Basic knowledge of LED display screen（5）

7. Technical indicators (1) Pixel loss rate The pixel out-of-control rate refers to the proportion of the smallest imaging unit (pixel) of the display that is not working properly (out of control). There are two modes of pixel out of control: one is the blind spot, that is, the blind spot, when it needs to be bright, it does not light, it is called the defect; the other is the constant bright spot, when it needs to be bright, it is always on, It is called the constant bright spot. Generally, the composition of the pixel is 2R1G1B (2 red lights, 1 green light and 1 blue light, the same reason), 1R1G1B, 2R1G, 3R6G, etc., and the loss of control is generally not the red in the same pixel. The green and blue lights are all out of control at the same time, but as long as one of the lights is out of control, we think that this pixel is out of control. For the sake of simplicity, we perform the statistics and calculation of the out-of-control pixels according to the respective primary colors of the LED display (ie, red, green, and blue), and take the maximum value as the pixel out-of-control rate of the display screen.

The ratio of the number of out-of-control pixels to the total number of pixels in the full screen is called "the whole screen pixel out of control rate". In addition, in order to avoid the out-of-control pixels concentrated in a certain area, we propose a "regional pixel runaway rate", that is, the ratio of the number of out-of-control pixels to the total number of regional pixels (ie, 10000) in a 100×100 pixel region. This indicator quantifies the requirement of “discontinuously distributed pixels are discretely distributed” in the “General Specification for LED Display” SJ/T11141-2003, which is convenient and intuitive.

The domestic LED display will be aging (bake machine) before leaving the factory. The LED lights of the out-of-control pixel will be repaired and replaced. The "full-screen pixel runaway rate" is controlled within 1/104, and the "region pixel out of control rate" is controlled. There is no problem within 3/104. Even some enterprise manufacturers' standards require that out-of-control pixels are not allowed before leaving the factory, but this will inevitably increase the manufacturer's manufacturing and maintenance costs and extend the shipping time. In different applications, the actual requirements of the pixel out-of-control rate can be quite different. Generally speaking, the LED display is used for video playback, and the index requirement control within 1/104 is acceptable and achievable; If used for simple character information distribution, it is reasonable for the indicator to require control within 12/104.

(2) gray level

Grayscale, also known as gradation or grayscale, refers to the degree of brightness of the brightness. For digital display technology, grayscale is the determining factor in the number of colors displayed. In general, the higher the gray level, the richer the displayed color, the finer the picture, and the more detailed the details.

The gray level depends mainly on the number of A/D conversion bits of the system. Of course, the system's video processing chip, memory and transmission system must provide support for the corresponding number of bits. At present, the domestic LED display mainly adopts an 8-bit processing system, that is, 256 (28)-level gray scale. A simple understanding is that there are 256 brightness changes from black to white. RGB × 256 × 256 = 16,777,216 colors can be formed by using RGB three primary colors. That is, the so-called 16 mega color. The international brand display mainly adopts a 10-bit processing system, that is, 1024-level gray scale, and RGB three primary colors can constitute 1.07 billion colors.

Although grayscale is the determining factor determining the number of colors, it does not mean that the larger the limit, the better. Because the resolution of the human eye is limited first, the increase in the number of bits processed by the system will involve changes in the system video processing, storage, transmission, scanning, etc., and the cost will increase sharply, and the cost performance will decrease. Generally, 8-bit systems can be used for civilian or commercial-grade products, and 10-bit systems can be used for broadcast-grade products.

(3) Brightness discrimination level

The brightness discrimination level refers to the brightness level between the blackest and the whitest images that the human eye can distinguish. As mentioned earlier, the gray level of the display is very high, which can reach 256 or even 1024. However, since the human eye has limited sensitivity to brightness, these gray levels cannot be fully recognized. That is to say, many grayscale human eyes of adjacent levels may look the same. And the ability to distinguish the eyes is different for each person. For the display, the level of human eye recognition is naturally as good as possible, because the displayed image is for everyone to see. The more brightness levels that the human eye can distinguish, the greater the color space of the display and the greater the potential for displaying rich colors. The brightness discrimination level can be tested with special software. Generally, the display can reach 20 or higher, even if it is a good level.

(4) Driving method

Common driving methods generally have constant voltage driving and constant current driving. Constant voltage driving is to drive and light the screen with a constant voltage input. This method is rare. The constant current drive is to drive and illuminate the large screen with a constant current. Common is the 1/8, 1/4, 1/16 scanning method. These so-called scanning methods mean that several similar lamps of the same color are driven and controlled by one driving chip. Of course, the more the driver chip has more brightness and the higher the refresh rate, the higher the cost. Usually 1/8 is a good choice.

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Basic knowledge of LED display screen (1)

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Basic knowledge of LED display screen（4）