What is HDMI 2.1a and why should you care?


Photo: Adam Clark Estes/Gizmodo

The HDMI license manager has announced a new HDMI 2.1 update, aptly named HDMI 2.1a, which is designed to improve picture quality when viewing content of varying quality.

I know: HDMI 2.1 is already kind of a mess. So why are there more numbers and letters to sort out? But don’t worry, we won’t get too deep into the technical weeds here. Let’s see what all this means and how it can improve your photo.

What is HDMI 2.1a?

Basically, it’s an updated version of HDMI 2.1 centered around a feature called source-based tone mapping.

What is source-based tone mapping?

Great question! Let’s define tone mapping first. This is a technique used in image processing where digital signals are matched to the correct brightness and color of the TV. In this way, you can create an image with a high dynamic range and present it on a monitor with a limited dynamic range while preserving details, contrast and colors.

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HDMI 2.1 allows part of the HDRM assignment to be performed by the source device, such as a set-top box, PC, or console, in addition to the on-screen assignment.

And why does this matter?

A source may send a video signal to a display that takes better advantage of that display’s HDR capabilities.

When dealing with HDR videos and photos, a display will usually do the mapping so that the content meets the limited capabilities. However, sometimes certain content contains different HDR levels. There can be an HDR element, an SDR (standard dynamic range) photo, and a base image. This is often the case when you are in picture-in-picture mode.

In this scenario, a fixed set of brightness and color ranges are typically determined for the display. Source-based tone mapping allows the source to adapt to the display by sending a video signal optimized for the panel’s HDR capabilities.

Give me an example.

Naturally. So you can open a streaming app and see some video thumbnails that support HDR and others that are in SDR. That same page can also contain base images. With HDMI 2.1, your streaming device can send your TV a signal that makes the most of its capabilities. The HDMI forum says that HDMI 2.1a is also designed for PCs and game consoles, and that it could improve picture quality in a multi-window scenario where you have a video on one side and text on the other.

I still do not understand.

I don’t blame you. I then go to the HDMI Licensing Administrator website, which describes HDMI 2.1a like this:

“SBTM is especially useful in cases where HDR and SDR video or graphics are combined into a single image, such as picture-in-picture or a program guide with an integrated video window. SBTM also enables PCs and gaming devices to automatically produce an optimized HDR signal to maximize use of the display’s HDR capabilities without manual user configuration of the source device.”

Should my source and display both support HDMI 2.1a?

Yes, unfortunately they do. The HDMI forum says most TVs can get the feature through a firmware upgrade, but it’s up to the device manufacturers to get one out.

When will I see HDMI 2.1a supported devices?

So the HDMI forum says it will continue to “refine and release” the HDMI 2.1a specification in 2022 and aim for a Q1 release. We should learn more about the spec in the coming weeks.


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