Introduction to LCD

LCD (Liquid Crystal Display) is the abbreviation for liquid crystal display. The structure of LCD is to place a liquid crystal cell between two parallel glass substrates. The lower substrate glass is equipped with TFT (thin film transistor), and the upper substrate glass is equipped with color filters. The liquid crystal molecules are controlled by the signal and voltage changes on the TFT. Rotate the direction, so as to achieve the display purpose by controlling whether the polarized light of each pixel point is emitted or not.

According to the different types of LCD, LCD can be divided into two types: CCFL and LED.

CCFL.

Refers to a liquid crystal display (LCD) with CCFL (cold cathode fluorescent lamp) as the backlight source. The advantage of CCFL is good color performance, but the disadvantage is higher power consumption.

LED.

Refers to a liquid crystal display (LCD) with LED (light emitting diode) as the backlight source, usually WLED (white light LED). The advantages of LED are small size and low power consumption. Therefore, using LED as a backlight source can achieve high brightness while taking into account lightness and thinness. The main disadvantage is that the color performance is worse than that of CCFL, so most professional graphics LCDs still use traditional CCFL as the backlight source.

1.Contrast.

The control IC, filter and oriented film used in LCD manufacturing are related to the contrast of the panel. For general users, a contrast ratio of 350:1 is sufficient, but such contrast in the professional field cannot satisfy users. Demand. Compared with CRT monitors that easily reach a contrast ratio of 500:1 or even higher, only high-end LCD monitors can achieve this level.

2.Brightness.

LCD is a substance between solid and liquid. It cannot emit light by itself and requires additional light sources. Therefore, the number of lamps is related to the brightness of the liquid crystal display. The earliest liquid crystal displays had only two upper and lower lamps, the lowest of the popular type was four lamps, and the high-end one was six lamps. The four-lamp design is divided into three types of placement: one is that there is a lamp on each of the four sides, but the disadvantage is that there will be dark shadows in the middle. The solution is to arrange the four lamps from top to bottom. The last one is the "U"-shaped placement form, which is actually two tubes produced by two lamps in disguise. The six-lamp design actually uses three lamps. The manufacturer bends all three lamps into a "U" shape, and then places them in parallel to achieve the effect of six lamps.

3.Signal.

The response time refers to the response speed of the liquid crystal display to the input signal, that is, the response time of the liquid crystal from dark to bright or from bright to dark (the time for the brightness from 10%-->90% or 90%-->10%) , Usually in milliseconds (ms). There is a phenomenon of "visual residue" in the human eye, and high-speed moving pictures will form a short-term impression in the human brain. Animations, movies, and other up-to-date games have applied the principle of visual residue, allowing a series of gradual images to be displayed in rapid succession in front of people, forming dynamic images. The acceptable picture display speed is generally 24 frames per second, which is the origin of the movie playback speed of 24 frames per second. If the display speed is lower than this standard, people will obviously feel the picture pause and discomfort. According to this index calculation, the time required for each picture to be displayed is less than 40ms. In this way, for the liquid crystal display, the response time of 40ms becomes a hurdle, and the display above 40ms will have obvious flickering phenomenon, which makes people feel dizzy. If you want the image screen to reach the level of no flicker, it is best to achieve a speed of 60 frames per second.

4. Viewing angle

In the LCD display principle, when the backlight source passes through the polarizer, liquid crystal and alignment layer, the output light has directionality. Most of the light is emitted vertically from the screen, so from a certain larger angle When you look at the LCD monitor, you cannot see the original colors, or even all white or all black.

The commonly used interfaces of TFT-lCD are TTL (RGB), LVDS, EDP, MIPI.

TTL (RGB)

TTL (Transistor Transistor Logic) is transistor-transistor logic, and TTL level signals are generated by TTL devices. The TTL interface is an interface for data transmission in parallel. When using this interface, it is not necessary to use a dedicated interface circuit on the driver board side and the LCD panel side of the liquid crystal display, but the TTL data signal output by the main control chip of the driver board through the cable Directly transmitted to the input interface of the LCD panel. The TTL interface has high signal voltage, multiple connections, and long transmission cables, so the circuit's anti-interference ability is relatively poor, and it is prone to electromagnetic interference (EMI). In practical applications, TTL interface circuits are mostly used to drive small size (below 15in) or low resolution LCD panels. The highest TTL pixel clock is only 28MHz.

LVDS

LVDS, or Low Voltage Differential Signaling, is a low voltage differential signal technology interface. A digital video signal transmission method developed to overcome the shortcomings of high power consumption and large EMI electromagnetic interference when transmitting broadband high bit rate data in TTL level mode. The LVDS output interface uses a very low voltage swing (about 350mV) to transmit data through two PCB traces or a pair of balanced cables differentially, that is, low-voltage differential signal transmission. Adopting LVDS output interface, can make the signal transmit at the speed of several hundred Mbit/s on the differential PCB line or balanced cable, because of adopting the low voltage and low current driving mode, therefore, realize low noise and low power consumption.