Since the color and luminous efficiency of LEDs are related to the materials and processes for making LEDs, red, green and blue are widely used at present. Due to the low operating voltage of the LED (only 1.5-3V), it can actively emit light and have a certain brightness. The brightness can be adjusted by voltage (or current), and it is resistant to shock, vibration and long life (100,000 hours), so it is large. Among the display devices, there is currently no other display mode that rivals the LED display mode.
Putting the red and green LEDs together as a pixel, the display screen is called a two-color LED display or a color LED display; the red, green and blue LED tubes are put together as a pixel display screen called a three-color screen. Or full color screen. The pixel size of the indoor LED screen is generally 2-10 mm. It is often used to package several LED dies that can produce different primary colors. The outdoor LED screen has a pixel size of 12-26 mm, and each pixel consists of several A variety of monochromatic LED components, the common finished product is called a pixel tube, the two-color pixel tube is generally composed of 3 red 2 green, and the three-color pixel tube is composed of 2 red 1 green 1 blue.
Regardless of whether a single-color, two-color or three-color screen is made with LEDs, the brightness of each LED that needs to constitute a pixel must be adjustable. The fineness of the adjustment is the gray level of the display screen. The higher the gray level is, the more delicate the displayed image and the richer the color, and the more complicated the corresponding display control system. Generally, the image of 256-step gray scale has a very soft color transition, while the color image of the 16th-order gray scale has a clear color transition boundary. Therefore, color LED screens are currently required to be more than 256 grayscale.
There are two ways to control the brightness of the LED. One is to change the current flowing through the LED. Generally, the LED tube allows the continuous working current to be around 20 mA. In addition to the saturation of the red LED, the brightness of other LEDs is basically proportional to the current flowing through; another method is to utilize The visual inertia of the human eye, using the pulse width modulation method to achieve gray scale control, that is, periodically changing the optical pulse width (ie, duty cycle), as long as the period of repeated lighting is short enough (ie, the refresh frequency is high enough), The eye does not feel that the illuminating pixels are shaking. Since pulse width modulation is more suitable for digital control, todayadays, with the widespread use of computers to provide LED display content, almost all LED screens use pulse width modulation to control grayscale and other orders.
The LED control system usually consists of three main parts: the main control box, the scanning board and the display control unit. The main control box obtains the brightness data of each screen pixel from the display card of the computer, and then re-allocates it to several scanning boards, each of which is responsible for controlling several rows (columns) on the LED screen, and each row (column) The LED display signal is transmitted in series. At present, there are two ways of transmitting and displaying control signals in series: one is to control the gray scale of each pixel point on the scanning board, and the scanning board decomposes the brightness values ​​of the pixels in each row from the control box (ie, pulse width modulation), and then The open communication number of each row of LEDs is transmitted in a pulse form (lighted to 1, not lit to 0) to the corresponding LEDs in series, to control whether they are lit. This method uses fewer devices, but the amount of data transmitted in series is larger because each pixel requires 16 pulses in the 16th order gray scale and 256 steps in the 256th order gray scale in a repeated lighting period. Pulses, due to the device operating frequency limit, generally only enable the LED screen to achieve 16-step gray scale.
Another method is that the content of the scan board serial transmission is not the switching signal of each LED but the luminance value of an 8-bit binary. Each LED has its own pulse width modulator to control the lighting time. Thus, in a period of repeated lighting, each pixel requires only 4 pulses in the 16th order gray scale, and only 8 pulses in the 256th order gray scale, which greatly reduces the serial transmission frequency. With this method of decentralized control of the gray scale of the LED, 256-step gray scale control can be conveniently realized.
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