Does Bitrate Dictate Streaming Video Quality?
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Learn more about streaming quality, players, and codecs at Streaming Media East Connect 2021.
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Will Law: There's more to quality than bitrate. Now, we're talking user-perceived quality. So, say your CDN is doing great and your player's great. And it's showing content to an end user. What's good quality and bad? A lot of the press and media associate bitrate with quality--more bitrate gives me better quality. And even if you look at player APIs, there's an API that doesn't say switch bitrates; it says switch quality level, which I find curious in 2021 is still there.
There are many examples where more bitrate is not better quality. I'll give you two. One is, there's a choice in the playlist manifest. You've got 1080p30 at 10 10Mbps with the AVC codec, or you've got 1080p30 at 8Mbps with the HEVC codec. Which one should you choose? The one with more bitrate? No. Your HEVC is going to give you better quality because the codec is that magical parameter that translates the bits, crossing the wire into the visual quality that you receive.
So another example is, you're sitting on your couch watching your 60-inch television, and you've got a choice between 4k at 16Mbps or 1080p at 8Mbps. Which one you choose? Well, it's a trick question, because they're both going to look the same to you. But one costs the provider twice as much as the other to send to the TV. So the players shouldn't choose the more expensive one if it knows the user can't perceive any quality benefit from that.
So there are many dimensions to visual quality and I'll go through some of the other ones. Everyone knows bitrate. We just discussed that.
Resolution can help. 4k is better than lower K as long as you're close enough and your television has enough visual DPI to consume it.
You might look at frame rate. 60 fps is becoming popular, certainly for sports. It's a real differentiator. Now you can actually see the tennis ball moving instead of just imagining yellow blur.
Color volume gives you much clearer colors going from Rec. 709 up to Rec. 2020. Color depth--eight-bit, graduations everywhere; 12-bit, much smoother dynamic range. We're getting used to having some notion of high dynamic range, be it static or dynamic metadata for that. But you know, standard range doesn't look so good anymore, and rather dark.
And then we throw on audio as well. Next-gen audio and object-based audio certainly create a more immersive experience.
So those are all multiple dimensions of perceived quality that are totally separate from bitrate. And they have to be taken into account from a player. People are trying to work on no reference metrics to estimate from the player's perspective, what would the user see? Is it better that I show a smaller video with a different codec? If I turn my phone around, change the aspect ratio, it has to go through a lot of complex calculations, and they're not simple ones for the player to make. So the notion of perceived quality is a much more complex one than this linear relationship to bitrate.
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