LCD liquid crystal display – principle

Basic concept

The common screen in life is called LCD (Liquid crystal display) LCD display has the advantages of low power consumption, small size, large amount of information and no eye damage, so it has become the mainstream electronic display device in life.

A liquid crystal is a special substance between a solid and a liquid. It is a kind of organic compound. It has the same regularity. When an electric field is applied externally to the liquid crystal, it will change its molecular arrangement, thus changing the propagation of light. , change the size of the voltage applied to the liquid crystal, and change the amount of light in a certain color.

Through this principle, you can make a controllable red (R), green (G), blue (B) light output intensity display structure, the three display structures are composed of a display unit. You can mix and match the unit to output different colors, change the ratio of the three colors, you can mix different colors, so a display unit is called a pixel. It is not luminous, it needs a back light lamp to provide a light source, then the color of the screen visible to the naked eye appears.

The brightness of the screen, that is, the light goes through a series of processing processes before reaching the output, so the intensity of the output light is much lower than the intensity of the light source. Relatively small, from the side view of the screen does not show its content.

It can be seen from this that, behind a small screen is the fusion of countless science and technology, the ring is mutually exclusive, and the huge development of today's film and television entertainment.

Basic parameters

What are the meanings behind the "nouns" introduced behind the screen?

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Pixels

Pixels are the most basic unit elements that make up an image. The pixel of a display refers to the smallest point of its imaging. A pattern is composed of countless points. The more pixels there are, the more delicate and beautiful the picture will be.

Rresolution

The screen resolution is usually expressed as "row pixel value x column pixel value". For example, a resolution of 1920x1080 means that each row of the display has 1920 pixels and each column has 1080 pixels, which can also be understood as 1920 columns and 1080 rows.

Color

Color depth refers to how many colors each pixel of the display can represent, usually expressed in "bits". Common display color depths are 16-bit and 24-bit.

Size

The size of a display is usually expressed in inches, such as 5 inches, 21 inches, 24 inches, etc. This length refers to the length of the screen diagonal. The smaller the size, the more science and technology it contains, and the more complex the structure. Kelai Optoelectronics can support ultra-small LCD display modules as small as 1.18 inches.

Dot Pitch

The distance between two adjacent pixels affects the fineness of the image quality and the viewing distance. For screens of the same size, if the resolution is higher, the smaller the dot pitch, the finer the image quality.

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Signal control

Now that we know the basic parameters, how does the screen work?

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RGB signal cable

There are usually 8 RGB signal lines each, so the RGB888 format is supported,with a total of 24 data lines, representing the red, green,and blue (RGB) color components of a pixel on the LCD screen.

Synchronous clock signal CLK

The LCD screen uses synchronous communication with the outside world,usually using the CLK signal as the synchronous clock.Driven by the synchronous clock, each clock transmits one pixel data.

Horizontal synchronization signal HSYNC

The horizontal synchronization signal is used to indicate the completion of the transmission of a row of pixel data on the LCD screen.Every time a row of pixel data on the LCD screen is transmitted, the HSYNC level will jump.For example, for a display with a resolution of 1920*1080, the HSYNC level will jump 1080 times when transmitting a frame of image!

Vertical sync signal VSYNC

The vertical synchronization signal VSYNC is used to indicate the end of the transmission of a pixel data of the LCD screen.Each time a frame of pixel data is transmitted, the level of VSYNC will jump."Frame" is the unit of image,and a set of images is called a frame.In LCD, a frame refers to a complete screen LCD pixel."/second"is often used to indicate the refresh characteristics of the LCD screen,that is, how many sequential images can be displayed per second by the LCD screen, also commonly known as "os". For example, when the LCD screen runs at a rate of 60 per second, the level of VSYNC changes 60 times per second,also known as 60 f0s.The number of frames displayed on a good screen is in line with the blinking frequency of the naked eye.

Data enable signal DE

The data enable signal DE is used to indicate the validity of the data. When the DE signal line is at a high level, the RGB signal line indicates that the data is valid.

Video Memory

Each pixel in the display screen is data. In practical applications, the data of each pixel needs to be cached and then transmitted to the LCD screen. This memory that stores display data is called video memory. Video memory can generally store at least one frame of display data of the LCD screen. For example, for an LCD screen with a resolution of 800x480, using the ob888 format for display, the data size is 300x480x3 = 1152000 bytes, 3 means that one pixel occupies 3 bytes of data (R: 8bit, G: 8bit, B: 8bit); if the RGB565 format is used, the data size of one frame is: 2x800x480 = 768000 bytes, 2 means that one pixel occupies 2 bytes (R: 5bt, G: 6bt, B: 5bit). Therefore, it can be seen that the higher the number of bytes occupied by a pixel, the richer the colors that can be represented!

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LED vs. OLED

In life, we often hear about LED and OLED screens. Do you know what the difference is? It's not just a difference in letters.

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LED screens do not require backlights. A single pixel of an LED dot matrix color display contains three LED lights, red, green, and blue. By controlling the brightness of these three lights, full-color output is achieved, and multiple pixels form an entire screen. Since each pixel is the light emitted by an LED lamp, it is also particularly clear outdoors during the day! This also brings a disadvantage. Due to the large size of LED lamps, the screen pixel density is very low, so it is generally only suitable for giant displays in squares. When we encounter a huge display screen in life, we can get closer and find many small LED lights on the screen.

OLED (Organic Light Emiting Diode) uses organic light-emitting diodes as its pixel unit, so the pixel density is much higher than that of ordinary LED dot matrix images, so the picture quality is also more delicate. 0LED displays also do not require backlights, have high contrast, are thin and light, have viewing angles, and have response speeds. The corresponding disadvantage is that their costs are rising.

Today’s science popularization ends here. I believe that everyone’s understanding of screens has reached a higher level. See you next time!