What is chroma subsampling?

Chroma subsampling is one of the most effective ways to watch high-resolution, high-refresh-rate content without requiring large bandwidth to transmit those signals. Like display stream compression, chroma subsampling is a compression technique that, when used effectively, is nearly indistinguishable from the original image.

It works by removing some color information from an image or video, while retaining all lighting or brightness information. This reduces bandwidth, opening up more space for higher resolution and refresh rate signals. This allows for UHD monitors with higher refresh rates, and without this technology, platforms like Disney+ and Netflix would not function properly.

Even high-end TVs like the Samsung S95C still use chroma subsampling on its HDMI 2.1 port. Zeke Jones / Digital Trends

What is chroma subsampling?

Chroma signal subsampling removes some color information from an image or video, while retaining brightness-related information. This reduces the overall bandwidth required to transmit the signal, but does not greatly affect the final image visually. Unlike Display Stream Compression, which is visually lossless, chroma subsampling sometimes produces artifacts, although they are usually hard to spot unless you look for them.

It’s true that the reason why chroma subsampling is so effective and widely used is because it tricks the human eye. Although humans are good at recognizing contrast – that is, the difference between light and dark – they are not good at recognizing differences in color. That’s why in chroma subsampled images and videos, full luminance data is transmitted, but color data is limited. While this reduces color variations in an image, it is very difficult for even a trained eye to recognize unless they are really looking for it.

That’s why most movies are broadcast with chroma subsampling used on all major streaming platforms, and even used with UHD Blu-ray, which is largely considered the best way to watch movies.

Chroma subsampling example.
Here you can see the effects of various chroma subsampling techniques. The image below shows the color resolution. Janke / Wikimedia

The image above shows how little difference there is when removing most of the color resolution with the more extreme chroma subsampling technique.

How does chroma subsampling work?

A standard video signal has two main components: lighting information, and color information. Completely uncompressed, such a signal is called 4:4:4, where each pixel has its own chrominance (color) value. This is the highest quality for a video, without any subsampling.

A marginally compressed signal is called 4:2:2, which has color information for every other pixel, and those other pixels copy the color information of their neighboring pixels. So in a two-by-two pixel block, each pixel has its own lighting data, while each group of two pixels shares color data. It is often used in professional video editing, where it reduces data rates without affecting visual quality at all.

Broadcasting, streaming, and UHD Blu-ray use more aggressive chroma subsampling of 4:2:0, where there is only one pixel with color data every four pixels. A two by two pixel block will have lighting data for each pixel, but share color data across pixels. Although this sounds dramatic (and its impact on bandwidth is dramatic) the final result is still almost impossible to distinguish from 4:2:2, or 4:4:4.

Chroma subsampling is usually not enabled by default on PC monitors, as it can cause problems with the readability of small text — something that is easier to see on a monitor than a TV.

Should you use chroma subsampling?

Chroma subsampling is one of several compression techniques that can be used to reduce the overall bandwidth required to transmit video. For consumers, DSC has largely replaced chroma subsampling, as it is completely lossless visually and unlocks the kind of additional bandwidth required for high refresh rates and resolutions.

However, this does not mean that chroma subsampling is redundant. It’s true, it’s still a key component in the functionality of modern streaming services, and modern UHD Blu-ray, which utilizes it to store much of the chroma and luminance data required for 4K resolution video.

Chroma subsampling is still useful when using older displays that don’t support DSC. It is also a key component of some static image algorithms, such as the way JPEGs are created.

Suffice to say, chroma subsampling is still useful and is used regularly by various industries and in specialized settings. However, for general consumers, DSC is arguably a more effective compression system. It’s much more modern and you’ll most likely find it enabled by default if you can use it, so you don’t have to use chroma subsampling manually.

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