In our last post, we talked about the three data channels used to send HDMI video, and how the resolution, framerate, and color bit-depth all contribute to the overall bandwidth of the signal. Because there are so many parts of an HDMI signal, it's often possible to compromise on one aspect to improve the overall image that reaches your TV.
You've probably heard of HDR, the latest big addition to HDMI. HDR uses Deep Color, along with a few other enhancements, to dramatically improve picture quality. HDR is often available on 4K TVs, and can be used on 4K video that runs at 60Hz.
After reading our previous article, you may be wondering how a 4K@60Hz video can include HDR if HDR requires Deep Color. The answer involves a type of compression called "chroma subsampling."
4K@60Hz video is often sent in the YPbPr format because RGB doesn't allow chroma subsampling. One of the benefits of YPbPr video is that the Brightness of each pixel is stored in its own sample. The human eye is much more sensitive to brightness than it is to color. Most of the perceived resolution in an image actually comes from the differences in brightness between adjacent pixels.
Because color is less important to the way we see video, it's possible to reduce the amount of data in a video stream by leaving out some of the color samples. That's what chroma subsampling is: a video stream with a brightness sample for every pixel, but color samples for less than every pixel.
When you're choosing a video format to send to your TV, resolution and framerate are fairly straightforward options. Chroma subsampling is much less straightforward. The options commonly available are 4:4:4, 4:2:2, and 4:2:0.
These groups of numbers actually describe a rectangle of pixels on your TV, and how each pixel will be assigned a color sample.
Notice that each format starts with a 4. That means the rectangle of pixels on your screen described by the format is four pixels wide. In theory, you can perform chroma subsampling on a wider rectangle. In practice, the rectangle is always four pixels wide and two pixels tall. The height is not specified, confusingly - only the width.
The second number in each group tells you how many color samples will be provided for the four pixels in the top row. In 4:4:4 video, for example, the second 4 means that all four of the pixels in the top row get their own color sample. In 4:2:2, the 2 means that only two samples are provided, and they are divided among the four pixels. The first two pixels both use the first color sample. The last two pixels both use the second color sample.
The last number tells us how many new samples are being provided for second row of pixels. In 4:4:4, all four pixels in the second row have unique color samples. That means there is no subsampling at all, and every pixel on the display is receiving its own three-part YPbPr color sample. In 4:2:2, two more samples are provided. Again, the first sample is used to color the first two pixels, and the second sample is used to color the last two pixels. In 4:2:0, no additional color data is provided for the second row of pixels. Each pixel in the second row is the same color as the pixel directly above it.
As you can see below, in the extreme case of 4:2:0, only two samples are being used to color 8 pixels. This is a 75% reduction in color data and 50% reduction in the size of the entire video stream.
This reduction is what frees up bandwidth for Deep Color in a 4K@60Hz HDR video stream.
Infinite Variety
Choosing the best options for your HDMI signal often depends on what you're using the display for. For example, an artist is likely to care more about color accuracy that framerate. Running a 4K display at 30Hz would allow the artist to use accurate RGB or YPbPr 4:4:4 color. A gamer is likely to care more about framerate that color accuracy, and might choose to run games at a resolution of 1080p with a framerate of 144Hz.
Ultimately, you'll need to determine which specific formats you want to view, then select a switch or cable capable of carrying the bandwidth required by the format. Some formats are standard enough that you can expect them to be supported. It would be difficult to find a switch that doesn't support 1080p@60, for example. Higher bandwidth signals, like 4K@60Hz with HDR, may require more expensive components.
