Computers are pretty great, but you need a display to be able to actually see what they’re up to. Most people buy a computer monitor for this task, but if you really want to go big, a 4K TV can be tempting. But that big screen comes with a compromise that might make you think twice about using a TV as your monitor.

Buying a big-screen TV does seem like the obvious approach, though. But when you come home with your new TV and plug it into your PC, you’ll likely be left disappointed. Don’t worry, your new TV isn’t faulty. You’re just running into a key technological difference between TVs and monitors.

An introduction to chroma subsampling

The hidden difference

Plugging a 4K TV into a computer is likely to yield fuzzy or distorted text, and that’s a problem. Unlike TVs, computer monitors are primarily used to display text, so they need to be pin-sharp and easy to read.

To understand why text can look so distorted on a TV, it’s important to first understand how that text gets there. For a 4K TV running at 3840 x 2160 resolution at 60 frames per second (60Hz), a large amount of data is required. This huge bandwidth requirement led engineers to look for a way to cut corners. They wanted to find a way to reduce the bandwidth required.

Armed with the knowledge that our eyes are more sensitive to changes in brightness (luminance) than to changes in color (chrominance), they devised chroma subsampling. Rather than transmitting brightness and color information for every pixel, chroma subsampling maintains full brightness fidelity while discarding up to three-quarters of the color data. This significantly reduces the bandwidth required to send a 4K image to the TV.

This technique is represented as a three-digit ratio such as 4:4:4, 4:2:2, or 4:2:0:

• 4:4:4 (Uncompressed): Every pixel receives its own distinct brightness and color data. This is standard for dedicated computer monitors.
• 4:2:2 (Half color bandwidth): Color information is shared horizontally across pairs of pixels, cutting the amount of color data by 50%.
• 4:2:0 (Quarter color bandwidth): Color information is sampled across a ‭2 x 2‬‭‬ grid of four pixels, discarding 75% of the original color data.

If you’re watching a movie or a TV show, or playing a video game, 4:2:0 compression is almost indistinguishable from 4:2:2 and 4:4:4. That’s why it makes sense for most TVs to use either of these approaches to save bandwidth. But it isn’t suitable for PC monitors.

Text never stood a chance

Compression kicks in

While 4:2:0 compression works great for the kind of content we use TVs for, computing is a different thing entirely. Computing requires a user interface and on-screen text to be pixel-sharp, with thin lines and high contrast as the key. Chroma subsampling isn’t conducive to either of those things.

When you use 4:2:0 to display thin lines or text, the algorithm that refactors the color data simply fails. If red text is displayed on a black background using the 4:2:0 method, for example, you can expect the two colors to bleed together. Halos around text are common issues when using a TV as a computer monitor, and it makes for a terrible experience. Especially when used at the kind of distance that we tend to use computer monitors.

The chroma subsampling issue is often compounded by the way modern TVs use internal post-processing systems to “improve” the images they display. These systems often add a sharpening effect to the image, often making any halos more pronounced than they already were.

TV makers won’t change anytime soon

They have just no reason to

By now, you might assume that chroma subsampling is something TV makers would try to avoid to improve their ability to display text. But that’s unlikely to happen any time soon, and with good reason.

At its core, this boils down to a matter of economics. A 4K display capable of uncompressed 4:4:4 color while maintaining a high refresh rate costs more to produce than one that isn’t. TV manufacturers are already working to make their products as affordable as possible, so anything that could minimize their margins is simply not an option.

It’s also important to remember that this isn’t an issue for most TV buyers. They’ll never connect their TV to a computer. It’ll spend its days on the other side of their living room, well beyond their ability to spot any text halos or color issues.

TVs are TVs, monitors are monitors

Ultimately, TV makers don’t need to worry about the effects of chroma subsampling on text because that’s not what TVs are for. And most of them also have PC monitor lines, with a full range of products better suited to the task at hand.

4K PC monitors aren’t the rare beasts that they once were, nor are they as costly as they used to be. If you’re set on a 4K resolution, do yourself — and your eyes — a favor and don’t be tempted by a TV. Buy a proper PC monitor instead.