With the growing maturity of LED video screen skills, the demand for large LED screens is also increasing, from monochrome to full-color. The following describes several control methods for two large LED screens:
One is to change the current flowing through the LED. Generally, the LED tube allows a continuous mission current of about 20 mA. Except for the red LED, the brightness of other LEDs is basically proportional to the current flowing; However, although this adjustment method is simple, with the gradual increase of LED large screen requests, it is not suitable for fast and accurate modulation. The following is a commonly used modulation method;
The other method is pulse width modulation (PWM), which uses the changing frequency that can be felt by the human eye to complete the gray scale control, that is, periodically change the light pulse width (i.e. duty cycle). As long as the cycle of repeated lighting is short (that is, the rewriting frequency is high), the human eye cannot feel the light emitting pixels shivering. Because PWM is more suitable for digital control, it has been widely used. The common method is to use a microcomputer to provide LED display. Now almost all LED screens use pulse width modulation to control the gray level.
LED control system is generally composed of main control box, scanning board and display control device. The main control box obtains the brightness data of each color of a screen pixel from the computer display card, and then allocates several scanning boards from the computer display card. Each scanning board acts as the control of several rows (columns) on the LED screen, and the display and control signals of LEDs on each row (column) are transmitted in a serial way. There are two ways to transmit display and control signals in series: one is to collect and control the gray scale of each pixel point on the scanning board, and the scanning board will stop the composition of the brightness value of each row of pixels from the control box (namely, pulse width modulation), and then transmit the conservative signal of each row of LED to the corresponding LED in a line by line serial way (the point on is 1, and the point off is 0) to control whether it can be lit. This method uses less equipment, but the amount of data transmitted serially is large. Because in a cycle of repeated lighting, each pixel needs 16 pulses at 16 levels of gray, and 256 pulses at 256 levels of gray. Because of equipment mission frequency constraints, LED screens can only achieve 16 levels of gray.