The HDMI Organization has outlined a revision of HDMI 2.1, enabling Source-Based Tone Mapping (SBTM). Implemented in the new HDMI 2.1a standard, SBTM isn't a new HDR format; rather, it facilitates matching existing HDR content to the display you are using. Notably, the HDMI Org says that many devices can be firmware updated to support SBTM; however, both devices must be aware and compliant with the standard for it to work.
HDR technology is gaining traction, and it is welcome for the sometimes greatly enhanced, vivid, and immersive visuals it can deliver. However, if there is an obvious problem with HDR content as it stands, it is often down to the proliferation of HDR standards (e.g., HDR10, HDR10+, HLG or Dolby Vision), as well as older SDR content.
When SBTM is useful
SBTM addresses some of the issues outlined above by allowing a source device to adapt its HDR output to match the capabilities of a connected monitor or TV. Modern displays can adapt to single HDR sources that use different HDR standards by mapping the signal sent to the brightness, contrast and gamut available. However, the HDMI Org says SBTM will come into play when a source device "needs to combine different types of content (HDR, SDR, dynamic HDR, graphics) together at the same time."
Mixing HDR standards might happen more than you think on modern systems. For example, a video streaming service might seek to load thumbnails from various HDR and SDR content, as well as the menu UI. At this point, a processor needs to kick in to render and optimize the thumbnails and menu for output. Another time you might welcome SBTM, specifically on PCs, could be when working in a multi-window environment with diverse mixed-media content.
These multiple HDR standard inputs cause less format friction with HDMI 2.1a, as SBTM "enables the source to send a video signal that takes full advantage of a specific display's HDR capability by adjusting its output to take better advantage of each display's potential."
The HDMI Org goes on to mention that the new, more intelligent device-display linking technology "can also be used by PCs and gaming devices to eliminate manual user optimization for HDR." Thus, this new smarter source-device link standard offers no manual mode.
It is worth noting that the HDMI Org reckons many existing devices, including TVs, will be able to use SBTM if the manufacturer issues a firmware update. For STBM to work, both device and display need to support it. No specific HDMI cable is required to make use of STBM, but quality high-speed cables are recommended. As HDMI 2.1a proliferates, expect to see devices sporting a sticker or a mention in the specs that they support HDMI with STBM.
Always check the specs and the small print
Last week we reported on the confusion, which will probably shortly ensue due to HDMI 2.0 certification being dropped. Technically unsupported devices are already being relabelled with the HDMI 2.1 logo since they qualify by supporting a subset of the enhanced standard. Please be wary of this unintuitive change, and also watch out for goodies like HDMI 2.1a with SBTM.