How to reasonably control the brightness of LED display screens. We know that if the brightness of an LED display screen is too high, it will cause damage to people's eyes. Generally, the brightness suitable for human vision is between 120cd/m2 and 150cd/m2. However, as the technology of LED display screens continues to mature, the requirements for large LED screens are getting higher and higher, from monochrome to full color.
The following introduces two control methods for two large LED screens:
Method 1:
Change the current flowing through the LED. The ordinary LED tube allows a continuous task current of about 20 mA. Except for the saturation of the red LED, the brightness of the other LEDs is basically proportional to the current flowing through; however, although this adjustment method is simple, as the requirements for large LED screens gradually increase, it is becoming less and less suitable for fast and accurate modulation. The following accepts a commonly used modulation method.
Method 2:
Use pulse width modulation (PWM), use the frequency of change that the human eye can feel, and use the pulse width modulation method to achieve grayscale control, that is, periodically change the width of the light pulse (i.e., duty cycle). As long as the period of repeated lighting is short enough (i.e., the refresh frequency is high enough), the human eye cannot feel the trembling of the light-emitting pixel.
Since pulse width modulation is more suitable for digital control, it has been widely used. The common method is to use a microcomputer to provide LED display mode. At present, almost all LED screens use pulse width modulation to control the grayscale level.
The control system of LED generally 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 various colors of a screen pixel from the computer display card, and then reallocates 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 serially: one is to collectively control the grayscale of each pixel on the scanning board, and the scanning board synthesizes the brightness values of each row of pixels from the control box (i.e. pulse width modulation), and then transmits the traditional signal of each row of LEDs in a pulsed manner (1 for lighting and 0 for non-lighting) to the corresponding LEDs in a row in a serial manner to control whether they can be 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 grayscale levels and 256 pulses at 256 grayscale levels. Due to the frequency limit of device tasks, the LED screen can generally only achieve 16 grayscale levels.
Another method is that the scanning board serial transmission method 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 requires only 4 pulses at 16 grayscale levels and only 8 pulses at 256 grayscale levels, which greatly increases the serial transmission frequency. This method of centralized control of LED grayscale can easily achieve 256-level grayscale control.