Brightness control method of LED large screen

There are two ways to control the brightness of LEDs. One is to change the current flowing through the LED. Generally, the continuous working current allowed by LED tubes is about 20 mA. Except for the saturation phenomenon of red LEDs, the brightness of other LEDs is basically proportional to the current flowing through them. The other method is to use the visual inertia of the human eye and use the pulse width modulation method to achieve grayscale control, that is, to periodically change the width of the light pulse (i.e., the duty cycle). As long as the repeated lighting cycle is short enough (i.e., the refresh frequency is high enough), the human eye cannot feel the jitter of the luminous pixels. Since pulse width modulation is more suitable for digital control, today when microcomputers are widely used to provide LED display content, almost all LED screens use pulse width modulation to control the grayscale level.

The control system of LED usually consists of three parts: main control box, scanning board and display control device. The main control box obtains the brightness data of various colors of pixels on a screen from the computer's display card, and then redistributes it to several scanning boards. Each scanning board is responsible for controlling several rows (columns) on the LED screen, and the display control signal of the LED on each row (column) is transmitted in serial mode. There are currently two ways to transmit display control signals in serial mode: one is to centrally control the grayscale 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 opening signal of each row of LED in the form of pulses (1 for lighting and 0 for non-lighting) to the corresponding LED in serial mode by row to control whether it is lit. This method uses fewer devices. , but the amount of data transmitted in serial is large, because in a repeated lighting cycle, each pixel requires 16 pulses at 16 gray levels and 256 pulses at 256 gray levels. Due to the limitation of the device operating frequency, the LED screen can generally only achieve 16 gray levels.

Another method is that the content of the scanning board serial transmission is not the switch signal of each LED but an 8-bit binary brightness value. Each LED 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 gray levels and 8 pulses at 256 gray levels, which greatly reduces the serial transmission frequency. This method of decentralized control of LED gray levels can easily achieve 256-level gray control.