Computers deal with RGB color spaces, mixing red, green, and blue combinations to render onscreen colors. But RGB isn’t suitable for storage and transmission as it carries redundancy.
Thankfully, human perception can be fooled with a technique known as chroma subsampling. It’s a type of compression for still images and video that discards some color information without degrading the perceived visual quality.
This helps reduce file size and bandwidth usage.
Follow along with us to learn how chroma subsampling works, how various sampling modes affect image quality, and how to best configure your devices.
What Is Chroma Subsampling? What Does It Do?
The human visual system has a much lower understanding of changes in color differences than brightness. Chroma subsampling leverages this fact by reducing the amount of color information in a source signal to allow more luminance data instead. In other words, color components are sampled at a lower resolution than brightness.
Reducing color information allows a video signal to travel efficiently via limited-bandwidth HDMI cables and over the internet. Because your eye is far less sensitive to color than brightness, you won’t perceive a major quality difference when enjoying your favorite movies.
Chroma subsampling is also used in JPEG encoding.
RGB vs. YUV vs. YCbCr
In digital video and photography, the brightness component—or luma—is usually denoted Y. The color data (called chrominance or simply chroma) consists of two different components: Cb (blue projection) and Cr (red projection). Their combination, known as YCbCr, is what defines the colors you see on the screen.
Imaging devices such as video cameras use composite analog signals (YUV), which can be digitally encoded as YCbCr. Because projection devices like TVs and the phone in your pocket deal with RGB instead, they convert the digital YCbCr signal back to the RGB color space before displaying images.
Common Types of Chroma Subsampling
The YCbCr color space supports several chroma subsampling schemes expressed as an A:B:C ratio. A is the horizontal width of the sampled region in pixels, B denotes the number of chrominance samples (Cr, Cb) in the first row, and C denotes vertical sampling.
For example, the 4:2:2 scheme defines a segment of four pixels horizontally in two rows. It samples four luma elements and mixes them with two chroma ones (one Cr and one Cb).
4:4:4, 4:2:2, and 4:2:0: What’s the Difference?
The following are the most common combinations for chroma subsampling:
- 4:4:4 is used for an uncompressed signal without color reduction. You get full horizontal and vertical color resolution without quality loss. There’s also 4:4:4:4, where the fourth four is a transparency channel.
- 4:2:2 has half the chroma of 4:4:4, cutting the horizontal color resolution in half compared with luma. It decreases the file size of an uncompressed signal by one-third, with little to no compression artifacts. As such, 4:2:2 is often used in gaming and professional video formats like Digital Betacam, DVCPRO 50, and MPEG-2.
- 4:2:0 transports one-quarter of the luma samples. Only half of the pixels in the first row are sampled, and the second row is discarded. With the sampling rate halved horizontally and vertically, 4:2:0 practically retains lossless visual quality. Those features have helped turn it into an industry standard for movies, TV shows, and sports.
There’s little visual advantage to using the 4:4:4 sampling for media. This is why most digital video cameras and many Blu-ray movies use 4:2:0 encoding. And for those times when you need to export a video from Premiere Pro on your computer, popular video codecs such as MPEG-1, MPEG-2, and H.261/263 all support 4:2:0 as well.
4:2:1 vs. 4:1:1 vs. 4:4:0
Other sampling schemes are not widely used, such as the examples below.
- 4:2:1 is a somewhat outdated sampling mode with the horizontal Cb resolution half that of Cr and a quarter of the horizontal Y resolution.
- 4:1:1 quarters the chroma resolution and halves the bandwidth, which does affect visual quality. While some digital video formats like DV, DVCAM, and DVCPRO use 4:1:1, this sampling mode isn’t broadcast quality.
- 4:4:0 halves the sampling ratio vertically but maintains it horizontally.
Chroma Subsampling in Text, Apps, and Media
Different chroma subsampling modes target specific use cases.
No one likes seeing compression artifacts on high-contrast, pixel-perfect content such as text atop a flat color. That’s why computers, phones, tablets, and gaming consoles exclusively use the 4:4:4 sampling. Encoding media with 4:4:4 is overkill—there’s no discernible visual difference, and file size goes through the roof.
How to Test for Chroma Subsampling
The simplest way to check for chroma subsampling would be a digital test card built into your TV, also known as a “startup test” or “test pattern.” Turning this feature on in the settings of your TV will output a set of patterns and calibrated color bars.
The colored bars on your TV not only let you check whether chroma subsampling is used but also ensure that the output is true to color and tone according to the original signal.
If you can’t find this option, Google “chroma subsampling test patterns” or use a chroma test image provided by RTINGS. Open this picture on a computer or view it on your TV. Be sure to set the scaling to 100% before looking for characteristical artifacts such as fuzzy text edges, color bleed, and blurring of lines and text.
If none are present, your TV uses the no-compression 4:4:4 sampling mode.
How to Choose the Best Chroma Subsampling
Armed with all this knowledge, you should have a basic understanding of chroma subsampling and how different compression modes function.
As a rule of thumb, your HDMI cables should support both 4:2:0 and 4:2:2. To enjoy uncompressed 4:4:4 video without chroma subsampling, you’ll need an HDMI Ultra High-Speed cable with a stated bandwidth in the 18-48 Gbps range.
If you’re connecting a computer, set-top box, or gaming console to a TV, be sure to set the video format to YCbCr/YUV (don’t worry about text fuzziness in games—today’s graphics engines account for chroma subsampling).
- LG: Go to the picture settings and turn on HDMI ULTRA HD deep color. Now click the Input button, choose All inputs, and pick the port to use this mode.
- TLC: 4:4:4 should be enabled by default when you connect a PC. If not, then be sure to set the input type to Computer in the settings interface.
- Samsung: Go to the Settings menu and click Expert Settings below the Picture Settings header. Now scroll down to HDMI UHD color at the bottom of the screen, choose the input which will use 4:4:4: and set it to PC mode.
- Sony: Open Settings and go to the external inputs menu, then choose HDMI enhanced format. You must also set your picture mode to Graphics in the picture settings.
- Vizio: Select your input in the Display settings of the SmartCast app. Now choose HDMI color subsampling, assign it to the desired input port, then set it to Computer.
For media streamers like the Apple TV, set your HDMI signal output to YCbCr in Settings.
Chroma Subsampling Matters After All
Understanding how chroma subsampling works won’t just help you decipher the meaning of those weird abbreviations printed next to I/O ports on the back of your TV but also choose the correct setting for optimal picture quality.
Without chroma subsampling, 4K video streams would stutter, and you’d need tons of storage just to save movies and images shot with your phone.
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