HDMI 2.1 - Everything You Need to Know Explained

The HDMI 2.1 specification has been in the works for some time now, having first been announced in January of 2017 at CES. The new spec is the result of an impressively forward-thinking effort by The HDMI Forum, an open trade association dedicated to promoting “broader industry participation in the development of future versions of the HDMI specification, and to further expand(ing) the ecosystem of interoperable, HDMI-enabled products.” The Forum’s members, which include many of the world’s leading manufacturers of consumer electronics — along with test labs, movie studios, and others — sought to develop a new HDMI standard that would be so capable as to be essentially future-proof (or the-foreseeable-future-proof, at the very least). Only at the end of 2020 will the group’s design work and planning finally come to fruition with the arrival of the first HDMI 2.1 source components, which will join a growing list of currently or soon-to-be available HDMI 2.1 audio components and displays, finally allowing the home user to assemble a complete chain of HDMI 2.1 gear.

HDMI 2.1 Cable Specs & Bandwidth Requirements YouTube Discussion

What’s all the fuss about? In a word, bandwidth. HDMI 2.0b, the predecessor to 2.1, maxed out at 18 Gbps (Gigabits per second) of bandwidth. That may have been a gracious plenty just a few years ago, but the AV signals produced by today’s gaming consoles and 4K blu-ray players can get awfully close to running out of room in the pipeline. These devices deliver 4K video at up to 60 fps (frames per second). Right now, the only source capable of outputting 4K video at more than 60 fps is a high-end gaming PC. But by the end of this year, we will begin to see sources that can deliver 8K video at 60 frames per second, or 4K video at a buttery-smooth 120 fps. In order to support higher resolutions and higher frame rates — along with some useful new features — HDMI 2.1 offers a whopping 48 Gbps of bandwidth. In addition to allowing for higher resolution and the smooth, fast-action detail provided by higher frame rates, this additional bandwidth will accommodate Dynamic HDR support, ensuring that every moment in a piece of video content is displayed with an ideal combination of brightness, contrast, and color. HDMI 2.1 also promises an improved audio experience and simplified system connectivity via a feature called eARC, or enhanced audio return channel. eARC ensures compatibility between AV devices while supporting the most advanced audio formats and highest-quality audio signals available. (More on eARC at the end of this article.)

The first two source components to support 8K at 60 fps and 4K at 120 fps will almost certainly be Sony’s upcoming Playstation 5 and Microsoft’s new Xbox Series X, both of which are slated to ship in time for the 2020 holiday season. It comes as no surprise that these cutting-edge gaming consoles should be the first HDMI 2.1 sources, especially when you consider some of the enhanced gaming and media features that have been included in the new spec — many of which focus on smooth motion and/or reduced input lag.

These include the following:

• Variable Refresh Rate (VRR), which reduces or eliminates lag, stutter, and frame-tearing for more fluid and detailed gameplay.
• Auto Low Latency Mode (ALLM), which allows a connected display (and AV receiver) to automatically enter low-latency mode — a.k.a “game mode” — when the user plays a video game, allowing for smooth, lag-free interactivity. The setting automatically returns to normal when ordinary video content, such as a TV show or movie, is played.
• Quick Frame Transport (QFT), which reduces latency in the signal delivery process from source to display, resulting in smoother no-lag signal transfer for gaming, virtual reality, and other interactive content.
• Quick Media Switching (QMS), which eliminates the delay that can result in blank/black screens before content is displayed after switching frame rates.
  

I will go into these features in more detail shortly, but I think it’s worth pointing out that you may very well find one or more of them useful even if you have no interest in gaming.

HDMI 2.1 Video Performance and Features

At CES 2020, 8K TVs were everywhere, and the ability to deliver an 8K signal at a refresh rate of 60 Hz was one of the more talked-about features of HDMI 2.1’s video capabilities. And to be fair, if you get the opportunity to see LG’s Signature ZX 88-inch 8K OLED TV, it’s sure to impress, with twice the horizontal and vertical resolution of a 4K display, and four times as many pixels (over 33 million). But the reality is that 8K content is virtually nonexistent for now, and very few of us will upgrade to an 8K TV in the next year or two. So for most, the real boon to video performance offered by HDMI 2.1’s increased bandwidth will be the ability to send 4K video at 120 frames per second, which is twice the frame rate offered by HDMI 2.0b. As Phil Jones of Sound United mentioned in his recent video interview with Audioholics founder Gene DellaSala, many gamers are excited by the ability to play 4K games at higher frame rates because the smooth motion allows for more fluid and effortless gameplay. The ability to display UHD images at 120 fps could also benefit some movie and TV content — think action films, nature documentaries, and sports — to which higher frame rates can lend a clearer image that remains crisp even during fast-moving scenes. The latest color spaces such as BT.2020 (also known as Rec. 2020) are supported, with 10 or 12 bits per color component. For commercial AV installations and industrial/specialty uses, HDMI 2.1 can support other resolutions, including 5K and even 10K. 

If you’re into the technical side of video transmission, you might be wondering whether compression is used to achieve these high resolutions and frame rates in the HDMI 2.1 specification. The short answer is maybe. The spec supports both compressed and uncompressed modes, leaving it up to the individual manufacturers to decide which mode(s) to implement. But even when compression is used, the specification incorporates VESA DSC (Display Stream Compression) 1.2a, which is a visually lossless compression scheme. DSC is required for all video with a resolution and frame rate higher than 8K at 60 fps, but that’s essentially irrelevant for the home user. In order to achieve higher uncompressed resolutions than what was possible with HDMI 2.0b (i.e. higher than 4K at 60 fps), the HDMI 2.1 spec uses a new signaling technology called FRL (Fixed Rate Link). This technology replaces TMDS (Transition Minimized Differential Signaling), which was used in previous versions of HDMI, but HDMI 2.1 is backwards compatible with devices that utilize TMDS, so the transition should be seamless and painless for the home user.

A note about bandwidth and video performance:
Most modern displays (TV or Projectors) are not capable of fully utilizing the maximum potential picture quality that could be provided by HDMI 2.1’s 48 Gpbs of bandwidth. Why? Currently the LCD and OLED panels used in higher-end consumer TVs are native 10-bit panels. To deliver 4K video at 120 fps with chroma 4:4:4 and 10-bit color requires 40 Gpbs. To deliver 8K at 60 fps with 10-bit color would also require a max of 40 Gpbs, uncompressed. (If DSC compression is applied, the necessary bandwidth drops to 18Gpbs.) As an example, all of LG’s 2020 OLEDs have HDMI 2.1 ports, yet they are limited to 40 Gbps, rather than 48 Gbps of bandwidth. But because they use native 10-bit panels, 40 Gbps is enough to max out the panels’ capabilities. Owing to these panel limitations, 40 Gbps can be considered the maximum bandwidth required for real-world scenarios — for now anyway.

HDR

Many experts agree that the most significant improvement to video quality to arrive in the UHD era was not the inherently higher resolution of 4K video, but instead the advent of High Dynamic Range, or HDR. With HDR, video content delivers a high-impact experience thanks to a more extended dark-to-bright contrast range. The result is brighter whites, deeper blacks, and improved detail in both the bright parts and the dark parts of a given image. This improved contrast, combined with greater detail within an extended color space, contributes to a more visceral video experience. An HDR solution can be static — meaning it uses a single set of metadata to optimize the picture brightness curve of a whole video — or dynamic, meaning it uses dynamic metadata to optimize the picture brightness on a scene-by-scene, or even frame-by-frame basis. Static HDR solutions (such as HDR10) and dynamic HDR solutions (such as Dolby Vision) can be passed over previous HDMI versions, but the HDMI 2.1 specification supports multiple static and dynamic HDR solutions in order to ensure that, going forward, the use of dynamic metadata will not necessarily be tied to a proprietary format like Dolby Vision, which requires manufacturers to pay licensing fees. HDR-enabled devices that implement HDMI 2.1 will transmit both static and dynamic HDR metadata over the HDMI interface in a standardized way, ensuring that the metadata will be delivered properly regardless of the products’ manufacturers. In order to make certain that the user will get all the benefits of Dynamic HDR without possible compatibility issues, all HDMI 2.1 devices must go through the same mandatory compliance testing.

HDMI 2.1 Sources & New Features YouTube Discussion

Gaming And Media Features

As described earlier, the HDMI 2.1 spec includes a number of features aimed at improving the gaming experience, including Variable Refresh Rate (VRR), Auto Low Latency Mode (ALLM), Quick Media Switching (QMS), and Quick Frame Transport (QFT). I’ll now dive a bit deeper into these features to explain what they’re all about, and how they may benefit the non-gamers out there as well.

Variable Refresh Rate (VRR)

VRR is designed to reduce or eliminate lag, judder, and frame-tearing, resulting in more fluid and detailed gameplay. But what does that mean exactly? When you’re playing a video game, the graphics processor (GPU) in the computer or gaming console renders images faster or slower depending on what’s happening on screen, and how much processing power is available. But traditionally, the TV or display has a refresh rate that is static, meaning it never varies from the specified frequency (60 Hz, for example). It doesn’t matter if the GPU requires more time to render a certain frame in a particularly complex scene — which happens often, depending on the horsepower of the GPU and the resolution selected — the display is going to refresh when it’s going to refresh. If the GPU hasn’t finished rendering the next frame by the time the display refreshes, the GPU must either repeat the previous frame or send the incompletely-rendered one. The result? A laggy, juddery, experience with the occasional “torn” frame displayed. VRR solves this problem by allowing the source and display to work together, varying the refresh rate as necessary. The GPU can wait until the next frame is ready before sending it to the display, and the display can wait to refresh until it has received the next frame. With the display’s refresh rate synced with that of the content being produced at the source, the gaming experience becomes smoother and more immediate. The refresh rate might be anywhere between 30 Hz and 120 Hz at any given time, depending on how taxed the GPU is at that moment. If you’re not a gamer, you might think that VRR isn’t relevant to your needs, but that might not be the case. As it turns out, lots of web-sourced video content ends up in unusual frame rates, perhaps because it’s produced using a variety of non-standardized hardware and software combinations. So if you like to watch YouTube and other internet content (and who doesn’t?), you may well benefit from VRR because your TV will be able to display the content at its native frame rate.

Auto Low Latency Mode (ALLM)

Auto Low Latency Mode is relatively simple. Many TVs (and some AV receivers, such as recent Denon and Marantz models) have a Low Latency Mode, or “game mode,” which reduces input lag — the time it takes for a TV or monitor to actually display a signal that it has received. If you’re playing a video game, input lag can be a major part of the delay experienced between pressing a button on the controller and seeing the character/game react on screen. In some competitive online gaming scenarios, having less lag gives you an advantage over other players. It’s like an extension of your own personal reaction time, which is just as critical to a gamer as it is to an athlete. But before ALLM, you had to manually set your TV to its game mode (by going into the menu system) before playing a game, and then when you wanted to watch TV or a movie, you had to manually set it back to its normal setting. (Since low-latency modes achieve their reduced input lag by cutting corners on image processing, the resulting picture quality isn’t optimal for normal viewing.) Auto Low Latency Mode allows a gaming console, PC, or other device to send a signal to the TV that automatically turns on the low-lag game mode. When the source no longer requires reduced lag — let’s say you stop playing a game on your PS5 and then use the console to stream a movie — the source will disable the signal and your TV will return to its normal settings for optimal image quality. Once again, this gamer-focused feature may prove useful for non-gaming activities, such as video conferencing (increasingly important during the coronavirus pandemic), or even karaoke.

Quick Frame Transport (QFT)

So, ALLM is about reducing the lag inside your TV — the time it takes for the TV itself to display an image that it has already received. Quick Frame Transport is also designed to reduce lag, but instead of being related to a setting in your TV, it is a system inside HDMI cables that reduces the time it takes for video frames to pass from the initial source device, like a gaming console or PC, to the display device — be it a TV, a computer monitor, or a virtual reality headset. The lag in question here is known as “display latency.” The term refers to the entire time necessary for a frame that is fully rendered and ready to leave the GPU  to then travel through the source’s output circuits, downstream across the cable, into the display, through the display’s processing circuits, and finally, to be shown on the screen. QFT transports each frame at a higher rate to decrease this overall display latency, further contributing to snappier, more responsive gaming and real-time interactive virtual reality.

Quick Media Switching (QMS)

Quick Media Switching is designed to solve one specific problem. Let’s say you’re watching movie trailers on a streaming service. It is not unusual for different trailers to use different frame rates, such as 60 Hz, 50 Hz, or 24 Hz. Also many streaming devices (Roku, AppleTV) use 60Hz for their menus, but then movies are in 24 Hz. When switching from one frame rate to another, all devices in the HDMI connection chain must change their clocking and re-sync, causing a momentary blackout known as a “bonk.” The viewer’s TV screen will go black before the content is eventually displayed. QMS uses the VRR mechanism to allow for these changes in frame rates to take place without interrupting the viewing experience. The whole system can quickly and smoothly change from 60 Hz to any lower rate down to 24 Hz, with no bonks.

A note about HDMI 2.1 features:
There is no official HDMI 2.1 certification for electronics, which is why you will not see it mentioned on any manufacturer’s website. The most a manufacturer can do is list the HDMI 2.1 features that are supported by each TV, AV receiver, or source component. Likewise, an 8K or 4K TV with HDMI 2.1 ports might not actually support all of the features mentioned above, so if you are considering a purchase, don’t just check to see if the product in question has an HDMI 2.1 port; be sure to check that it supports the specific features that matter to you.

HDMI 2.1 Managing Bandwidth & Cables YouTube Discussion

HDMI 2.1 Cable: The Ultra High Speed HDMI Cable

Unlike previous versions of the HDMI specification, the 2.1 spec includes a new cable called the Ultra High Speed HDMI Cable, which is the first cable defined by the HDMI Forum. This new cable is “the only cable that complies with stringent specifications designed to ensure support for high resolution video modes (including uncompressed 8k@60 and 4K@120) and all other HDMI 2.1 features,” according to the HDMI Forum. The cable’s increased bandwidth capability supports up to 48 Gbps, though the specification does not indicate a cable length, which may max out at around 3 meters for passive cables before bandwidth begins to suffer. The new cables will also “exceed the requirements of the latest international EMI standards to significantly reduce the probability of interference with wireless services such as Wi-Fi.” All Ultra High Speed HDMI Cables will have to pass the mandatory Ultra High Speed HDMI Certification Program, which will include testing at an HDMI Forum Authorized Testing Center (ATC). The certification program is intended to ensure the quality and feature-support/compatibility of all Ultra High Speed HDMI Cables that reach the market. Qualifying cables will have an official Ultra High Speed HDMI Certification Label affixed to their packaging, and the cables’ outer jackets will also be labeled.

The Ultra High Speed HDMI Cable is the only way to ensure all the features and capabilities of the HDMI 2.1 Specification are delivered from a source device to a display. Ensuring Ultra High Speed HDMI cables are compliant with the HDMI 2.1 Specification is essential to the HDMI ecosystem. The HDMI Forum’s mandatory ATC-only certification requirements are designed to ensure cables are compliant with the HDMI 2.1 Specification. And the anti-counterfeit Ultra High Speed HDMI Certification Label and its scanning results provide a visible verification of certification that a product meets the HDMI Forum’s requirements.

 — David Glen of Advanced Micro Devices, Inc. and President of the HDMI Forum

Do you really need to upgrade your cables?
Compared to HDMI 2.0b, the HDMI 2.1 spec allows more than two-and-a-half times as much data to be sent down a cable at one time. So it stands to reason that in order to take full advantage of HDMI 2.1, the use of a new Ultra High-Speed Certified HDMI cable would be necessary. And it’s true that, eventually, you will need to upgrade to the newer cables in order to benefit from all of the features described above. But the reality is that many people will not need all new cables to enjoy the features that matter most to them right now. Some of the spec’s most important features, such VRR, ALLM, end eARC, do not require the new cable. And that leads us to…

HDMI 2.1 Audio: ARC vs eARC

On the audio side of things, HDMI 2.1 ushers in mandatory support for a feature called eARC, or enhanced audio return channel. As the name suggests, this is a newer and more capable version of the HDMI ARC protocol, which was added to the spec-sheet in 2009 and introduced as part of HDMI version 1.4. Before we get into the details of eARC, let’s do a refresher course on ARC. Prior to the advent of ARC, an HDMI cable was a one-way street for an audio/video signal, which traveled downstream from a source, often through an AV receiver or processor, and finally onto a display. Consider my setup from about a decade ago. My sources included a blu-ray player, a cable box, and a Roku HD-XR streamer. My housemates were gamers, so there was often a gaming console connected as well. All of these sources sent their signals via HDMI cables into my Denon receiver, and then on to my Pioneer TV. This was all fairly typical, and everything worked just fine. But things began to get complicated when Smart TVs came along, and people began using the streaming apps built into their TVs as main sources of video content. In a setup like mine, even though the TV and receiver were already connected via HDMI, a digital audio signal could not be sent out from the TV and into the receiver via that cable; doing so would require the signal to travel “upstream” along the one-way HDMI cable. And so, in order to get the TV’s audio routed to an AV receiver or sound-bar, instead of to the TV’s internal speakers, a second cable was needed — almost always an optical Toslink cable. Not only did this add yet another unsightly cable to deal with, it had sound-quality limitations as well. Some TVs could pass a multichannel audio signal to a receiver or sound-bar via optical Toslink, but many others sent only stereo signals. (These days, only a handful of Sony TVs will send a multichannel surround signal out via Toslink.)

In an attempt to solve all of these problems, the audio return channel feature was created. ARC effectively turned HDMI into a two-way street, allowing the TV to send its audio back along an HDMI cable to an AV receiver, processor, or sound-bar. With ARC, it (theoretically) became easier to enjoy the audio from your TV’s streaming apps, or from an antenna. And it also meant that you could (theoretically) choose to attach all your HDMI sources directly to your TV instead of to your audio device, if that was more convenient for your setup. But ARC had its own limitations and problems. First of all, it sometimes worked automatically, and sometimes required the user to tweak a variety of settings that might be buried in a TV’s menu system. It also allowed manufacturers to hand-pick which elements of the protocol they wanted to include, and which to leave out. For example, some TVs could send a 5.1 Dolby Digital or DTS soundtrack to a receiver via ARC, while others still only supported two-channel stereo soundtracks. Lastly, ARC was not able to carry high-quality lossless codecs such as Dolby TrueHD and DTS-HD Master Audio. If you connected your blu-ray player directly to your TV via HDMI and then tried to use ARC to send the audio to your receiver, you would get, at best, a degraded data stream that relied on lossy codecs. And while ARC could theoretically support object-based immersive audio formats like Dolby Atmos when streamed from Netflix or Amazon Prime Video, these too relied on lossy codecs, like Dolby Digital Plus, instead of the full-fat, lossless versions. ARC simply didn’t have the capability — or the required bandwidth — to handle uncompressed and lossless audio.   

Enhanced Audio Return Channel (eARC)

Enhanced Audio Return Channel aims to solve all of ARC’s problems by improving sound quality capabilities while providing greater ease of use. With eARC, the original, full-resolution audio signal can be sent “upstream” from a TV to an audio device. Dolby TrueHD, Dolby Atmos, DTS-HD Master Audio, and DTS:X soundtracks can all be delivered in their true, lossless forms. Thanks to the vastly increased bandwidth of HDMI 2.1, eARC can handle 32 channels of uncompressed audio, or up to 8 channels of 24-bit/192kHz audio. The TV’s audio-processing capabilities are no longer a limitation because eARC requires a no-compromise approach to compatibility. Whether the audio signal is coming from the TV’s internal apps, or from a blu-ray player or gaming console connected via HDMI directly to the TV, eARC will deliver the full-resolution sound signal back to the receiver, pre/pro, or sound-bar.

Another benefit of eARC is that the technology incorporates an improved “handshake” process between compatible devices. The original ARC protocol relied on HDMI CEC (Consumer Electronics Control), which often had to be activated manually, and didn’t always work properly. The new eARC standard obviates the need to activate CEC, allowing the user to get up and running without extra steps. It also means that the user should be able to control various functions on various devices (such as turning on the TV’s power, and then adjusting the receiver’s volume) without using multiple remotes. This functionality was supposed to be possible with the original version of ARC, but it wasn’t really ready for prime time before the arrival of eARC.

As mentioned above, eARC does not require the new Ultra High Speed HDMI Cable. According to HDMI.org, a standard HDMI cable with Ethernet will work, as will a High Speed HDMI cable with Ethernet. Some very old (and/or very long) HDMI cables might struggle because of the extra bandwidth needed for advanced audio formats over eARC, but you should definitely try it out with your current cables before spending money on new ones. The bad news is that some existing ARC-enabled products may not work with new products that use eARC. It is entirely possible for manufacturers to design products that are compatible with both ARC and eARC, but backwards compatibility is not a mandatory feature. In order for eARC to work for certain, both the TV and the audio device must have compatible HDMI eARC sockets. The good news is that eARC (along with Auto Low Latency Mode and Variable Refresh Rate) isn’t strictly limited to brand-new HDMI 2.1 devices. These features, which are some of the most important parts of the HDMI 2.1 spec, can be (and have been) added to some HDMI 2.0 products via firmware updates. Some HDMI 2.0 TVs introduced in 2019 and early 2020 have already been updated to support eARC, as have AV receivers from Onkyo, Pioneer, Sony, Denon, and Marantz.

Conclusion

There’s certainly a lot of information to unpack in the HDMI 2.1 specification, but the main takeaway is that the spec will offer improved audio and video, along with an improved user experience. It remains to be seen how many HDMI 2.1 sources will arrive in 2020, or even in 2021, given the current state of the industry within the context of the global COVID-19 pandemic. CES 2020 was packed with HDMI 2.1 products, but many manufacturers have faced significant delays in their production and shipping schedules. Still, there are already products out there that are ready for the future. All of LG’s 2020 OLED TVs have HDMI 2.1 ports, as do two of Sony’s top LCD TV’s, and Samsung’s flagship 85-inch Q950TS 8K QLED TV. Yamaha has said that its 2020 receiver lineup will support HDMI 2.1 and will be available in time to handle the sound for your new Playstation 5 and Xbox Series X games this holiday season. (Again, check each product’s feature list to make sure it supports the features that you care about!)

Are you planning to upgrade your system to support HDMI 2.1? Which features interest you the most? Share your thoughts in the related forum thread below.

Acknowledgement

Many thanks to Phil Jones from Sound United for supplying the HDMI 2.1 education in our YouTube interview series.

For more information, please visit the Sound United Training Channel.