LED large screen brightness control method
There are two ways to control LED brightness. One is to change the current flowing through the LED. Generally, the continuous working current of the LED tube is about 20 mA. Except for the saturation phenomenon of the red LED, the brightness of other LEDs is basically proportional to the current flowing through. The other method is to use The visual inertia of the human eye uses the pulse width modulation method to achieve grayscale control, that is, the light pulse width (ie duty cycle) is periodically changed, as long as the period of repeated lighting is short enough (ie the refresh frequency is high enough), the human The eye cannot perceive the shaking of the light-emitting pixels. Because pulse width modulation is more suitable for digital control, almost all LED screens use pulse width modulation to control gray levels today when microcomputers are generally used to provide LED display content.
The LED control system usually consists of three parts: the main control box, the scanning board and the display control device. The main control box obtains the brightness data of each color 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 rows (columns) on the LED screen, and each row (column) The display control signal of the upper LED is transmitted in a serial manner. At present, there are two ways of serially transmitting display control signals: one is to centrally control the grayscale of each pixel on the scanning board, and the scanning board decomposes the brightness value of each row of pixels from the control box (ie, pulse width modulation), Then, the turn-on signal of each row of LEDs is transmitted to the corresponding LEDs in the form of pulses (1 when light is on, 0 when not lighted) to the corresponding LEDs in a serial manner to control whether they are lighted or not. This method uses fewer devices, but the amount of data transmitted serially is larger, because in a cycle of repeated lighting, each pixel needs 16 pulses under 16-level grayscale and 256-level grayscale. 256 pulses, due to the limitation of the operating frequency of the device, generally only the LED screen can achieve 16-level grayscale.
Another method is that the content of the serial transmission of the scanning board is not the switching signal of each LED but an 8-bit binary brightness value. Each LED has its own PWM to control the lighting time. In this way, in a cycle of repeated lighting, each pixel only needs 4 pulses under 16-level grayscale, and only 8 pulses under 256-level grayscale, which greatly reduces the serial transmission frequency. 256-level grayscale control can be easily realized by this method of decentralized control of LED grayscale.