Shedding Light on Pixel Sharing
The facts behind the figures
35mm, 20mm, 16mm – every manufacturer of LED digital color displays loves to pitch their “pitch” when it comes to their products, and the race is on as to who can be the next to offer the smallest number. Strictly speaking, pitch is the distance between the clusters of red, green, and blue LEDs that make up a single pixel in a sign. The tighter the pitch (i.e., the smaller the number), the greater the resolution and image quality in a board. But the “pitch” that some companies offer actually utilizes a technique called pixel sharing to achieve the pitch in a “virtual” manner. Before you buy your LED display product, you should first understand more about pixel sharing and determine if this approach will work for your application.
Figure 1 Physical pixels
Figure 2 “Shared Pixels” (circled in yellow)
Figure 3 Boards that use shared pixel technology also switch between lines of LEDs, generating additional rows of virtual pixels as shown in blue.
Figure 1 diagram is a common 32mm pixel arrangement that may be represented as a tighter 16mm “virtual” pitch, which is achieved through running software that shares the LEDs in different pixel combination. Figure 2 and 3 show how the LEDs become “shared”, creating new virtual pixels. When the sign is powered on and generates images, these rows of LED pixels fire in staggered rows alternately. The human eye then interpolates this visual data into perceived color.
Table 1 compares the pixel density for this “virtual” 16mm board with a real 20mm board. Although the “virtual” 16 mm board has 22% fewer LEDs, it seems to deliver higher resolution. The initial impression that these figures seem to show is that this type of board can effect a 16mm resolution with 22% fewer LEDs. At first glance, this seems like great news! From the manufacturer’s point of view, this new product has 22% fewer LEDs compared to a true 20mm product and 50% fewer LEDs than a true 16mm product. This means that the product will cost about 50% less to manufacture than a true 16mm product, or 22% less than a true 20mm product. From the end user’s point of view, getting a higher resolution at a lower cost is a simply a terrific bargain.
But is this really the case?
Table 1: LEDs per square foot
Weighing Your Choices
First, consider the overall brightness. How can a product with half the LEDs be as bright as one with twice as many? The answer is that it can only be brighter if the LEDs are driven harder or kept on longer. Over-driving LEDs dramatically reduces product life and quickly deteriorates the color and brightness.
Second, if I have a static picture, how can I have a LED that is shared in 4 pixels operate correctly, or at 4 discrete levels? The answer is you can only if you multiplex this pixel, or share the “on time” of a pixel with its neighbors. Multiplexing further reduces the light available for each of the specific pixels. So, once more, to achieve a bright image, you will have to resort to overdriving the LEDs again.
So are there any cases where pixel sharing makes sense? The answer is “yes” of course. If your display is running video , you can get a better picture at a lower cost and a lower weight, but at the cost of a decrease in the overall life of the product. Perhaps you have a video replay board that is only operating for a few hours on a weekend. Maybe brightness isn’t an issue for you. You might want a big sign but are concerned about overall weight. Or you have an extremely tight budget, and think you can live with the drawbacks. Any of these situations might make a strong case for pixel sharing. It’s certainly a decision that seems confusing and contradictory on many levels.
However, for the end user who wants to make the most economical choice without sacrificing performance, there are alternatives. If the people viewing the board are more than 200′ away, you might consider going to a 20mm or 23mm pitch. Since they contain fewer LEDs than a true 16mm, these displays are cheaper per square foot, lighter, and brighter, and actually yield better static image quality than a 32mm product that utilizes pixel sharing.
Making the Decision
Pixel sharing does save on power and weight, but the price you pay is a decrease in content resolution, image crispness, and product longevity. The virtual pixel must share LEDs from its neighbors, flashing at a high frequency so the eye cannot pick up on it. This works effectively for moving video images, but lends an annoying flicker to static images.
You might ask, “How can I protect myself when considering a product with pixel sharing?” Firstly, you should run your own content, not the content the manufacturer might suggest. Run this content in an environment that your sign will be in once installed (in direct sunlight, south facing, for example). If the sign looks good in this installation, then this might be a viable choice for your purposes. Most importantly, ask for a warranty that guarantees at least 5 years of an
image quality that has not deteriorated.
With advances in LED technology in recent years, such as the incorporation of lighter materials and more efficient electronics, products offering “real” 16mm and smaller pitches are beginning to come on the market at lower prices than ever before. As end users become more sophisticated as to the ins and outs of the new generation of LED displays, the “virtual pixel” may well be on its way out. However, the well-informed consumer owes himself -and his business – a basic understanding of the many meanings behind the concept of “pitch”.
The Pitfalls of Pixel Sharing?
Reduced product life
Poor static image quality