HDMI 2.1a to Address HDR Variances With Source-Based Tone Mapping

HDMI 2.1a introduces source-based tone mapping (SBTM), an HDR feature that allows a portion of the HDR mapping to be performed by the source device in addition to the tone mapping done by the display device, said HDMI Forum President Chandlee Harrell. SBTM enables the source to adjust its video output to “take better advantage of the display’s potential.”

Harrell noted current TVs are capable of displaying far wider ranges of color and brightness than the CRT displays of the 20th century. HDR is used to map colors of both standard and high dynamic range content into “immersive” color images, he said. SBTM enhances video, giving a wide range of colors to provide deeper blacks, brighter whites and details in shadows and bright areas. It also provides a broader color palette, he said.

SBTM addresses the disparate HDR capabilities in the market, said Harrell. Not all displays have the same HDR capability, he said, referencing varying ranges of color and brightness levels. “That’s fine when it’s one video stream of HDR video such as HDR10 and HLG, because the television will typically map that HDR content into the display’s capability to get the maximum effect for the color and contrast,” he said.

But today’s content has a lot of mixed imagery, which could be a video stream mastered in HDR or older content in SDR, dynamic HDR, graphics or a user interface, Harrell said. All are transmitted from the source to the display at the same time. “This can provide some difficulty for the one-stream HDR that has been available to date,” he said.

SBTM allows the source to combine different content by sending a video signal that takes advantage of the display’s capabilities, taking all the information into account, Harrell said. “Rather than using a fixed set of color and brightness ranges for the aggregate content, it uses a separate set of parameters for each of those different content types.” That allows the source to adapt the content to different displays’ capabilities, he said. With gaming, PCs and game consoles have required the user to manually adjust content to optimize such issues for HDR, he said.

All types of set-top boxes can implement SBTM, Harrell said, along with PCs and game consoles. In a multi-window environment, one window can display rich video while another is optimized for black text on a white background -- without the need for sliders or manual user configuration, Harrell said. The transmitting and receiving devices must support SBTM for it to work; many devices, including TVs, could become capable of supporting SBTM via a firmware upgrade, he said.

In addition to adding SBTM, the 2.1a spec incorporates three amendments and five errata that have been introduced since 2.1, including HDMI cable power, said Harrell. He also cited enhancements to the quick media switching features.

During 2022, the HDMI Forum will continue to refine and release the HDMI 2.1a spec, plan the technology road map, conduct test events and plug fests, promote the spec’s new features and the Ultra High Speed Cable program and recruit new members “to shape the future of HDMI technology,” Harrell said.

Responding to a question in Q&A on why chipset providers and TV makers on their own couldn’t solve the connectivity issues addressed by SBTM, Harrell noted that HDMI Forum is made up of many of the TV makers and chipset providers. Various companies in the forum proposed and developed the new features to “allow this kind of signaling,” he said. SBTM doesn’t make a selection about the type of image processing: “It provides the communication and transport mechanism to allow the best results from each of the different kinds of HDR standards that are out there,” he said.

Responding to a question on “fake HDMI” questions that have emerged in the marketplace, HDMI Licensing President Rob Tobias said the topic pops up when a new specification reaches the market. “Since day one, HDMI has been developed by consumer electronics and PC companies to meet their needs,” he said. Companies like the flexibility of having optional features so they can design their products to meet the needs of a particular market segment.

HDMI 2.1 was a big step forward in features, said Tobias, and gaming has quickly embraced many of those features. TV makers have a broad product range, he said, from small screens at low price points to very large screens with premium feature sets. “The manufacturers don’t want to be hamstrung”; they want to select features depending on the target customer. HDMI is just a part of the TV system, which also requires a compatible panel, SoC, memory and processing power that can handle HDR, variable refresh rate (VRR) and higher end feature sets for games, he noted.

There are different requirements for high-end TVs, such as Dolby Atmos, and in that case, a TV maker would need only enhanced audio return channel and not necessarily 4K at 120Hz, auto low-latency mode or VRR. Optional features enable the system manufacturer to decide which are most important, he said.

From the HDMI LA and HDMI Forum side, “We understand that it’s simple to say it’s HDMI 2.1, and consumers expect all the features, but reality doesn’t enable that,” Tobias said. “So we require systems manufacturers -- when they claim a version number of the spec -- to also list which of the features they support. That way consumers can be informed if they’re buying their TV set that claims HDMI 2.1, that it is supporting eARC but not some of these other features.”

To help consumers better understand which features are supported in HDMI versions, the HDMI LA spends “a lot of time educating our adopters and resellers" about requirements for listing supported features, Tobias said. “Unfortunately, not everybody does that. A lot of them take the bullet-point approach when they list the features.” In those cases, HDMI LA contacts manufacturers to “encourage them to help consumers out, identify which features are supported on 2.1 on that particular product,” he said.