Although LED displays are becoming more widely used in the performing arts industry, many in the industry still know very little about this ever-changing technology, even if people are on the stage and on the street every day.
Today, many people will ask "Is this high-definition" when I see the LED display. The answer is not as simple as "yes" or "not", far more complicated. Therefore, you need to know the basics of the display screen to distinguish whether it is HD or just HD images.
LED and pixel
The LED display is made up of light-emitting diodes. The display uses red, green and blue LED tubes to make pixels to mix millions of colors, similar to other light mixing phenomena. Some manufacturers claim that their products can mix hundreds of millions or even trillions of colors, and point out that a wider color gamut can improve visual effects to some extent. However, the human eye can only distinguish about 750,000 colors.
Depending on the method of production, LED displays are divided into two broad categories, both of which use groups of LEDs to produce bright, clear individual pixels.
The in-line LED display uses a single LED tube that passes through the board and is soldered to the back of the board. There are many combinations of LEDs for this type of display. For example, three and four LED tubes are used as one pixel. They use single or multiple red, green and blue LED tubes to improve the color mixing and brightness of the display. . Some displays even take 5 LED tubes as one pixel to meet the brightness, color and contrast requirements of a more demanding outdoor environment.
Because the in-line LED display emits very bright light and its board and LEDs can be packaged together to protect it from water and other objects, such displays are often used. Outside. Most outdoor displays have a brightness of 5 000 nt (cd/mz), although the brightness of the updated LEDs may be much brighter than this.
Surface-mount (SMD) LED displays use RGB LED chips with three very small chips soldered to a single node on the board's surface. Because the brightness of the surface-mounted display is not as good as that of the in-line display, the application field is mainly indoors not long ago. A typical surface-mounted indoor display has a brightness of 1 000 nt to 2 000 nt, which is much brighter than a normal LCD TV with a brightness of 400 nt.
In the past few years, the new surface-mount display has been put on the market, its brightness is more than enough for outdoor environments, and some are waterproof, so it can be protected from external objects without having to install an extra lens or protective layer. The immersion.
Recently, some LED manufacturers have also begun to produce four-in-one (RGGB or RRGB) or all-in-one surface-mount displays, while they used only a single red, green and blue LED tube at each node. Pixel.
Pixel pitch
LED displays are usually described by their pixel pitch. It refers to the distance between pixels and pixels on the display, in millimeters (mm). For the in-line LED display, it refers to the distance between the LEDs of the same color; for the surface-mounted LED display, it refers to the distance between the center points of the surface-mount components.
For example, the (P16) LED display with a real pixel pitch of 16~ indicates that the distance between pixels is 16 mm, as shown in Figure 1. Some manufacturers approximate the measurement results. For displays with a pixel pitch of 15.5 mm or even 16.5, the pixel pitch is still 16 mm. Some manufacturers also give virtual pixel spacing or effective pixel spacing, which is obtained by compressing the source image information and sharing the information between the pixels. 
Figure 2 shows a schematic diagram of pixel sharing, which shares image information through LED four-in-one pixels.
Manufacturers often claim that their virtual pixel display can double the resolution and provide a 16/8 mm or 24/12 mm LED display that “effectively†reproduces at lower resolutions. image. In fact, effective resolution can increase display resolution by 60% to 75%, as some manufacturers have pointed out in their technical literature, while some manufacturers still use doubled resolution. Although the performance of some products has improved, people still have different opinions on the real performance of such products.
Resolution and its importance
Resolution is a very important parameter for the display, and the line of sight depends on it. Whether for a real pixel display or a virtual pixel display, the number of pixels must be such that they display recognizable letters or numbers. Most commonly used letters and numbers can be displayed in a small matrix. The seven-segment digital tube is a commonly used display device, as shown in Figure 3.
Many of the displays that use the light bulb as a dot matrix use 3x3, 4x4, and 5x5 matrices to show clearer characters. Simply put: the more points, the clearer the characters. If the number of available pixels is larger, the display of characters and numbers becomes clearer, and even various fonts can be displayed. At present, the number of rows and columns of dot matrix displays commonly used for displaying text is at least 7×5.
There are some formulas for calculating the line of sight corresponding to the character size. To answer these questions accurately requires some algebraic knowledge. The author recommends that the character height be 1/200 of the line of sight, which is also the calculated value.
Many smartphones and laptops have very small screens, so they look great when viewed at their display resolution. The iPhone 4 has a display resolution of 960 x 640, according to the specifications provided by the manufacturer. Everyone thinks that this resolution is quite good. People can read mobile phone information and emails on the way to work (on the train or bus), enjoy family photos, and watch videos. But imagine, if there is a LED display with a pixel pitch of 10 mm, the width and height are 9.6 m and 6.4 m, respectively, in the stadium or shopping center. Its size is large, but as we all know, size does not determine everything, but more importantly resolution.
The 960 x 640 resolution display provides a number of lines for alphanumeric characters, but more importantly, it provides a number of pixels to clearly view ads and instant replay information just like viewing a phone. In this respect, the images that viewers see and how they are produced and how they are converted become critical.
Line of sight
The author now introduces the relationship between pixel pitch and line of sight (the distance between the viewer and the display). The position of the viewer can make the adjacent pixels achieve the color fart well in the human visual organ. Then, the distance between the LED display and the viewer is the ideal viewing distance, and the human eye can no longer distinguish the independence. The pixels are the same as the TV screen in the home.
Most manufacturers rely on experience to evaluate the nearest line of sight by multiplying 1 000 (distance factor) by the pixel spacing to get the desired result. For example, the P10 display is best viewed from a distance of at least 10m (10 mm x 1 000 to 10 000 mm = 10 m). This conclusion is based on: at the line of sight obtained by multiplying the distance factor (near the vicinity), the eyes and brain of the average person "fill" the gap between the pixels to produce a single image (rather than a point) ) visual perception.
In addition, consideration must be given to ensuring that the viewer can see the farthest viewing distance of the display screen content. The P20 display with the same resolution as the iPhone 4 mentioned above has a size of 19.2 mx 12.8 m and an area of ​​245.7 mZ. The farthest viewing distance available area is multiplied by 20 (in this case about 4 900 m). Perhaps the content of the picture can be seen at this line of sight, but in fact it cannot be seen clearly.
In order to see the screen content of the display, take 10 or a smaller number as a factor, which means that when the character height is about 1/200 of the line of sight (in this case, about 2 450 m), the viewer can start. Identify characters and images.
These formulas are really easy to use, and the determination of the line of sight of the display screens used on roadsides and buildings, as well as on the stadiums, playgrounds, and stages, is often based on these formulas. Of course, depending on the conditions, these formulas should be corrected accordingly, such as: whether the LED display is in-line or surface-mount, the brightness of the display, whether it is a virtual pixel or a real pixel, between the pixel and the background. Contrast and other factors.
It is worth noting that the displayed image also requires pixel pitch and display resolution. The better the quality of the input image, the better the quality of the displayed image.
Is this display HD?
When buying a TV set, the concept of HD is often mentioned. The salesperson will say that it is "Full HD" and can display high definition images. This is not something that can be seen at a glance. Many "Full HD" products are displayed after the image has been processed and reduced to its actual physical resolution. Resolution conversion is the key to HD.
When a high-definition image is shot or produced at a resolution of 1 920 x 1 080, its quality is generally considered to be optimal. For example, Blu-ray DVD is full HD, and now some movies and TV shows are also full HD, especially for sports events and major events. The more information that is transmitted, the greater the bandwidth required. That's why HD programming and 3D HD programming are still not popular.
The video display called "Full HD" has three resolutions: 720p, 1080p and 1 080 i. The letters and "p" and "i" are related to the image scanning mode of the display (p for progressive scanning and i for interlaced scanning). The number is related to the number of vertical pixels. It can be seen that a display with a fixed resolution of 1 920 x 1 080 is considered high definition. Similarly, a display with a resolution of 1 280 x 720 is also considered high definition. Other "quasi" HD displays can have a resolution of 1 366 x 768, and even older computer monitors with a resolution of 1 024 x 768 can be considered "quasi" HD.
So everything goes down to whether the real pixels of the display meet the requirements of a high-definition display, or whether the display can convert high-definition images to the actual physical resolution of the display.
The iPhone 4 mentioned above is not HD. Because the image is compressed onto the display, it makes the visuals quite good.
Resolution conversion
Image resolution conversion is the key. Most digital video systems can adapt to the size of the screen by increasing the pixels of the low resolution image or reducing the pixels of the high resolution image. Readers may have heard of pixel mapping—the source image is matched or mapped to the output display; this often results in the image being in the clearest state because there is no information sharing or interpolation between the image and the pixel.
If the input source image resolution is higher than the actual physical resolution of the display, then it will lose some image detail, although the final viewing is still good. If the source image resolution is lower than the actual physical resolution of the display, the image quality will not be any better due to the high resolution of the display.
Therefore, whether the gleaming LED display in the stadium or shopping center is high-definition, the answer depends on the pixel.
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