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How to improve the gray level of LED display

LED grayscale can also be called LED brightness. Gray scale, also known as half tone, is mainly used to transfer pictures. There are three levels: 16, 32, and 64. It uses matrix processing to process the file pixels into 16, 32, and 64 levels to make the transferred pictures clearer. Whether it is monochrome, bicolor or tricolor screen, to display images or animations, it is necessary to adjust the light intensity of each LED that constitutes the pixel. The fineness of the adjustment is what we usually call the gray level.

There are two methods to control the LED gray scale: one is to change the current flowing through it, and the other is pulse width modulation. 1. Change the current flowing through the LED. Generally, LED tubes allow a continuous working current of about 20 mA. Except for the saturation of red LEDs, the gray scale 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 to realize the gray control by using the pulse width modulation method, that is, periodically change the light pulse width (i.e. duty cycle). As long as the cycle of repeated lighting is short enough (i.e. the refresh rate is high enough), the human eye cannot feel the light emitting pixels shaking. Because PWM is more suitable for digital control, almost all LED screens use PWM to control the gray level today when microcomputers are widely used to provide LED display content. The LED control system is usually composed of the main control box, scanning board and display and 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 the display and control signals of LEDs on each row (column) are transmitted in a serial manner.

At present, there are two methods of serial transmission of display control signals:

1. One is to centrally control the gray level of each pixel point on the scanning board. The scanning board decomposes the gray level 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 to the corresponding LED in the form of pulse (1 if it is lit, 0 if it is not lit) in line serial mode to control whether it is lit. This method uses fewer devices, 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. Due to the device's operating frequency limitation, LED screens can only achieve 16 levels of gray.

2. One is pulse width modulation. The scanning board serial transmission content is not the switch signal of each LED, but an 8-bit binary gray value. Each LED has its own pulse width modulator to control the lighting time. In this way, in a cycle of repeated lighting, each pixel only needs 4 pulses at 16 levels of gray and 8 pulses at 256 levels of gray, greatly reducing the serial transmission frequency. With this method of decentralized control of LED grayscale, 256 level grayscale control can be easily realized.

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