HDMI (High-Definition Multimedia Interface) is the most popular medium for transporting both audio and video information between two digital devices. In the past two decades, HDMI technology has evolved from HDMI 1.0 to HDMI 2.0. In 2017 HDMI 2.1 introduced enhanced gaming and media features such as Variable Refresh Rate (VRR) and Auto Low Latency Mode (ALLM) to eliminate lag, stutter, and tearing, adding smoothness to the gaming and video experience. Recently the HDMI Forum has announced a new version, HDMI2.1a, that brings a standout gamer-friendly feature, Source-Based Tone Mapping (SBTM).
What is SBTM?
In our previous blog, Dynamic HDR in HDMI 2.1: The Ideal Display for TV and Mobiles, we shared how High Dynamic Range(HDR) enhances video with a wide range of rich colors, from the deepest of blacks to the brightest of whites. The Sink reproduces the HDR video on the screen with the help of color information (static and dynamic metadata) sent by the Source. SBTM is a new feature that allows the Source device to share a load of tone mapping HDR, which previously was solely managed by the display.
What are the limitations of Sink tone mapping?
1. Not all displays are alike in terms of the luminance range support for HDR.
The color range and levels of brightness will vary from display to display. But for HDR video, the display will still map HDR content beyond its capabilities.
2. The metadata information might not always be useful, especially in a scenario where different kinds of content such as Static Dynamic Range (SDR), HDR, dynamic HDR, and graphics are combined.
Consider any streaming website like YouTube, Netflix, or Amazon Prime. The image above shows the home page layout of the Netflix account. It consists of the main video preview, video thumbnails, and different On-Screen-Display (OSD) menu options. The video preview might be dynamic HDR, the video thumbnails might be SDR or HDR and the OSD menu will be graphical in nature. The graphical menu should not flicker in response to the dynamic metadata associated with the HDR video.
Another example of composite content is the gaming screens which consist of the game along with a camera window to display the gamer’s video image and a chat window. Here, the game might be in HDR, the camera window will be SDR and the chat window is graphical.
3. Picture control options provided through Source devices are confusing for users.
The user control options are confusing because the user does not understand whether to adjust the manual options in the Source device or the Sink device. For e.g. to get the desired picture quality, the average TV user will be confused about whether to adjust the options in the set-top box (Source) or the TV(Sink).
How does SBTM help?
SBTM allows the Sink device to indicate to the Source about the target color volume it can receive. This helps the Source to optimize the image with respect to the Sink’s capability. Thus, the Source adapts to a specific display instead of adopting a fixed color range or brightness level. The Source uses a new SBTM extended metadata to inform the Sink about the mode, type of video, minimum, and peak luminance, used by Source during tone mapping. SBTM will also be useful in a multi-windowed PC, where one window can display HDR while another can simultaneously display on-screen graphics.
For SBTM to work, both the Source and the Sink devices should support SBTM. The good news is that this feature will be available via a firmware update to many devices.
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