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.
]]>TV screens are made up from millions of individual rectangular cells called "pixels". When you buy a 4K TV, the term "4K" is there to tell you the number of pixels in the screen, which determines its clarity. A 4K TV contains 3,840 columns of pixels (~4,000, or 4K), and each column contains 2,160 individual pixels, for a total of 8,294,400 pixels. Each of those pixels is usually composed of three even smaller cells - subpixels - each of which is lit to a different brightness to produce the overall color for that pixel.
When you use HDMI, video is sent in three channels. One sample is needed from each channel to color one pixel on the screen, but there are two different three-channel formats you can choose between:
Normally, the samples for each channel use 8 bits of data. HDMI 1.3 added a feature named Deep Color. Deep Color just means that each channel is using 10, 12, or 16 bits for each sample, allowing each pixel on your TV to take be colored more precisely. The obvious downside is that 12-bit sample require 50% more bandwidth than an 8-bit sample, and that increase applies to the entire signal.
As an example, a Premium High-Speed HDMI cable, rated for 18Gbps, must be able to pass 4K video at 60 frames per second using 8 bits per channel. Such a signal will use about 17.82Gbps, which is very close to the 18Gbps testing limit of the cable. If the same 4K video is sent at 10 bits per channel, it will require 20.05Gbps, which exceeds the speed rating of the cable, and is likely to fail.
The framerate, usually displayed in hertz (Hz), is the number of different images a TV can display in one second. A higher framerate allows smoother motion, while a low framerate can look choppy. If that same Premium High-Speed HDMI cable is used to send a 4K video at 30Hz (30 frames per second), the bandwidth requirement will be cut in half. At 30Hz, there is plenty of bandwidth available for 10 bits per sample, or even 12 bits per sample.
As you can see, it's hard to give a complete answer to the compatibility question in one sentence. An HDMI switch may support 4K video at 60Hz and support 4K video with Deep Color, but not support both at the same time.
]]>We receive a lot of questions about compatibility, many of which are about HDMI 2.0. Customers often want to know if our switches support HDMI 2.0, or are compatible with their HDMI 2.0 devices. Our switches are compatible with all HDMI 2.0 devices, but the reason is worth exploring.
Most HDMI devices, like Blu-ray players and TVs, support a specific version of HDMI. The version number tells you that the device supports a specific set of features. For HDMI 2.0, these features include CEC extensions, dual video streams, and 32-channel audio. Support for these features requires up-to-date chips and code on the devices being used, because the are actively participating in the creation and display of your video.
Switches are different. Switches don't participate in the creation or display of video, and they don't transform it as it passes through. In that sense, switches are much more like cables than they are like other HDMI devices. This can make it difficult for customers to determine what a switch will support, because determining what an HDMI cable will support is, in itself, a bit tricky.
HDMI cables do not have HDMI version numbers. There is no such thing as an HDMI 1.4 cable, or an HDMI 2.0 cable. In fact, HDMI LLC does not allow cable manufacturers to market cables using HDMI version numbers.
HDMI cables are rated by category. To qualify for a category, a cable must achieve a certain speed. This is the current line-up of HDMI cable categories, as of mid-2018:
Category | Speed | Name |
---|---|---|
Cat. 1 | 4.95Gbps | Standard |
Cat. 2 | 10.2Gbps | High-Speed |
18Gbps | Premium* | |
Cat. 3 | 48Gbps | Ultra High-Speed |
*Premium certification means a Category 2 cable is guaranteed to pass 18Gbps, but is not technically a category. |
When it comes to cables and switches, everything depends on bandwidth. Different HDMI features don't require explicit support, but every feature you use adds data to the signal you're trying to pass through the switch. Stay tuned for our 3 part series coming over the next few weeks and months.
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