Buying a TV with HDMI 2.1 does not guarantee that you will be able to take full advantage of the new PlayStation 5, Xbox X Series consoles, or the latest graphics cards. Now, if that wasn’t enough, a new version of the video standard is appearing on the horizon: HDMI 2.1a.

It’s just that the way TV manufacturers are implementing HDMI specifications is confusing. And, unfortunately, it’s not a whim of the companies, but a set of guidelines set by HDMI Licensing Administrator, Inc, the organization that licenses and oversees the video standard.

The mess of versions and features

Since HDMI is not an open standard, manufacturers interested in adopting it must have a license, and this one comes with a very strict set of rules. One of them states that newly certified products must have the latest version. This is because older versions are superseded and are no longer available.

However, (and this is where the problems begin) manufacturers are not obliged to implement all the features of a version for it to be considered compliant. For example, all (yes, all) of the new HDMI 2.1 features are now completely optional.

This means that feature support can vary from device to device, even within the same version of HDMI. You can have an HDMI 2.1 connector, but it may not support a single one of the features that were released with that version, i.e. it could be a sort of HDMI 2.0 in disguise.

FEATURESHDMI 2.0 – 2.0BHDMI 2.1
FULL HD BLU-RAY DISC AND HD DVD VIDEOYesYes
CONSUMER ELECTRONIC CONTROL (CEC)YesYes
DVD-AUDIOYesYes
SUPER AUDIO CD (DSD)YesYes
AUTO LIP-SYNCYesYes
DOLBY TRUEHD / DTS-HD MASTER AUDIO BITSTREAM CAPABLEYesYes
UPDATED LIST OF CEC COMMANDSYesYes
3D VIDEOYesYes
ETHERNET (100 MBIT / S)YesYes
AUDIO RETURN CHANNEL (ARC)YesYes
4 AUDIO STREAMSYesYes
2 VIDEO STREAMS (DUAL VIEW)YesYes
PERCEPTUAL QUANTIZER (PQ) HDR OETF (SMPTE ST 2084)YesYes
HYBRID LOG – GAMMA (HLG) HDR OETFYesYes
STATIC HDR METADATA (SMPTE ST 2086)YesYes
DYNAMIC HDR METADATA (SMPTE ST 2094)NOOptional
ENHANCED AUDIO RETURN CHANNEL (EARC)NOOptional
VARIABLE REFRESH RATE (VRR)NOOptional
QUICK MEDIA SWITCHING (QMS)NOOptional
QUICK FRAME TRANSPORT (QFT)NOOptional
AUTO LOW LATENCY MODE (ALLM)NOOptional
DISPLAY STREAM COMPRESSION (DSC)NOOptional

As we explained in detail a few days ago, the Xiaomi Fast LCD Monitor is the clearest example of this situation. The device states that it has two HDMI 2.1 ports and, although no one can deny it, these connectors offer HDMI 2.0 features.

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Now, this same scenario will be repeated with the arrival of HDMI 2.1a. Once the specification is available, it will replace HDMI 2.1. New TVs will, by default, come with “the label” of the new version, although the only feature associated with it may not be included by many manufacturers.

HDMI 2.1a focuses on HDR

HDMI 2.1a will be a minor update, but it will arrive with an interesting feature that seeks to balance the HDR content playback experience in all possible scenarios. This is source-based tone mapping, also known as SBTM for short.

SBTM

This feature will add additional capabilities to existing dynamic range display systems and will process some of the content on the source device delivering optimized images to the target displays and avoiding the user having to perform manual calibrations.

Also, instead of adopting a fixed set of color and brightness ranges, SBTM will be dynamically adaptable. As a result, it will deliver better results in mixed content playback. For example, when video thumbnails are HDR and other on-screen elements are presented in SDR.

As The Verge notes, HDMI 2.1a will be unveiled at CES 2022. The organization in charge of the standard will allow devices sold with HDMI 2.1 to enjoy the benefits of HDMI 2.1a. However, for this to happen, device manufacturers will have to allow a firmware upgrade.

Keeping an eye on the fine print

The present feature salad of HDMI 2.0, HDMI 2.1, HDMI 2.1, and 2.1a leaves us with a clear lesson: we cannot rely on the connector version when buying a new TV or monitor.

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However, the standard’s adoption agreement obliges manufacturers to list which features are supported by the connectors included in their devices, in addition to indicating the version of the specification in question.

In that sense, we will have to take the trouble to verify whether the HDMI features we are interested in are supported, although the information may be hidden behind an asterisk (*) and in small print.

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