HDMI Data Rates for 4K HDR

By David Meyer posted 4 days ago

  

Last year I wrote a blog about HDR and implications of delivery through HDMI. But then I just recently received a great message from Simon Fulstow of Sona Projects in the UK, which prompted the need for further clarification. He asks;

I wonder if you could guide me to a resource (or resources) where I could really clarify the various video formats and data rates specifically as applicable to HDR and Dolby Vision. I understand the concept of resolution, refresh rate, colour depth and chroma sampling but am lost how this applies with HDR added in and how to apply this to video distribution products ... which claim to support video formats and resolutions that don't seem possible based on the data rates required.

We have a project at the moment where the client wants to ensure his system is Dolby Vision compatible - but trying to establish products that are, is proving to be an absolute minefield. [Some claim to] support 4K60, 4:4:4, HDR (which if I assume is 10bit has a data rate of just under 23Gbps) but not Dolby Vision (even though we could have a 4K60, 4:2:0: 12bit signal with a data rate of under 14Gbps) - Does Dolby Vision require a certain chroma sampling..?

Thank you Simon! The fundamentals of our resulting conversation we both thought were worth sharing. But then since we had this conversation I've seen two more products highlighted in industry newsletters claiming to support 4K/60 4:4:4 with HDR, one of which included Dolby Vision. The problem with such claims is that they imply each of these parameters - 4K, 60fps, 4:4:4 chroma subsampling and HDR - are available together,  but the current generation of products claiming such support can only do so with 3 of the 4 features at any given time. That is, 4K/60 4:4:4 but drop HDR, or 4K/30 4:4:4 with HDR (drop to 30fps), or 4K/60 4:2:2 or 4:2:0 with HDR (drop the 4:4:4).

So what to do?

Firstly, recapping the previous blog, there is no 8-bit HDR. The minimum bit depth required for HDR is 10-bit. That's the "10" in HDR-10 or Samsung's HDR-10+. Dolby Vision is optimized for 12-bit, but can also work (really well) at 10-bit. Secondly, HDR including Dolby Vision can work with 4:4:4, 4:2:2 or 4:2:0 chroma subsampling.

In summary;

  • HDMI products supporting up to 9Gbps (maybe stated as 10.2Gbps High Speed) can support 4K/30 HDR with 4:2:0 or 4:2:2 only. HDBaseT is fundamentally compatible too.
  • HDMI products supporting up to 18Gbps (those advertised as "4K/60 4:4:4") can also only support 4K/60 HDR with 4:2:0 or 4:2:2 only. 
  • There are currently no products to support 4K/60 4:4:4 with HDR.
  • Formats operating above 9Gbps but less than 18Gbps require compression to work through HDBaseT. And here's the catch - the VESA Display Stream Compression (DSC) system that's employed is some products can't (yet) support 4:2:0. Hmmm.

I've created a Google sheet to show data rates with different combinations. See the screen grab below, or check it out the original sheet here

Conclusion

HDMI 2.1 products will come out in due course, which will resolve this issue. But until then if you have a client asking for Dolby Vision support, you should advise them that installing CATx &/or fiber infrastructure can provide a possible upgrade path when required. If pulling long length active cables, including HDMI AOC, be aware of its data rate limit as most can only support 18Gbps. Being aware of the infrastructure capability could unlock the option for not only 4K/60 4:4:4 with Dolby Vision, but even 120fps or 8K!

As for hardware, up to 30fps UHD Blu-ray with Dolby Vision is no problem as long as your UHD-BD player doesn't convert the output to 4:4:4, rather outputting the native 4:2:0 as on the disc. Gaming systems such as X-box One S/X do have a menu option to change output to 4:2:2 to give a free ride (from 8- to 12-bit) for HDR-10 or Dolby Vision content. 

If anyone has any questions, feel free to post a message on the forum, or contact me directly.


Google Sheet screenshot
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