The Challenging World of 4K HDR Streaming

HDR places considerable restrictions on both the screen it’s displayed on and the player doing the playback. It also restricts the method of connection.

The single biggest variable in the HDR effect is the quality of the screen. Better quality screens support more brightness levels (measured in nits) and thus are able to produce a better HDR effect.

There are three main 4K HDR standards in use today:

HDR10 is free-to-use for manufacturers. Every HDR supporting TV screen can decode it and every HDR streamer can stream it. Pretty much all HDR content has an HDR10 version of it. Just like HDR10, HDR10+ is also an open standard but offers a higher picture quality than HDR10 by adding dynamic metadata as an extra layer of information for each scene.

On the other hand, Dolby Vision is a proprietary standard licensed by Dolby, and it comes with a license fee. That license fee does include a service whereby Dolby helps with getting the most out of the technology. Because of the license fee, there is less content available, but all the major studios still support and use it.

From a technical perspective, the difference that matters between these three standards is how they deal with color and brightness metadata. HDR10 uses static metadata - color and brightness are set for the entire movie, while HDR10+ and Dolby Vision support dynamic metadata, enabling scene-by-scene and even frame-by-frame settings. Another difference is the maximum brightness. HDR10 tops out at 1000 nits, HDR10+ up to 4000 nits, while Dolby aims at 10 000 nits of brightness.

In short, HDR10+ and Dolby Vision are able to produce better-looking visuals with their use of dynamic metadata and higher brightness. Still, HDR10s static metadata produces content that is leaps and bounds ahead of standard video. The bottom line is that any HDR content simply looks better than standard-definition video, no matter the standard used.

While HDR is more common for streaming video, there is a recognized HDR standard developed by the BBC and NHK specifically for live broadcasting - Hybrid Log-Gamma or HLG for short. It’s a royalty-free standard that, unlike other HDR standards, is backward compatible with non-HDR or standard dynamic range displays.

The availability of HLG content is on the rise, with support from device manufacturers and the adoption of HLG in broadcasting major sporting events held worldwide. For example, Sky broadcasted live 4K HLG action from the Tokyo 2020 Olympics and continues to do so with the 2021/22 Premier League season.

With the massive increase in file size for 4K HDR videos comes another problem - compression for streaming. Existing codecs like the widely used H.264 are simply not good enough to enable 4K HDR streaming. The files are still too big.

This is where new codecs like HEVC (High-Efficiency Video Coding), better known as H.265, and VP9 come into play. They’re both around twice as efficient as H.264 and thus are suitable for streaming 4K HDR content. Both of these codecs support HDR10, HDR10+ Dolby Vision, and HLG.

File size and streaming support become especially important in disconnected scenarios on different vessels like airplanes with their seatback devices and BYOD (Bring Your Own Device) support. In July 2019, ANA became the first commercial airline to launch 4K TVs on airplanes. While it currently does not support HDR, it’s only a matter of time.

Besides the fact that 4K has 4-times the pixels of Full HD and thus enormous file sizes, which put constraints on physical hard drive space required, 4K HDR also has other hardware requirements. Hardware support has three sides - the screen, the device/player doing the playback, and the cable connecting them.

As a first step, the content must be available in 4K HDR by the content provider and accessible by the user. Moreover, it must also be encoded in an HDR standard that the current hardware can use.