LED display knowledge lecture

LED display composition and working principle
The LED display is composed of LED dot matrix, which displays text, picture, animation, video and content through the LED light. It is divided into three categories: monochrome, two-color, and full-color.

According to the LED display function, it can be divided into graphic LED screen and video LED screen. The graphic LED display can display text content, graphic pictures and other information content, which can be displayed offline. The video LED screen can be connected with the computer to play various information in real time, synchronously and clearly, and the pictures and images are beautiful. It can also display 3D animations, videos, TV, VCD shows, and live live events.

1, the composition of the LED display

A complete LED display system is mainly composed of display system, control system, editing system and other peripheral systems. According to whether it is offline (computer), it is divided into asynchronous screen and synchronous screen. The display content of the asynchronous screen is relatively simple, there is an SD card or a hard disk, and the looping is played; the synchronous screen is synchronized with the computer, that is, the computer screen displays the content, and can be displayed in the LED display at the same time, which is convenient for switching the playing content.

1.1 Asynchronous screen

Generally, it consists of a display unit board (module), a strip screen card, a switching power supply, and a HUB board (optional). Connect to the computer through the serial cable to change the display text, then you can disconnect the computer.

1.2 Synchronization screen

Synchronous screen system is more complicated, the system can be large or small, generally consists of computer, DVI graphics card, data transmission card, synchronous data receiving card, HUB board, network cable, LED display and so on. The system always needs to work on the online computer to display the image text on the computer on the big LED screen.

1.3 Detailed description of each component

(1) screen body

Display body: controllable display panel composed of LED light-emitting devices (display unit and grayscale system)

Structural skeleton: screen support part, including display unit fixing frame, outer frame, etc.

Power supply unit: distribution box, 220V switching power supply

(2) Control system

The system is composed of a computer, a multimedia card or a DVI graphics card, a control card, etc., and is mainly used for editing, processing, and distributing display data, and controlling display of the display screen.

Control host: PC

DVI graphics card: display card with DVI output

Control card: data sending card, data receiving card, HUB adapter board

(3) Editing system

The function of the editing system is to complete the arrangement, arrangement and design of the display content of the display. After the editing, the display content must be sent to the control system by the editing system for effective playback.

Edit computer: PC

Editing software: LED display editing software, multimedia production software, etc.

(4) Peripheral system

The functional peripheral part of the LED display peripheral expansion, mainly for the video peripheral part and the audio peripheral part;

Video peripherals: set-top boxes, DVDs, cameras, etc.

Audio peripherals: audio, amplifier, etc.;

Data cable: cable, cable, cable, etc.;

2, the main working principle of the display

LED display system is mainly composed of display system, control system, editing system and other peripheral systems, but in its work, its leading role is control and editing system;

First of all, through the upper computer editing software of the editing system on the PC, combined with the pixel density of the actual LED display and the control mode of the driving IC, the large screen is divided into several different areas (actually, different display module boxes) body);

Then, the transmission card in the control system acquires the brightness data of each pixel of the computer from the graphics card, and divides the data into data required for each receiving card, and packages the data through the network cable to send each piece to different a receiving card for the display area;

Once again, after receiving the data, the receiving card converts the brightness data of each pixel into the display data of the column (row) required by the corresponding driving IC;

Finally, by transferring the HUB card and the signal line, the data is transmitted to each driver IC, and the corresponding control chip is coordinated to illuminate the entire LED display.

1, pixel, pixel diameter, dot spacing

1.1 pixels

The minimum illumination unit of the LED display has the same meaning as the pixel described in the ordinary computer display; each of the LED display panels that can be individually controlled is called a pixel.

1.2 pixel diameter

The pixel diameter ∮ refers to the diameter of each LED illuminating pixel, in millimeters.

1.3 pixel pitch

The center distance between two pixels of the LED display is called the pixel pitch, also called the dot pitch. The denser the dot pitch, the higher the pixel density per unit area, the higher the resolution, and the higher the cost. The smaller the pixel diameter, the smaller the dot pitch.

2, resolution

2.1 module resolution

The number of horizontal pixel points of the LED module is multiplied by the number of vertical pixels.

2.2 screen resolution

The number of horizontal pixels in the LED display is multiplied by the number of vertical pixels.

2.3 LED display module

The smallest unit consisting of several display pixels, which is structurally independent and can form an LED display screen, typically has 32*16, 32*32, 64*32 modules.

The higher the resolution of the screen, the more content that can be displayed, the finer the picture, but the higher the resolution, the more expensive the cost.

32*16 unit module

32*32 unit module

64*32 unit module

3, brightness

The intensity of illumination per unit area in a given direction. The unit of brightness is cd/m2. The brightness is proportional to the number of LEDs per unit area and the brightness of the LED itself.

The brightness of an LED is proportional to its drive current, but its lifetime is inversely proportional to the square of its current, so the drive current cannot be excessively increased in pursuit of brightness.

At the same point density, the brightness of the LED display depends on the material, package form and size of the Led Lamp used. The larger the LED chip, the higher the brightness; conversely, the lower the brightness.

4, gray

The brightness level of the LED display that can be distinguished from the darkest to the brightest in the same level of brightness. For display screens for displaying video images, each primary color should have 256 levels (8 bits) of grayscale processing capability.

Grayscale depends on the number of bits processed by the video source and control system. At present, the domestic LED display mainly adopts an 8-bit processing system, that is, 256 (28)-level gray scale. A simple understanding is that there are 256 brightness variations from black to white. RGB × 256 × 256 = 16,777,216 colors can be formed by using RGB three primary colors. That is, the so-called 16 mega color.

Image comparison of different grayscale data

5, contrast

The ratio of the maximum brightness of the LED display to the background brightness under a certain ambient illumination.

Contrast = brightness when emitting light (lighting brightness) / brightness when not emitting light (reflecting brightness)

In order to be able to display text and images of uniform brightness, the screen should have sufficient contrast without being affected by ambient light. For LED displays, the contrast should be 4096:1 or better.

6, white balance

LED display white can be proportionally mixed by RGB three primary colors, R:G:B ratio is 3:6:1

Color coordinates---CIE chromaticity diagram

7, pixel out of control rate

It refers to the proportion of the smallest imaging unit (pixel) of the display that is not working properly (out of control).

Pixel out of control has two modes

a) blind spot, which is a defect, it does not light when it needs to be bright, it is called a defect;

b) Highlights, when it needs to be off, it is always on, it is called constant bright point.

Generally, the pixels are composed of 2R1G1B (2 red lights, 1 green light and 1 blue light), 1R1G1B, 2R1G, etc., and the loss of control is generally not the same as the red, green and blue lights in the same pixel. But as long as one of the lights is out of control, we think that this pixel is out of control.

According to the SJ/T11141-2003 industry standard, the pixel out-of-control rate of the indoor screen should be no more than three-tenths of a percent, and the pixel out-of-control rate of the outdoor screen should be no more than two thousandths and is discretely distributed.

8, the frame rate change rate

The number of times the display screen information is updated per unit time; generally 25Hz, 30Hz, 50Hz, 60Hz, etc. The higher the frame change frequency, the better the continuity of the changed image.

9, refresh frequency

The LED display shows the number of times the data is repeatedly displayed every second. Usually 60Hz, 120Hz, 240Hz, etc., the higher the refresh rate, the more stable the image display.

10, static screen and scan screen

10.1 static screen

When displaying text, images, and video, the light points on the LED display are illuminated at the same time;

10.2 Scan screen

Using the visual persistence characteristics of the human eye, each row of the LED display is illuminated in a short period of time.

At this stage, the scanning screen is generally 1/4 scan, 1/8 scan, 1/16 scan, 1/32 scan; also set to other scans to adapt to different resolutions;

Scan screens and static screens also have different requirements for display brightness. In the case of the same brightness of the same light tube, the static screen is higher than the brightness of the scan screen, so the static screen is often used outdoors, and the scan screen is often used indoors.

Scan screen captures pictures at high speed shutter

11, perspective, perspective, best perspective

11.1 perspective

The brightness of the viewing direction drops to 1/2 of the brightness of the normal of the LED display. The angle between the two observation directions of the same plane and the normal direction is divided into horizontal and vertical angles of view.

11.2 Angle of view

Just can see the direction of the image content on the display, and the angle formed by the normal of the display.

11.3 Best perspective

Can just see the content on the display, and not flat, the sharpest direction of the image content and the angle formed by the normal.

12, the best line of sight, minimum line of sight

12.1 Best line of sight

Can just see the content on the display completely, and not flat, the clearest position of the image content relative to the vertical distance of the screen.

12.2 Minimum line of sight

For two spots with a certain shape, brightness, and distance, the minimum vertical distance from the position of the two points to the two points cannot be distinguished.

The factors that affect the minimum line of sight and the minimum angle of view are: the shape, brightness, and distance of the spot.

Best line of sight = point spacing / (0.3-0.8). Unit: m

For example: P4 (φ3) optimal viewing distance is 5--13.3 meters;

If the distance is too close, the pixels of the display screen can be distinguished, and the graininess is relatively strong. If the station is far away, the human eye can not distinguish the features of the detail.