Published On: July 27, 2020

Everything You Need to Know about HDMI 2.1 (Including Stuff You Might Not Ask)

Published On: July 27, 2020

Everything You Need to Know about HDMI 2.1 (Including Stuff You Might Not Ask)

HDMI 2.1 brings with it a lot of new features, and officially codifies a few features we've had for a while now. But do you need it? And if so, when?

For two years, one of the most common remarks in the comments section of my AV receiver reviews has been a version of, "I'm not buying a new AVR until HDMI 2.1 arrives." Indeed, 2019 was a sparse year for new AV receiver releases, and two different manufacturers confidentially hinted to me that it was because HDMI 2.1 was on the horizon, and they worried that fewer people would be buying HDMI 2.0b-capable units so late in the lifecycle of that spec.

Well, it's summer 2020 now, and HDMI 2.1 has started to mosey into the marketplace. We've just received our first two HDMI 2.1-equipped receivers, and full reviews will be coming along shortly. But in the meantime, we thought it would be worthwhile to break down the features of this new specification, discuss what advantages it has over previous versions of HDMI, and help you decide whether you need to upgrade soon or if you're okay waiting until your current AVR craps the bed.

Or maybe you'll decide you don't need HDMI 2.1 anytime soon, and take this opportunity to get a good deal on one of the remaining HDMI 2.0 receivers, which still dominate the market but are on the way out.

The only right answer is the one that's right for you, and we're here to help you figure it out.

To make this simple, we'll break down the biggest features of HDMI and discuss how they may or may not influence your decision to upgrade.

It's all about 8K (isn't it?)


Most of the talk about HDMI 2.1 so far has been about support for 8K resolution (7,680 x 4,320) video. Indeed, manufactures like Denon -- first to the market with new 2.1-capable AVRs -- are spelling out the support for 8K right in the descriptive names of their new models, such as the AVR-X6700H​ 11.2 Ch. 8K AV Receiver with 3D Audio, HEOS Built-in and Voice Control.

But the fact remains that few of us have 8K TVs at this point, 8K sources are exceedingly rare outside of Japan (amounting mostly to high-end PCs for those of us in the States), and 8K video content is unlikely to be a widespread thing anytime soon.

In other words, you shouldn't be making your AV receiver shopping decision based on the need for 8K support.


A more intriguing (and potentially valuable) element of the HDMI 2.1 spec is support for other resolutions and refresh rates. One example is [email protected], which means 4K (more accurately "UHD," 3,840 x 2,160) video at 120 frames per second -- the framerate of Ang Lee's Gemini Man and twice the refresh rate supported by HDMI 2.0b.

Indeed, HDMI 2.1 supports a wide variety of resolutions and refresh rates, including:

  • 4K50/60
  • 4K100/120
  • 5K50/60
  • 5K100/120
  • 8K50/60
  • 8K100/120
  • 10K50/60
  • 10K100/120

Of course, standard- and high-definition, as well as 4K 24 and 30 frames per second, are also supported by the HDMI 2.1 spec. But for reasons we'll dig into in a bit, devices that support HDMI 2.1 don't necessarily support all of these resolutions. And just because the HDMI 2.1 specification includes several new features, that doesn't mean you'll only find those features on HDMI 2.1 devices.

1080vs4Kvs8K.jpg

That's a lot to digest, but here's a simple question: Will I need a new HDMI cable?


This may be the only question with a simple answer. Its increased resolutions and framerates mean HDMI 2.1 gets a boost in bandwidth, from 18Gbps to 48Gbps. As you may have guessed, that necessitates a new cable, officially dubbed "Ultra High Speed" but labeled by most cable manufacturers and dealers as "Ultra 8K."

Those of you who are quick with a calculator may have noticed that even 48Gbps isn't technically enough to pass resolutions higher than 8K at 60 frames per second with 4:2:0 chroma subsampling. Anything higher than that will require a new form of transmission compression known as DSC (Display Stream Compression) 1.2. Lower resolutions may also use DSC depending on the modes supported by your new TV.

Wait, what's all this business about modes?

In short, you will soon start to see the resolution and refresh rate capabilities of new displays followed by a subscript A or B. 4K120A means that a TV supports 4K resolution at 120Hz only in uncompressed mode. 4K120B means that a TV supports 4K resolution at 120Hz only in compressed mode. 4K120AB means the display supports both.

So, if I don't need 8K or 120Hz 4K, I don't need HDMI 2.1, right?

Well, unfortunately, it's not so simple. There are other features of HDMI 2.1 that may enhance your home theater experience, even if you're perfectly happy with your old 4K TV. Truth be told, though, many of these features will appeal primarily to gamers.

VRR_3_500wide.jpgPerhaps the most enticing new gaming-centric feature of HDMI 2.1 is VRR, or Variable Refresh Rate, a technology that PC gamers may be familiar with thanks to Nvidia's G-Sync and AMD's FreeSync. The simplest explanation is that Variable Refresh Rate will enable the next generation of video game consoles, such as the PlayStation 5 and Xbox Series X, to deviate from the rigid refresh rates of our old TVs to deliver a smoother gaming experience.

To understand why that's a big deal, imagine you're playing a video game on your PS4 connected to your HDMI 2.0b-capable TV. You're running along at a regular clip, and the game console's video processor is sending new frames to your TV sixty times a second (60Hz). But suddenly you enter a new environment or the action gets intense, and for a few brief seconds the graphics processor can't crank out new frames at a rate of sixty per second.

This can result in "screen tearing," in which two different frames are displayed on the screen simultaneously. Or you might see a stuttering effect as the display repeats the last frame it received until the console is ready to send a new one.

Variable Refresh Rate flips the script. Instead of the refresh rate being dictated by your display, it's dictated by the source device (up to the display's maximum refresh rate at a given resolution). So, if your gaming console needs to slow down to 54 frames per second momentarily, a display that supports Variable Refresh Rate will be capable of doing so without introducing tears and stuttering.

Another gaming-specific technology provided by HDMI 2.1 is Auto Low Latency Mode (ALLM). You probably already have something called Game Mode on your current display, which turns off certain video processing and other features to reduce latency as much as possible. ALLM enables your console to send a message that effectively says, "Hey, switch to that mode while I'm gaming, but turn it off when it's time to watch movies or TV."

Other game-enhancing features of HDMI 2.1 include Quick Frame Transport (QFT) and Quick Media Switching (QMS) which will, respectively, reduce latency and allow for quick transitions between different frame rates.

But what if I'm not a gamer?

Even if you don't "pwn the n00bs" on a regular basis, you'll be able to appreciate features such as the aforementioned Quick Media Switching. You know how switching from, say, the disc player to the satellite receiver results in your staring at a black screen for a few seconds? That's a thing of the past thanks to Quick Media Switching, which should make input switching nigh-instantaneous. And technically, you won't need an HDMI 2.1-equipped TV or source component to enjoy it, only a compatible AV receiver or preamp.

QMS_700wide.jpg

Other HDMI 2.1 features, you may already be familiar with. Enhanced Audio Return Channel (eARC), for example, allows you to send lossless audio and object-based audio formats like Dolby Atmos and DTS:X from (or through) your TV via its Audio Return Channel. With eARC, if you rely on apps built into your smart TV, or for some reason route your source components through your TV instead of your receiver, you can still enjoy Atmos audio.

And then there's the new HDCP 2.3 copy protection supported by HDMI 2.1. This may soon become the thing that prompts a lot of people to buy a new receiver, even if they care little for the higher resolutions and other features supported by the spec.

Some of those features sound familiar. Don't I already have them?

It's true: We've seen some new features of the HDMI 2.1 spec trickling down to HDMI 2.0 devices for a while now, most notably eARC and Auto Low Latency Mode, as well as HDCP 2.3 copy protection. If that makes the HDMI 2.1 spec seem a little fuzzy, well, it is. This is the first time we've seen features of a new HDMI spec appear on devices that officially only support the old spec. It gets more complicated than that, though, when you consider that devices don't have to support all of HDMI 2.1's features to be considered HDMI 2.1-compliant.

For example, those new HDMI 2.1-equipped Denon AV receivers mentioned above? They only support 8K video up to 60Hz, and do not support 10k resolutions at any refresh rate. Again, that's not a significant concern for now, because, like, where are you even getting 10K video to begin with and what are you using to display it?

Just understand going forward that simply saying a device has HDMI 2.1 inputs or outputs doesn't really tell you what features it supports. In the era of HDMI 2.0, you could tell if a video component supported Hybrid Log Gamma (a type of HDR used in TV broadcasts and occasionally on YouTube) by whether or not it had a "b" after the HDMI 2.0 designation. (At least theoretically. Truth be told, though, in the last year or so I've noticed that it's usually the other way round: You can only tell if a device is 2.0b-compliant based on whether or not "HLG" is listed amongst the supported formats.)

At least for now, it looks like that trend will continue. According to HDMI.org's messaging to licensees, "You can only use version numbers when clearly associating the version number with a feature or function as defined in that version of the HDMI Specification. You cannot use version numbers by themselves to define your product or component capabilities or the functionality of the HDMI interface."

In other words, from now on it won't simply suffice to say whether or not an AV receiver, preamp, source device, and TV reviews device boasts HDMI 2.1 input, output, or passthrough. Instead devices will require laundry lists of supported features, and you'll likely have to do a little more homework to determine if Display A is capable of receiving everything that Source Device B is capable of sending. And vice versa. And whether the AV between them can pass the signal along. But don't worry -- we're here to help you sort it all out.

Additional Resources
• Read HomeTheaterReview's AV Receiver Buyer's Guide (June 2020 Update).
• If you want more in-depth coverage the new HDMI 2.1-compatible receivers (coming soon), keep an eye on our AV Receiver category page.

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