On the display function of LED screen

Because the color and efficiency of LED display are related to the material and technology of LED display, red, green and blue are

widely used at present. Due to the low working voltage (only 1.5-3V) of LED display screen, it can actively emit light and has

certain brightness. The brightness can be adjusted by voltage (or current), and it has shock resistance, vibration resistance and

long service life (100000 hours). Therefore, there is no other display mode to match the LED display mode in large display


The LED display screen made by putting red and green LED displays together as a pixel is called two-color screen or color

screen; the LED display screen with red, green and blue LED display tubes as one pixel is called three color screen or full-color

screen. The pixel size of indoor LED display screen is generally 2-10 mm. Several LED display tube cores which can produce

different primary colors are often packaged into one. The pixel size of outdoor LED display screen is mostly 12-26 mm. Each pixel

is composed of several monochromatic LED displays. The common finished product is called pixel tube. The two-color pixel tube

is generally composed of 3 red and 2 green, and three-color pixels The tube is composed of 2 red, 1 green and 1 blue.

No matter using LED display screen to make single color, two-color or three-color screen, the brightness of each LED display

screen that needs to form pixels to display images must be adjustable, and the fine degree of adjustment is the gray scale of

LED display screen. The higher the gray level is, the more delicate and colorful the displayed image is, and the more complex

the corresponding display control system is. Generally, the color transition of 256 gray scale images is very soft, while that of

16 gray scale images is very obvious. Therefore, color LED display screen is required to be 256 gray scale or above.

There are two ways to control the brightness of LED display. One is to change the current flowing through the LED display

screen. Generally, the continuous working current of the LED display tube is about 20 mA. Except for the saturation phenomenon

of the red LED display screen, the brightness of other LED display screens is basically proportional to the current flowing through;

the other method is to use the visual inertia of human eyes to realize gray-scale control by pulse width modulation, that is, to

change the light pulse width periodically As long as the repetition period is short enough (that is, the refresh rate is high enough),

the human eye can not feel the flicker of the luminous pixels. Because pulse width modulation (PWM) is more suitable for digital

control, nowadays, computer is widely used to provide LED display content. Almost all LED display screens use pulse width

modulation to control gray scale.

The control system of LED display screen is usually composed of main control box, scanning board and display and control device

. The main control box obtains the luminance data of a screen pixel from the display card of the computer, and then redistributes

it to several scanning boards. Each scanning board is responsible for controlling several lines (columns) on the LED display screen

, and the display and control signals of the LED display screen on each row (column) are transmitted in series. At present, there

are two ways of serial transmission of display control signal: one is to centrally control the gray scale of each pixel on the scanning

board. The scanning board decomposes the brightness value of each row of pixels from the control box (i.e. pulse width

modulation), and then transmits the turn-on signal of each line of LED display screen to the corresponding LED display screen in

a pulse type (light = 1, not bright = 0) in a row Control whether it is on or not. This method uses less devices, but the amount of

data transmitted in serial is large, because in a repeated lighting cycle, each pixel needs 16 pulses at 16 gray level, and 256

pulses at 256 gray level. Due to the limitation of device operating frequency, the LED display screen can only achieve 16 order

gray scale.

Another method is that the scanning board transmits a brightness value of 8-bit binary instead of the switch signal of each LED

display screen. Each LED display has its own pulse width modulator to control the lighting time. In this way, in a repeated lighting

cycle, each pixel only needs 4 pulses at 16 order gray level and 8 pulses at 256 order gray level, which greatly reduces the serial

transmission frequency. The 256 order gray scale control of LED display can be easily realized by this method. [LED display


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