Dynamic HDR in HDMI 2.1 for TV and Mobiles

The latest buzzword in the world of TVs and smartphones is High Dynamic Range (HDR). Many of us might already know that an HDR TV improves the viewing experience by offering better picture quality, just like people who use the latest smartphones know that turning on the HDR mode in the camera helps in capturing more lively pictures. In November 2017, the HDMI forum officially released HDMI 2.1 adding more to our joy, by offering the new and improved HDR. The announcement goes on to say “Dynamic HDR support ensures every moment of a video is displayed at its ideal values for depth, detail, brightness, contrast and wider color gamuts—on a scene-by-scene or even a frame-by-frame basis”. Before we explore HDR and Dynamic HDR in detail, let’s first understand how Standard Dynamic Range (SDR) works.

Standard Dynamic Range

It all started with SDR. In fact, it is technically still the current standard for video and picture displays. However, to give you a frame of reference, a 4K SDR TV is not capable of displaying the level of depth and contrast as 4K HDR TV. The 4K here refers to the horizontal display resolution of approximately 4,000 pixels. So, one might ask, if the resolution is same, what affects the contrast?

Contrast is determined by the difference between the brightest whites and darkest blacks a TV can display; measured in candelas per square meter (cd/m2). The SDR is based on the conventional Cathode Ray Tube (CRT) display, meaning it uses 8-bit color depth. An 8-bit TV will have 256 shades of red, green, and blue, so the maximum luminance allowed is 100 cd/m2.

High Dynamic Range

With HDR, there are two types of displays available – Liquid Crystal Display (LCD) and Organic Light Emitting Diode (OLED). HDR requires a minimum of 10-bit color depth, so a 10-bit TV would have 1,024 shades of red, green, and blue. The HDR display can support a peak brightness of over 1000 cd/m2 and a black level less than 0.05 cd/m2 (a contrast ratio of at least 20,000:1) or a peak brightness of over 540 cd/m2 and a black level less than 0.0005 cd/m2 (a contrast ratio of at least 1,080,000:1).

The HDR video is made possible by sending additional information along with the video. This additional information is called metadata. The metadata contains the entire color information sent to the display so that the video is reproduced on the screen precisely. It tells the display how to match the content to its own color volume limit. HDMI 2.0 supports Static Metadata, meaning the information will be applicable to every scene and every frame of the entire movie. Dolby Vision and the HDR10 are the two most popular HDR formats. The HDR10 is an open standard and prompting many TV manufacturer giants to adopt it initially.

Dynamic HDR

Even though the HDR technology is a clear improvement over the SDR technology because the metadata is static, the brightest scenes or the darkest scenes do not appear their best. Often, the images appear dark. What if the metadata changed itself dynamically for every scene or every frame? Then it would be possible to reproduce every frame as close as possible to the original image and the metadata information could vary throughout the entire content. This metadata is called Dynamic Metadata and is supported in the 2.1 version of the HDMI standard. This way, every moment is for real! The image below helps us understand how the image quality has improved from SDR to Static HDR to Dynamic HDR.