HDMI 2.1a adds source-based tone mapping (SBTM): HDMI 2.1 isn’t what it once was. The standard was created to label the latest features in audio and video. Since the end of last year that can go in the trash because hdmi 2.0 no longer exists and devices using HDMI 2.0 are now allowed to use HDMI 2.1 as a label.
HDMI 2.1, however, is now extensive with a new version; HDMI 2.1a. This version adds a new feature called Source-Based Tone Mapping (SBTM). Simply put, this means that a source device can automatically adjust its HDR output to match the capabilities of a television. It is not a new HDR format such as Dolby Vision or HDR10+ but allows communication between source and screen for optimal reproduction of HDR-material.
According to the organization behind the HDMI standard, already released devices can easily be provided with a firmware upgrade that adds Source-Based Tone Mapping (SBTM). Both the source (a player) and screen (a television) must support SBTM. It has not yet been announced whether firmware upgrades will be rolled out to televisions and players.