First, what is the LED?

In some of the PN junction semiconductor materials, into a small number of the carrier and the majority of the carrier when the complex will be redundant in the form of light energy to release in order to directly convert electrical energy into light energy. PN junction reverse voltage increases, the minority-carrier injection is difficult, it is not luminous. This use of injection-EL produced by the principle of diode called light-emitting diodes, commonly known as LED.

LED light and color LED light-emitting efficiency and the production of materials and technology relating to the current widespread use of the red, green and blue. As the low-voltage LED (only 1.5-3V), to take the initiative to have a certain brightness light-emitting and brightness while using voltage (or current) regulation, impact resistance itself, anti-vibration long life (100,000 hours), so in the large-scale Display equipment, there is no other display LED display with the match.

The red and green LED together as a pixel screen is called the production of two-color screen or color screen; the red, green and blue LED tubes of the three together as a pixel screen is called tri-color screen or Full-color screen. The production of indoors LED screen pixel size is generally 2-10 mm, often used to produce several different color of the LED die into one package, outdoor LED screen pixel size to more than 12-26 mm, as each -From several kinds of LED components single, common products that pixel cylinder, two-color pixel by 3 normally red drum 2 Green, three-color pixel 2 red tube with a green component of Lan 1.

Whether used to produce LED monochrome, dual-color or tri-color screen, the images need to want to show that constitute each pixel brightness of the LED must be able to adjust its regulation of the level of fine is the display of gray level. The higher the level of gray, showing images of the more delicate, more color-rich, shows that the control system is also more complex. General 256 gray image, the transition has been very soft colors, and 16 of the gray color image, color transition line is very clear. Therefore, the current color LED screen all requests made of 256 gray.

Second, control LED brightness:

There are two ways to control LED brightness. One is to change the current flowing through LED, LED tube to allow for general work in the current around 20 mA, in addition to the red LED there is the phenomenon of saturation, other LED brightness and basically flow of current proportional; Another method is to use Human visual inertia with PWM control method to achieve gray, which is cyclical changes in pulse width (that is, the duty cycle), as long as the light of the repeated cycle of short enough (that is, the refresh rate high enough), who Eye is not feeling light-emitting pixels in the jitter. As the PWM is more suitable for digital control, it is commonly used in the computer to provide the LED display today, almost all of the LED screen and are based on the PWM to control the levels of gray.

The LED control system is usually dominated by the box, and the board significantly Scan control device composed of three parts. From the control box in the computer's video card to obtain a wide screen pixel brightness data, and then re-allocated to a number of pieces of plate scanning, scan each board is responsible for controlling LED screen on a number of firms (out), and each line (out) LED on the control signal is marked with the serial transmission. At present, there are two serial transmission of control signals in the show: a kind of centralized control board is to scan the pixel grayscale point, the board will be scanned from the control box of the line pixel brightness value decomposition (that is, PWM), And then the opening line of LED signal to the form of pulse (1 for light, not-for 0) by way of serial transmission line to the LED, whether or not to light up its control. In this way the use of fewer devices, but the serial transmission of large amounts of data, because in a repeat cycle of light, each pixel in the 16 required under the gray-Pulse 16, in 256 gray needed Pulse 256, as the devices limit the operating frequency, so that the LED screen in general can only do 16 gray.

Another method is to scan the contents of the board serial transmission of each LED is not a switch but a signal 8-bit binary value of brightness. Each LED has a pulse width modulator own to control the light time. In this way, in a repeat cycle of light, each pixel in the gray-under 16 only 4 pulse, 256 gray just under 8 pulse, greatly reduced the frequency of serial transmission. This decentralized control of the gray LED can easily achieve 256 gray-scale control.