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Liquid crystal display module (LCD module), also known as liquid crystal display screen and liquid crystal panel in the industry, is the main component of liquid crystal display, and its function is to display a clear picture. Liquid crystal display module mainly includes liquid crystal panel, LCD drive circuit, timing control circuit, backlight module, TTL or LVDS interface circuit and so on. These parts are usually encapsulated in the same shell.

The liquid crystal panel (Cell) is composed of a polarizer (including upper and lower polarizers), a color filter, two glass substrates (liquid crystal material filled in the middle), and an alignment film, wherein, a number of TFT (so called TFT array substrates) are made on the lower glass substrate, similar to the layered structure of sandwiches.

A Polarizer acts like a fence, allowing only components parallel to the fence (light) to pass through, while components perpendicular to the fence (light) are blocked and cannot pass through.

The function of color filter (Color film, that is, CF) is to color the liquid crystal display (LCD), usually on a transparent glass substrate. The color filter is composed of black matrix, color filter film (red, green, blue) and ITO film (also known as transparent electrode).

The function of the alignment film (Alignment film) is to arrange the liquid crystal molecules in a predetermined direction and angle.

The thin film transistor in the TFT (Thin Film Transistor, thin film transistor) array substrate is used to control the direction or steering of the liquid crystal.

The LCD driving circuit consists of a row driving circuit and a column driving circuit.

The column driver circuit processes the pixel signal, which adds a row of parallel pixel signal to the column electrode line line by row under the control of the pulse.

Row drive is to control the release electrode line from top to bottom, so as to display pixel signals line by line from top to bottom, and finally form an image. The circuit is relatively simple, which is a “displacement switch” controlled by row pulse.

The timing control circuit TCON (Timing Controller) is responsible for determining the order and time of pixel display and transmitting the signal to the drive circuit.

The backlight assembly (Back Light Unit, that is, BLU) is composed of a lamp tube or LED, a light guide plate, a reflector, etc. Because the liquid crystal display is a passive display device, the liquid crystal itself does not emit light, so it depends on the modulation of external light to display the image. Among them, the backlight is mainly used for external light, and the backlight is mainly divided into two types: cold cathode fluorescent lamp (CCFL) and light emitting diode (LED). Therefore, the cold cathode fluorescent lamp and the light emitting diode are the light source of the backlight assembly, and the function of the light guide plate and reflector is to cover the light of the backlight all over the screen.

At present, TTL (Transistor Transistor Logic, transistor-transistor logic) and LVDS (Low Voltage Differential Signaling, low voltage differential signal) are most widely used in LCD interface circuit.

TTL interface circuit is a kind of parallel bus interface circuit, which is used to drive TTL liquid crystal display screen. According to the different resolution, there are 24-bit and 48-bit parallel digital display signals.

LVDS receiving circuit is a kind of high-speed serial bus interface circuit, which is used to drive LVDS LCD. This kind of interface has higher transmission rate, lower electromagnetic interference and less data transmission line than TTL interface, which simplifies the circuit structure.

LCD is mainly composed of LCD screen, main control board, power supply board, high voltage board, key control board, interface and data line, etc. Figure 1 shows the composition structure of liquid crystal display, and figure 2 shows the block diagram of the composition structure of liquid crystal display.

The main control board (also known as the driver board in the industry, which is not confused with the driving circuit of the LCD screen here) is mainly used for receiving and processing analog (VGA) or digital (DVI) image signals sent from the outside, generating the timing and voltage needed to control the deflection of liquid crystal molecules, and sending out driving signals through the screen line to control the LCD panel to work.

The main control board mainly consists of signal input part (connected to data line), signal output part (connected to screen line), AME D conversion circuit (connected to VGA interface), TMDS receiver (connected to DVI interface), signal processing part (main control chip), microcontroller part (MCU and memory), control menu and high voltage board interface.

The signal input part mainly receives the analog (VGA) and digital (DVI) signals output by the computer graphics card.

The signal output part is the connection between the drive board and the display screen. The commonly used ones are parallel bus TTL interface and low voltage differential LVDS interface.

The AME D conversion circuit is used to convert the analog signals output from the analog VGA interface into R, G, B digital signals, and send them to the image scaling circuit (Scaler) for processing.

TMDS (Transition Minimized Differential Signaling) is a differential signal transmission mode, which converts 8-bit R, G, B data signals into 10-bit row field synchronization signals, clockwise signals, data DE signals, error correction signals, etc., and then uses differential signals to transmit data signals after DC balance. Compared with LVDS and TTL, it has better electromagnetic compatibility, thus realizing long-distance and high-quality digital signal transmission. The TMDS receiver is used to decode the TMDS data and clock signal, and then send it to the image scaling circuit for processing.

The signal processing part refers to the image scaling circuit of the driving board, whose function is to convert the signals of different resolutions into the resolution corresponding to the LCD screen through the scaling circuit, so as to ensure that the LCD screen displays a normal image picture, because the pixel position and resolution of the LCD screen are fixed, while the resolution of the TV signal and the external input signal is variable.