PS4 Pro: "2160p - YUV420" or "2160p - RGB"?

Setting up a PS4 Pro to take full advantage of 4K @ 60fps can be difficult (see my previous post). There are multiple options that don't look all that different on casual inspection. What's the difference, and what should you choose?

There are three different options in the PS4 Pro menu which are able to provide 4K support:

• Automatic
• 2160p - YUV420
• 2160p - RGB

If everything's set up correctly, the best choice is actually Automatic. Assuming you have compatble equipment, you'll know everything is set up right when Automatic gives you this:

The big question, though, is what does that really mean? What's the difference between RGB and YUV420? (And what in the world is YUV422??)

First things first, let's start with RGB. I won't spend much time on this since it's pretty simple and easy to find information on the web. TVs and monitors use separate red, green, and blue components to display colors in an image; RGB simply sends red, blue and green color information directly.

Original:

RGB:

Historically, though, televisions have rarely used RGB. Instead, the signal is split up into luma and chroma—this is called YUV (or pedantically speaking, Y′CbCr). The luma component is the Y component, and represents the pixel's brightness; it looks like a black and white version of the image. Black and white televisions supported only luma.

Luma:

When televisions were upgraded to color, they didn't want to break existing TV sets, so they kept broadcasting the luma information just as before. To add color, they added a chroma component, the UV information, which basically contains the color with the luma/brightness information removed. Chroma data is very hard to parse visually; you can compare it to the original above and see that colors are basically preserved, but with no bright or dark spots. Our visual system just sees the chroma image as a blur, mostly fuzzy and indistinct.

Chroma:

Here's another image split into luma and chroma.

Original:

Chroma:

Luma:

Compared to RGB, a pure YUV signal could theoretically present the exact same quality image with the same amount of bandwidth. In a normal SDR picture, RGB takes 8 bits per channel, which means 24 bits for a pixel. Pure YUV data takes 8 bits for the luma and 16 bits for the chroma on a per-pixel basis, so that's still 24 bits per pixel. This signal would be called YUV444.

However, the PS4 does not actually use YUV444—it's more complicated than RGB and technically has no advantage (or disadvantage). In cases where there is sufficient bandwidth, the PS4 will simply switch to RGB mode.

YUV420 mode is a smart compromise that takes advantage of the limitations of our visual cortex. Remember how the luma component of an image is very distinct, but it's difficult to see details clearly in the chroma component? Well, it turns out that you can shrink down the chroma channel to 1/4 of its original size, and it's extremely difficult to tell the difference. In other words, for a 4K 2160p signal, the luma component is sent in full 4K quality, but the chroma component is sent at 2K 1080p resolution. In this case, on a per-pixel basis, the luma still takes 8 bits per pixel. We send 16 bits of chroma information at 25% size (50% reduction on each axis), so that averages out to 4 bits per pixel. So YUV420 needs only 12 bits per pixel, meaning it only needs half the bandwidth and delivers an image that most people find indistinguishable from pure 4K. Given the choice between RGB and YUV420, RGB is the superior "no compromise" choice, but most viewers will struggle to see a difference in typical usage.

— more to come —