HDMI made the entire Internet mad recently by introducing a confusing new branding scheme when the old one was serving everyone just fine. But hold on, there's more, because it's about to get more confusing with the introduction of HDMI 2.1a.
So let's talk about what's new in the HDMI 2.1a spec. And as you may have gathered from the fact that it's called 2.1a instead of 2.2, the list of changes is rather short. In fact, there's only one new feature, and it's called source-based tone mapping.
That might sound like some kind of gimmick you'd pay extra for at a tanning salon, but it actually has to do with the way devices handle HDR. Tone mapping is the process of taking HDR signal and adjusting it to match the capabilities of whichever display you happen to be watching that content on.
You see, many HDR-capable TVs actually are not able to show an HDR image as it's originally mastered. Oftentimes the original video is mastered at 1,000 nits of brightness or even higher, but most consumer displays, even many that are marketed as HDR, cannot get that bright. Additionally, HDR content is often mastered for a wider color gamut than many TVs can show, meaning they can't display as many colors as are contained in the source material.
Without tone mapping, the TV would clip the image. That is, it would simply discard the information it can't display from the video signal, and the image would instead have very incorrect colors, or just parts with no information, and portions that would be blown out to absolute crap so you can't even see them at all.
So tone mapping effectively takes color and brightness information and maps it to a value that the TV can actually show. This allows the TV to approximate the original image accurately enough so that it looks okay on your screen instead of a distorted mess. So for example, a signal with 1,000 nits peak brightness can be reduced so that the brightest parts of the image are now, say, 400 nits. Everything goes in line with that.
Now back to HDMI 2.1a. Traditionally, tone mapping is handled by the display itself, but the source-based tone mapping in HDMI 2.1a offloads some of this responsibility from the display and moves it to the source. Whether the source is a GPU, a streaming box like an Nvidia Shield, or a Blu-ray player. But why exactly would we want this?
Dolby Vision HDR actually already has a proprietary option where you can select either TV-led or player-led tone mapping, and tests have shown that tone mapping looks better when it's handled by the display itself,as the TV is adapting to its own characteristics, as well as data from its ambient light sensor if it has one of those built in. Having the source handle tone mapping means that it's doing the mapping based simply on what the TV reports its capabilities to be, meaning the adjustments the source makes may not be as accurate.
So why do we want this? Well, it's 'cause there is a big plus to source-based tone mapping; lower latency. Even if something might look a bit better if the TV is handling the tone mapping, this additional processing that the TV has to do adds latency, which can be a significant drawback for gamers who want as small of a delay between pressing a button on the controller and seeing an action happen on the screen as possible.
So source-based tone mapping, or SBTM, is a sensible addition to HDMI's increasingly gamer-focused feature set. Additionally, source-based tone mapping does allow more of a plug and play experience for the user. As, since it allows the source to read and understand the display's capability, the user won't have to spend as much time adjusting picture settings to get their HDR content to look right.
For now, it sounds like SBTM support can be added to existing devices via firmware updates, but that'll be up to device manufacturers. So, if you want the feature without spending extra on new equipment, your mileage may vary, as AV manufacturers have a track record, let's say, of being very slow with updates.
Additionally, HDMI 2.1a is going to suffer from the same stupid naming issues we saw with the original HDMI 2.1. All current HDMI devices will transition over to the HDMI 2.1a name, but they don't all have to support every new feature of HDMI 2.1 or 2.1a, so make sure you read the spec sheet to see if the features you care about are actually present. I mean, you never spend money without reading the fine print first, right?
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