Many times, when people think about video quality, the first things to come to mind are resolutions like 4K and 8K UHD, and higher refresh rates such as 120 Hz and 240 Hz.
Another important part of the story is how to support those resolutions along with multiple refresh rate options, while managing to design and specify for the required bandwidths. Three vital considerations in system design and equipment selection are full chroma versus chroma subsampling, compression, and color bit rate. In general, most gear and systems support many options already, but it is helpful to keep them in mind.
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A reduction in color bit rate or utilizing chroma subsampling or compression allows more options to increase any of those three variables, along with improving resolution and refresh rates. Lower the color bit rate, and now the system may be enabled for uncompressed content delivery, or compression could allow for higher refresh rates, higher color bit rate, etc. For example, the HDMI 2.1b Specification supports uncompressed 8K60 with subsampling, but with the higher 96 Gbps bandwidth of HDMI 2.2 Specification, uncompressed 8K60 is now supported with full 4:4:4 chroma, and 4K240 is supported with full 4:4:4 chroma at 10 and 12-bit color.
Brightness, Color, and Subsampling
Chroma subsampling is a technique used to reduce the amount of color information in a signal to save on bandwidth without significant degradation of visual quality. Subsampling formats in common use include 4:4:2 and 4:2:0. The numbers refer to how many samples are taken of each pixel for luminance and chrominance. Thus, 4:2:2 means that for every two luminance samples, there is a single chrominance sample. A 4:2:0 subsampling rate indicates that for every four luminance sample, there is one chrominance sample. The designation 4:4:4 indicates there are four samples for each pixel for both luminance and chrominance — in other words, no subsampling.
In terms of application and visual quality, 4:4:4 is being used for professional production, post-production, and some niche consumer applications in which no subsampling is employed and thus full color resolution is delivered at the cost of file size and bandwidth.
Contrast that with 4:2:0 delivering very good quality while offering bandwidth savings. How good is very good? Well, good enough for broadcast television, camcorders, Blu-ray player standards (although many have 4:4:4 options, too), and streaming — all while offering bandwidth savings.
When it comes to residential system sales and design, chroma subsampling tradeoffs boil down to file size and bandwidth versus color fidelity, which can be especially noticeable in high-detail areas, as well as artifacts.
Color Bit Depth
Color bit depth accounts for how many shades of color are available. The greater the number of shades, the smoother the transition between different colors on screen.
A bit depth of 8 bits per channel (24-bit RGB) equates to 256 levels per color, or 16.7 million colors in total. A bit depth of 10 bits per channel (30-bit RGB) equates to 1024 levels per color, or 1.07 billion colors, and 12 bits per channel (36-bit RGB) offers 4096 levels per color or more than 68 billion colors.
For residential system installers and their customers, color bit depth matters because a greater number of colors available means smoother color gradients, which are especially noticeable in video images of the sky and in shadowy areas of pictures.
Having more colors available is also crucial for displaying high dynamic range (HDR) video, which calls for a color palette that theoretically exceeds the capacity of the human sight and color perception.
As with color subsampling, there are tradeoffs involved with color bit depth. The lower the bit depth, the less data required, which lowers the bandwidth needed. On the upside, a lower bit depth can help deliver higher resolution and refresh rates more efficiently. A lower bit depth can also make it possible to avoid signal compression, giving consumers the benefit of uncompressed content delivery.
Display Stream Compression
Display Stream Compression (DSC), a visually lossless compression standard from the Video Electronics Standards Association (VESA), enables 4K, 8K, and HDR content to be transmitted at high refresh rates over limited bandwidth.
DSC uses delta PCM (pulse code modulation) and a YCoCr-R color model to compress video in real time. It focuses on removing redundancies and subtle visual data that the human eye won’t notice.
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Offering benefits like slimmer devices and longer battery life, DSC is frequently an option used in 8K TVs, AR/VR headsets, gaming monitors, and GRUs, as well as vehicle displays and smartphones and tablets. DSC also allows HDMI Cables to deliver signals at longer lengths due to the reduced bandwidth. But check the device specs, because in many cases gear supports both compressed and uncompressed signals.
Putting It All Together
In almost all cases, the communications between source devices and displays determine which combinations of resolution, refresh rate, color bit rate, compression, and chroma sampling are enabled to deliver the best end-user experience the system can display. With a better understanding of how these factors work together, along with bandwidth efficiency, designers of residential systems can make more informed choices as they plan projects.
Finally, HDMI® technology supports a wide range of these options, giving device manufacturers and residential systems designers the flexibility they need to deliver the best possible viewing experience.
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