There are two ways to control LED brightness. One is to change the current through the LED, LED allows continuous work current around 20 mA, except the red LED has saturated phenomenon, the other LEDs brightness are essentially proportional to the current; Another way is to use human visual inertia, using pulse width modulation method to achieve gray scale, that is, changes the pulse width cyclically (ie occupation), as long as the repeated cycle is short enough (ie, high enough refresh rate), human eyes do not feel light-emitting pixels in jitter. For the pulse width modulation is more suitable for digital control, now computer are widely used to control the LED display, almost all of the LED screen are based on pulse width modulation to control the gray level.
LED control system usually consists of the main control box, scan board and display device. Main control box receive pixel brightness from the computer monitor, and then re-assigned to a number of scan boards, each board responsible for controlling the number of scanning rows (columns) on LED screen, and signal in each row (column) on the LED display is transmitted using a serial manner.
There are two way to transmit display signals: one is the centralized control pixel grayscale by scanning card, the card decompose pixel brightness value from the control box (ie, pulse width modulation), Then transmit the signal to the relative LEDs in pulses (light on 1, light off 0) by using serial connection. This way used less devices, but the serial connection transmit big volume of data, because in a repeating cycle of light, each pixel in the 16 gray scale requires 16 pulses and in 256 gray levels requires 256 pulses, because of restrictions of device frequency, the LED screen usually use 16 gray scales.
Another method is the transmission of content from scanning board is not on & off signal for each LED, but an 8-bit binary brightness value. Each LED has its own pulse width modulator to control the light up time. Thus, in a repeating cycle of light, each pixel in the 16 gray scales only requires 4 pulses, 256 gray scales only requires 8 pulses, greatly reducing the frequency of serial transmission. LED can achieve 256 gray scales easily by use this decentralized control methods.
