4K Streaming: Guessing at Quality
By Chris Heinonen on
CES 2014 provided many announcements about 4K streaming services coming to us this year. The largest player is Netflix, as they have the largest subscriber base, but Amazon, Comcast, DirecTV, and more jumped onto the 4K streaming bandwagon. While these announcements are often light on the details, Netflix came out at the Sony press conference to say that you only need 15 Megabits/second to provide 4K streaming. With that in mind, I wanted to look at what sort of performance we might expect to see.
When it comes to video today and on 4K, there are three primary compression codecs that come into play: H.264, H.265, and VP9. Almost every Blu-ray uses H.264 compression (also commonly referred to as MPEG-4) while older DVDs use MPEG-2. H.264 provides good video compression but not enough for 4K content right now. So we have two new formats, H.265 and VP9.
H.265, also called High Efficiency Video Coding (HEVC) is the successor to H.264. It provides better compression ratios without a loss in image quality. You can fit more data into the same size file and not lose picture quality with it. VP9 is a competitor to H.265 and promoted by Google. Unlike H.265 it is free to use which can make it attractive to companies.
At the Sony press conference, they said they would use VP9, but everyone else has stated they are like to use HEVC. We can look at both and see what the difference is. The key for 4K streaming is that it has to provide a better picture than a current Blu-ray disc does. If 4K targets those that want the best image, and it can’t provide an image better than Blu-ray, is there much reasoning behind it?
Looking through recent popular titles, the average video bitrate for Blu-ray discs ranges from 23-28 Megabits/second. That’s well above the 15 Mb/sec that Netflix mentioned for 4K. Now if we assume that 4K discs will use the same bit depth for image data (8 bits per pixel) we have four times as much data to store as a Blu-ray image. Of course future 4K content that follows the Rec. 2020 standard will need 10-12 bits per pixel, which is 50% more data on top of that!
So now we have a video bitrate of 92-112 Mb/sec that we need to stream. This is where HEVC comes into play as it offers better compression than H.264. How much better? Going off the research here, which compares them along with VP9, we see 39-43% reductions in bandwidth. Even if we take the 43% number, that is 40-48 Mb/sec to get Blu-ray quality images, streaming, for 4K with HEVC.
Netflix wants to use 1/3rd of that amount. So what is this going to mean? More artifacts in your image compared to a Blu-ray disc. Macroblocking, those large colored blocks you often see in shadows, will be even more common. Fine textures and facial details won’t be there. Gradients will show banding and other flaws. You might get some objects that are sharper due to the 4K resolution, but you’ll see far more flaws in the image as well.
For a test, I’ve added two images from a Finding Nemo Blu-ray. One is compressed, and one is compressed to use 1/3rd the bitrate of that one. To show the detail I’ve zoomed in to 200% on a part of the image. Dropping that bitrate leads to blocky areas and a loss of fine details. While we are not certain how Netflix will look, this provides an idea of what that kind of bitrate reduction can cause.
VP9 looks even worse from this research. It does not offer a benefit over H.264 in their testing, and takes longer to process as well. Looking at data from Google on it, it seems to do really well at tiny bitrates (0.38 Mb/sec) compared to H.264 but that has nothing to do with the 4K bitrates that Netflix is looking into. It might be free of patents and royalty fees, but it doesn’t have the quality to make it a reasonable alternative. I should also note that none of these bitrates include audio, much less lossless audio. Blu-ray will still have every service beat here as lossless audio can take up 0.4-0.6 Mb/sec of data that would lead to even worse image quality.
Of course, you need the ability able to stream 15 Mb/sec continuously to see this at all. I have a 30 Mb/sec cable connection and have trouble streaming Netflix past 480p at nighttime. I can see 1080p SuperHD sometimes during the day, but not always. I can’t imagine a 15 Mb/sec 4K streaming being more reliable that this is. People with access to FIOS or Google Fiber are likely fine, but most people are likely stuck with 1080p images or worse at peak viewing hours.
Better Than Blu-ray? Probably not.
We should see 4K streaming this year and people will have something to show on their new TV. Will we see a picture better than Blu-ray? I doubt it. It seems that the Blu-ray group is finally getting together for a 4K standard to release later this year potentially, but what it will include we do not know. Maybe it will have support for Dolby Atmos and 21:9 screens to really target the videophile crowd as it knows most people will just opt for Netflix and its competitors. Or maybe it will have support for Dolby Vision or Rec. 2020 to offer features that 4K streaming cannot. Whatever it has I imagine will look better than any streaming we can get, I just don’t know if most consumers will care.