When you change the rules don’t forget to go back to the why not the what!
By Rick Clucas, Head of Innovation, V-Nova
What is an Adaptive Bitrate ladder?
The clue is in the name! An Adaptive Bitrate (ABR) ladder is a list of encoded video streams at different bitrates allowing a player to play video to match the bandwidth it has available.
Today when people talk about ABR profiles you would think they are talking about Adaptive Resolutions as they will ask “what bitrate you can do resolution ‘x’ at?”. The whole point of ABR is to maximise the visual quality experience as you drop the bitrate. It’s true that one way of achieving this is to drop the resolution, but this is a short-sighted approach not least because the viewing resolution of the display showing the video is typically high resolution or above e.g. 1920 x 1080.
As I mentioned in my recent article, ‘The PERSEUS Plus story’ (https://www.v-nova.com/perseus-plus-story/,) we changed the rules by creating a Next Generation codec that could run on existing devices while still maintaining full compatibility with the whole ecosystem including DRM, streaming protocols, packagers etc. PERSEUS Plus is so compatible that a player that doesn’t know anything about PERSEUS Plus can still show video! But very often we find ourselves in the situation that typical ways of working have forgotten why they are doing it that way, ABR is an excellent example.
Returning to the principle of ABR
When a customer wants to define an ABR ladder what they are trying to do is deliver video across a wide range of operating points in a way which is unnoticeable or unobtrusive to the viewer. With H.264 this means trying to avoid block artefacts e.g. ‘collapsing,’ every viewer finds objectionable. Interestingly these block artefacts seem to have been a major focus of HEVC which instead ‘smudges’ the image. On first viewing, this looks better, but has the effect of removing the all-important ‘texture’ that makes an image look real so it works well for metrics but is terrible for the quality of viewing experience.
One of the biggest problems with changing the resolution to reduce bitrates is that it doesn’t really deliver video at the operating points that matter, instead it is limited by what the codec can deliver which usually means the bitrates for a good viewing experience don’t meet the bandwidth constraints of the network, limiting the reach of the service and reducing the overall quality of viewing experience for the user.
Working with customers, we have identified the bitrates that matter from various network constraints and focused our wonderful engineering team’s efforts these key operating points.
The PERSEUS Plus difference
At V-Nova, we passionately believe in maximising the overall visual quality experience. In particular, we aim to move the operating point at which the video starts to collapse as far down as possible. This can be measured subjectively or objectively, using frame-by-frame metrics. With frame-by-frame metrics it is easy to spot the points where the video will collapse as these frames will show large drops in the metric. Many people focus on the ‘average’ quality across a clip which is completely useless because a few frames of collapse will be very noticeable to the viewer but won’t show up in the ‘average’.
Consistency of the video creates a much better visual quality experience than a video that is constantly changing between good quality and bad quality which means that a video with a lower ‘average’ metric will actually look better to a viewer.
So how should we create an ABR ladder using PERSEUS Plus?
There are two ways of creating your ABR ladder. One way is to focus on creating the best quality at the bitrates that matter, which will enable you to offer a good visual quality experience to the maximum number of users, e.g.
Profile 1 1080p 2 Mbps
Profile 2 1080p 1 Mbps
Profile 3 720p 300 Kbps
Profile 4 360p 100 Kbps
The other way is where you want to maintain the same theoretical visual quality experience you have today (assumes that people can get the maximum bitrates) but want to reduce the bitrate.
With this approach the first thing to do is focus on bitrate not resolution, start with your current highest profile, e.g. 1080p at 5 Mbps, and find the frames where the metrics drop. You can now encode using PERSEUS Plus at various bitrates and again look at the frame by frame metrics. Selecting the bitrates where the ‘drop points’ match, will give your PERSEUS Plus encode a similar overall visual quality experience at a lower bitrate.
A recent customer test used the same commercial encoder with and without PERSEUS on live football content and showed that their top profile, 5 Mbps without PERSEUS was considered comparable at 2 Mbps with PERSEUS Plus. As you can see from this table, moving the top operating point down to 2 Mbps leapfrogged over the 2nd highest profile.
The 3rd h.264-only profile was 1.2Mbps at 480p. We found that 1080p at 1.0Mbps with PERSEUS Plus looked better than the 480p image and was still lower bitrate.
The rule change with PERSEUS Plus is to keep the resolution as high as possible and drop the bitrate to meet your operating points that matter.