What Is HDMI 2.1a and Why Should You Care?

What Is HDMI 2.1a and Why Should You Care?
Photo: Adam Clark Estes/Gizmodo

The HDMI Licensing Administrator announced a new HDMI 2.1 update, appropriately called HDMI 2.1a, that is designed to improve picture quality when you’re viewing content of varying quality.

I know, I know: HDMI 2.1 is already kind of a mess. So why are there more numbers and letters to figure out? Don’t worry, though, we won’t get too deep into the technical weeds here. Let’s break down what this all means and how it can enhance your picture.

What is HDMI 2.1a?

In short, 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! First, let’s define tone mapping. This is a technique used in image processing where digital signals are matched to the proper luminance and colour of the TV. This way, you can take a high dynamic range image and present it on a monitor with a limited dynamic range while preserving details, contrast, and colours.

With HDMI 2.1, a portion of the HDRM mapping can be performed by the source device, like a set-top box, PC, or console, in addition to the mapping being done on the display.

And why does this matter?

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

When you’re dealing with HDR videos and photos, a display will typically do the mapping so content conforms to its limited capabilities. Sometimes, though, certain content will contain different levels of HDR. There might be an HDR element, an SDR (standard dynamic range) photo, and a basic graphic. This is often the case when you’re in picture-in-picture mode.

Typically, in this scenario, a fixed set of brightness and colour ranges is determined for the display. With source-based tone mapping, the source can adapt to the display by sending a video signal that is optimised for the HDR capabilities of the panel.

Give me an example.

Sure thing. So you might open a streaming app and see some video thumbnails that support HDR and others that are in SDR. That same page might have basic graphics as well. With HDMI 2.1, your streaming device can send your TV a signal that makes the best of its capabilities. The HDMI Forum says HDMI 2.1a is also designed for PCs and gaming consoles, and that it could improve image quality in a multi-window scenario where you have a video playing on one side and text on the other.

I still don’t understand.

I don’t blame you. Let me then turn to the HDMI Licensing Administrator website, which describes HDMI 2.1a in these terms:

“SBTM is especially useful in cases where HDR and SDR video or graphics are combined together into a single picture, 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 optimised HDR signal in order to maximise the utilization of the display’s HDR capabilities without manual user configuration of the Source device.”

Do my source and display both need to support HDMI 2.1a?

Yes, unfortunately, they do. The HDMI Forum says most TVs can gain the feature via a firmware upgrade, but pushing one out is up to device manufacturers.

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 spec in 2022 and is aiming at a Q1 release. We should learn more about the spec in the weeks to come.