical scanning frequency


Also known as refresh rate, is the number of times the display refreshes the screen per second in Hz. The lower the field frequency is, the more flickering and jittering the image is. But the scanning frequency of LCD display screen is different from CRT. It refers to the number of times that the display receives signals in unit time and updates the picture. Because the status of LCD display pixels changes only when the content of the picture changes, so even if the scanning frequency is very low, it can ensure stable display. Generally, 60 Hz is enough. But in some industry applications such as medical treatment and monitoring, the refresh rate of LCD is required to reach 70 Hz or even 85 Hz. The main requirement is to be able to read and display data at a faster frequency.





Bandwidth represents a comprehensive indicator of display capability. It refers to the number of pixels scanned per second, that is, the sum of frequency points displayed on all scanning lines in a unit time, in MHz. The larger the bandwidth, the stronger the display control ability and the better the display effect.


The detailed formulas for calculating bandwidth are as follows: theoretically, bandwidth B = r(x)*r(y)*V


R(x) denotes the number of primes on each horizontal scan line


R(y) denotes the number of horizontal scanning lines per frame


V represents the refresh rate per second (i.e. field frequency)


B represents bandwidth


A kind of


Liquid Crystal FFS Technology


Technical Principle of FFS


FFS technology is a TFT-LCD technology mastered by Korea HYDIS (which was stripped from BOE in 2006 and acquired by PVI-Taiwanese Yuantai in 2007). It is called Fringe Field Switching (FFS). FFS is a wide-view technology developed by LCD industry to solve large-scale, high-definition desktop display and LCD TV applications, which is now commonly known as hard-screen technology. FFS technology generates edge electric field between the electrodes in the same plane, so that orientational liquid crystal molecules can rotate in the plane direction (parallel to the substrate) between the electrodes and above the electrodes, thus improving the transparency efficiency of the liquid crystal layer. FFS technology overcomes the conventional IPS (In-Plane-Switching, plane side)

Type of liquid crystal plate