Published on 26th Jan, 2022 by Cable Chick

HDMI 2.1 Essential Features Explained

HDMI 2.1 Essential Features Explained

Check out the impressive new capabilities of HDMI 2.1 hardware, the fancy cables needed to link them, and learn who will benefit the most.

HDMI 2.1 Feature Review

Don't be fooled by the seemingly insignificant number change from HDMI 2.0 to 2.1. What might seem to most people a minor revision actually hides a big leap forward in functionality that will benefit most home theatres - even those that will likely never see 8K content. This upgrade requires a new Ultra High Speed cable that you'll need to enjoy all those tasty features.

The Big News

By far the most important improvement of HDMI 2.1 is the greatly increased bandwidth over HDMI 2.0. Up from 18Gbps to 48Gbps, this additional data can do more than just push higher resolution pictures. While 8K movies, TV or even gaming content may be a fair way off for the general public, HDMI 2.1 allows us to combine more high-end HDMI features together to improve next-gen 4K sources right now.

In the old HDMI 2.0 era, hardware often touted many glorious features, like 4K, 120Hz, HDR, chroma 4:4:4, RGB and 16-bit colour. But 18Gbps wasn't enough to drive all of these technologies at once, and you often had to choose a compromised mix of features. Even 4K60 with HDR (a sweet spot for gaming) was only possible with YUV420 compression on last-gen games consoles. This is no longer the case with HDMI 2.1.

Ultra High Speed HDMI Certification Sticker from Avencore

The New HDMI Cable

HDMI 2.1 now requires Ultra High Speed cables capable of 48Gbps. With such a high bandwidth requirement compared to older revisions, have developed a certification programme to help consumers make informed cable purchasing decisions when performance is critical. You can spot a certified HDMI cable by the hologram sticker on the packaging, and scan it with the Certification app using your smartphone to verify it.

Not all 48Gbps cables include or require the certification, and many reputable brands have offerings that save on the cost of certification to offer a better price point. So long as you trust the brand, the certification is not essential to achieving 48Gbps. Soon, we will see hybrid fibre optic HDMI cables which will exceed the 5m limitation of copper 48Gbps cables, and these too will not be eligible for certification due to the nature of the technology that drives them.


Gamers Get the Goods

With the video gaming industry already out-performing Hollywood, HDMI 2.1's biggest feature push caters mostly to gamers. HDMI is now comparable to DisplayPort for PC gaming features, and has opened up the taps for games consoles to enjoy many of the same bells and whistles their PC counterparts have been used to for years. Your Xbox Series X and PlayStation 5 will thrive when paired with a HDMI 2.1 display thanks to 4K60+HDR now available together at YUV444.

While we don't expect to see 10K gaming monitors or graphics cards which can drive them any time soon, 5K may become the de-facto standard in the high-end desktop gaming space. The leftover bandwidth can then be leveraged for 'quality of life' features like HFR, VRR and ALLM which will improve gameplay feel and image fidelity across the board to make HDMI TVs a suitable option alongside DisplayPort gaming monitors.


High Frame Rate (HFR)

High Frame Rate Diagram (Simulated)
High Frame Rate means smoother gameplay and faster reaction times

24fps is fine for movies and TV, but gamers benefit greatly from much higher frame rates. 60 is considered the minimum for smooth gaming, and rates as high as 240 are favoured for competitive play where fast reaction times are critical. HDMI can now deliver these frames at matching refresh rates, with 4K reaching 144Hz (240 with DSC compression) and 5K at 60Hz (120 with DSC).


Variable Refresh Rate (VRR)

Until now, a discrepancy between your game's frame rate and the display's refresh rate caused screen tearing (the top half of the old frame and the bottom half of the new frame displayed together) which was very apparent during fast gameplay. Variable Refresh Rate is the HDMI tech (similar to FreeSync or G-Sync) which allows a display to show newly rendered frames as they are created. Basically, it matches the refresh rate of the screen to the ever-changing in-game FPS.

VRR (in any form) is also preferable to Vsync, which creates input lag by holding or skipping new frames to deliver them when the refresh occurs. VRR can't introduce input lag - it can only reduce it.

Variable Refresh Rate Diagram (Simulated)
Tearing occurs when the frame rate and refresh rate do not match


Display Stream Compression (DSC)

If you're familiar with compressed audio or compressed JPGs, you may not like the sound of Display Stream Compression. However, DSC is different. It is visually lossless. While not perfect, it offers a better fidelity image than chroma sub-sampling, and runs so fast it only introduces a single scanline of latency per frame (less than 8 micro seconds). This means that not only is the image visually comparable to the uncompressed source, but you won't notice a difference in lag either, so it should be suitable for high-res gaming if we see things like 5K 120Hz monitors.

Chroma Subsampling Diagram (Simulated)
Chroma Subsampling is a type of compression that favours luminance data over colour data. In Movies and TV, the differences can be imperceptible from the couch, but for gaming and desktop use, using Chroma Subsampling to save bandwidth comes with a major drop in image fidelity due to the lossy compression used on the colour data.

Dynamic HDR

Dynamic HDR Diagram (Simulated)
Dynamic HDR can change over time, between scenes or even frames

HDMI 2.0 already supported High Dynamic Range, but only with static metadata - meaning there was one HDR 'look' for the whole piece of content. Dynamic HDR (such as HDR10+) allows each scene, or even each frame, of a movie or game to have different HDR data applied in real-time. As displays improve with the likes of QD-LED and MicroLED technologies, TVs and Monitors will offer greatly improved HDR visuals for the content that supports it.


Automatic Low Latency Mode (ALLM)

Almost all HDMI TVs feature sophisticated image processing like frame interpolation, upscaling and colour correction to improve the look of live action video from traditional sources like TV stations and Blu-rays. This processing takes time, and for pre-recorded and broadcast footage the audio is synced up and you're none the wiser. For gaming, this delay is noticeable because it sits between your controller input and seeing its effect on-screen. Many high-end TVs offer a dedicated 'gaming' mode you can select to skip most of these processes and reduce the delay, but ALLM will put control into the hands of the source hardware so the lowest latency mode is 'forced' when running gaming content.


Enhanced Audio Return Channel (eARC)

The old ARC could only handle 5.1 compressed audio streams. Now, HDMI 2.1's eARC can transmit uncompressed 5.1 and 7.1 channel audio, as well as technologies like Dolby Atmos and DTS:X in their original forms. This means better audio format support across the board for home theatres with high-end surround sound or multi-speaker Atmos setups. If your speaker setup was limited because you were using ARC, you'll now be able to invest in a more capable sound system and enjoy its benefits to the fullest.


Movies, Pre-Recorded and Broadcast Video Content

Apart from eARC, HDMI 2.1 sadly doesn't include much that will impact the current crop of Blu-rays, DVDs, free-to-air TV and streaming services.

They never really came close to the ceiling of even the old 10.2Gbps bandwidth limit. Live action video encoded with 4:2:0 subsampling running at 24 frames per second was relatively cheap, with even 4K movies using under 6Gbps with that compression.

The benefit of this low bandwidth requirement was basic cables could run up to 20m long and stay reliable, which is something HDMI 2.0 and 2.1 can only match with more expensive fibre optic cables.

Until we see a truly new Blu-ray type format with bigger images and Dynamic HDR as standard, HDMI 2.1 doesn't change much when it comes to Movie Night or Netflix Binging.



There's a lot to unpack with HDMI 2.1 and the future of 8K. If you have any questions or comments, please contact us and we'll do our best to help.

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