HDMI 2.1a announced, making yet another ‘standard’ mess

As if things were not already enough of a mess with the various incompatibilities involved in the supposed USB-C standard, HDMI is also getting more complicated …


HDMI (High-Definition Multimedia Interface) is a standard for transmitting video signals from one device to another. It is a very flexible protocol that can handle raw video data, as well as both compressed and uncompressed digital data. It is also backward compatible with the DVI standard that preceded it.

HDMI 1.0 dates back to 2002, and subsequent versions were focused on increasing bandwidth to handle higher resolution TVs, monitors and projectors. Version 1.4 offered support for 4K monitors, while 2.0 increases the 4K video capacity to 60Hz at 24-bit color depth. The latest current version, HDMI 2.1, added support for both 4K and 8K video at 120Hz.

Enter HDMI 2.1a

The edge reports that HDMI 2.1a is now being introduced – and it’s going to create even more confusion.

Let’s start with the good: HDMI 2.1a is an upcoming revision of the HDMI 2.1 stack and adds a great new feature, Source-Based Tone Mapping or SBTM. SBTM is a new HDR feature that transfers some of the HDR tone mapping to the content source (like your computer or set-top box) along with the tone mapping that your TV or monitor does.

SBTM is not a new HDR standard – it is not here to replace HDR10 or Dolby Vision. Instead, it is intended to help existing HDR setups work better by letting the content source better optimize the content it sends to the screen or by eliminating the need for the user to manually calibrate their screens to HDR by letting the source device configure the content for the specific Display.

And a whole new mess

HDMI 2.1 is already a mess – and that’s because you have no idea what you’re getting when you buy a device or cable that claims to support it. In theory, version 2.1 means you get support for 10K resolution and variable refresh rates – but in practice it may not. This is because, like USB-C, only one label is used for any device that supports it anyone of the possibilities in HDMI 2.x.

According to the HDMI Licensing Administrator, now that HDMI 2.1 exists, there is no HDMI 2.0 standard anymore: All new HDMI 2.0 ports should be bundled in the HDMI 2.1 branding, even though they do not use any of the new features included in the “new” 2.1 standard.

The same will be the case with HDMI 2.1a.

HDMI 2.1a will work in a similar way: Once the standard is released, all new ports will in theory be labeled HDMI 2.1ai according to the rules of the HDMI Licensing Administrator – but they do not have to offer the new SBTM or even all HDMI 2.1 features .

HDMI Forum’s argument is that this is always how its standards have worked, and that optional features allow manufacturers to have flexibility in what functionality they offer (an entry-level set, for example, probably does not need ports that supports 8K 120Hz VRR games). And the group says companies are required to indicate what features their hardware supports, so it’s clear to customers what their hardware is capable of, beyond the number of expectations.

Which is nice.

The 2021 MacBook Pro received an HDMI port as part of the redesign, but is limited to HDMI 2.0. This also applies to Mac mini (from 2018) and 2019 Mac Pro. Older Macs with HDMI ports offer HDMI 1.4.

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