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8K Compression and UX Tests by bcom

Last week, Zaxel announced the delivery of a Zaxtar 5 to b<>com so that the latter could undertake some 8K testing on its technology. So naturally, we were intrigued and spoke to Jean-Yves Aubié, Manager of Advanced Media Contents Lab of b<>com, to find out more.

Zaxel Systems is a US-based company with offices in Japan that started in 1998 as a 3D video specialist but has more recently shifted towards professional video, including solutions to manage uncompressed 8K video. The Zaxtar 5 that was the subject of their press release is used in the b<>com tests as a server of uncompressed 8K streams.

b<>com is a private French R&D outfit working across many markets, including the creative industries and our video sector. Of their many areas of expertise, we noted the b<>com solution for the automatic conversions between SDR and HDR and some cutting-edge technology on what they call “Variable Frame Rate”. The idea here is only to include the extra frames – say between 30 and 120 fps – only when they provide a noticeable difference, thus saving up to 35% of storage space for lightly compressed or uncompressed video. Frustratingly for us, these two solutions are not yet commercially available for 8K video, but the 8K Monitor will be the first to let you know as soon as they are.

(C) Fred Pieau

b<>com’s initial interest in 8K focuses on compression. They are looking into 8K delivered with two streams—a 4K base layer and an enhancement layer. The company typically resells the “how-to” to encoder vendors.

Aubié told us that they are using an AI approach to create the enhancement layer. He believes that 4K resolution will become more and more common in the coming years, especially with the terrestrial network of their home market of France, which after much testing in 2020 and 2021, should be scheduled to carry commercial 4K channels from 2022-2024. This offers the opportunity for an enhancement layer delivered OTT, or by embedding metadata in-band. Indeed, once a critical mass of 8K TVs is present in French households, there might be new services and enough demand from something “extra” beyond upscaled 8K, and that’s when the OTT enhancement layer would make sense.

The Zaxel server is the starting point of the testing workflow b<>com has set up. An AstroDesign device is then used to convert 4 HDMI 2.0 inputs into an HDMI 2.1 output connected to an 8K Sony TV. Aubié told us of a trick they found to avoid the TV potentially downscaling to 4K followed by upscaling back to 8K by using the TV screen’s “photo” or “graphics” mode.

The testing underway has three distinct goals.

  1. To evaluate the compression gains in using VVC over HEVC with 8K and confirm (or not) that they represent 40% to 50% improvements for the same visual quality,
  2. Determine the required bitrate for transparency, i.e. for there to be no visible differences before and after the encoding,
  3. Confirm that regular viewers can see the difference between 4K and 8K resolutions and measure that difference.

The test will use the recognized BT500 standard and DSCQS methodology for the results to be recognized by other researchers. In addition, the subjective test will use SDR content in the same spirit of industry recognition, which we regret as that doesn’t represent the full 8K experience.

Most of the 8K content comes from the Japanese organization ITE, so many of the clips are the same as those seen in recent NHK booths. Fraunhofer has also supplied a few of the test clips.

Aubié told us that there would not be much content with fast movement. When we discussed this further, he agreed that the optimal refresh rate for 8K would eventually be 100/120 fps but it is hard to produce fast-moving content in 8K now. The fundamental limitation is in the lens technology and the big sensor size. Most of the time, shooting with 8K cameras will be a real challenge due to a very shallow depth of field. It is therefore challenging for a camera operator to track a fast-moving object correctly. This limitation is visible in those rare demos you may have seen, usually of 8K sports, where there are often brief moments in which the subject is out of focus. 

The 8K Monitor sees the testing initiative by b<>com very positively and is looking forward to reporting the first results in the fall, so stay tuned.