So you want to upgrade to a 4K system? There are several options when it comes to the type of signal your 4K system can send to display 4K images. To understand which option your AV application might require, it helps to understand what can make up a 4K image signal.

The Definition of 4K

Let’s start with the basics: What does “4K” really mean? There are different answers to this question, depending on the industry. The broadcast/video industry defines 4K differently than the motion picture industry. Both industries refer to their versions as 4K images, but they are slightly different.

The motion picture/film industry has been shooting movies in digital high definition for some time now, moving away from using actual film. If you’ve been to a movie theater recently, chances are good that you watched a 2K Digital Cinema Initiatives (DCI) native resolution image, which has 2048 pixels horizontally and 1080 pixels vertically on the screen. The 4K version of this is 4096x2160 (four times the size of the 2K). On the other hand, the broadcast/video industry’s standard for 4K is four times a 1080p (1920x1080), which is 3840x2160 pixels. The majority of U.S. consumers are working with the broadcast/video industry image, where a 4K image is more precisely referred to as ultra-high definition (UHD).

4K Frame Rates

Remember when you flipped through the pages of a book with slightly different pictures on each page as a child? As you turned the pages, the images looked like they were moving. But there is no real movement there; your mind creates a sense of movement by looking at a series of sequential still pictures. Movies and videos work according to the same principle: They show a series of still pictures, and your mind creates the movement called the persistence of vision. Isn’t it interesting that this trait is one only humans possess? 

The number of images displayed in one second is referred to as the “frame rate.” Traditionally, frame rates were 24 per second for movies to balance between film costs and creating what seemed to be a “moving picture.” Over the years, there has been an increased demand to display more frames per second to better capture action and sporting events. Because of this, you now have the option to double the frame rate to 60 frames per second. A frame rate of 120 is even possible. But an important caveat here: Sending twice as many pictures takes twice as much bandwidth.  

Color Depth

Now let’s examine the 4K image itself. Color video works based on a principle you learned in elementary school: “Combining the three primary colors you can make any color you want.” This same idea applies to lighting: Using red, green and blue lights, combine different levels to create any color. The more color levels you use, the more realistic the images are.

Traditionally, each color is represented by 8 bits of information, which equates to 256 choices per color (2 to the power of 8 choices). With all three colors making up a picture, you have over 16 million choices (2 to the power of 24 (3x8=24). For even more options, the market now offers 10, 12 and 16 bits of information per color. If you do the math, this gives you well over 100 trillion choices. Choosing more bits is often referred to as “deep color”; needless to say, this increases the bandwidth required of your signal.  

Cabling to Support 4K UHD HDBaseT™ Applications - White PaperLet’s put another spin on 4K image color. In the early 1950s, a scientist discovered that the human eye was much more receptive to brightness than color, so they developed another way to break down a signal. Rather than by color, the signal was broken down by brightness or luminance (Y) and subtracted out color, (called chrominance (C).

Instead of sending three colors, luminance and chrominance red and blue (or YCrCb) are sent. Both color patterns are used for 4K images. By using this technology, you can focus your attention on the brightness of the information, which will result in a more perceivable change in picture quality. Why is this important? A traditional TV has a brightness range of about 150 nits. The industry has developed a new TV with a standard called high dynamic range, or HDR, which offers a brightness range of 1,000 nits. The industry is embracing this change because it offers a more perceivable difference in the images being presented. But, like everything else, sending more brightness information increases bandwidth requirements. 

Chroma Subsampling

Regardless of which options you select, you’re dealing with a lot of information and bandwidth requirements. In fact, video signals are often combined with audio, control and sometimes Ethernet signals, increasing bandwidth requirements even more.  

Bandwidth requirements can be in excess of 18 Gbps. This causes a problem when it exceeds the limits of today’s systems, requiring users to make compromises. You can limit the chrominance information, which is the least perceptible part of the image – as we talked about earlier in the blog.

You do this by having pixels share chrominance information with adjacent pixels. (If two pixels share chrominance, this is called 4:2:2 sampling; if four pixels share chrominance, it’s called 4:2:0 sampling.) This can help you reduce bandwidth requirements and make it compatible with today’s systems.

chroma subsampling chart

Another option is to encode more information into one bit of signal. Because this compression requires encoding and decoding, it can delay the signal. You also run the risk of losing some of the signal.  

The Future of 4K

Remember: This is cutting-edge technology, and it’s anyone guess as to where the improvement will stop. In fact, the industry is already working on 8K. For all we know, maybe someone down the road will develop smell-a-vision (or maybe that will provide the viewers with a little too much information!).

Watch for future blogs that will cover your choices in technology for 4K image signals. In the meantime, to learn more about how Belden can help you manage your 4K applications, visit www.belden.com.

Do you think you understand the complexities involved with 4K image choices?
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