It’s all very good saying “let’s implement CMAF”, but what’s implemented so far and what can you expect in the real world, away from hype and promises? RealEyes took the podium at the Video Engineering Summit to explain.
CMAF represents an evolution of the tried and tested technologies HLS and DASH. With massive scalability and built upon the well-worn tenants of HTTP, Netflix and a whole industry was born and is thriving on these still-evolving technologies. CMAF stands for the Common Media Application Format because it was created to allow both HLS and DASH to be implemented in one common standard. But the push to reduce latency further and further has resulted in CMAF being better known for it’s low-latency form which can be used to deliver streams with five to ten times lower latencies.
John Gainfort tackles explaining CMAF and highlights all the non-latency-related features before then tackling its low-latency form. We look at what it is (a manfest) and where it came from (ISO BMFF before diving in to the current possibilities and the ‘to do list’ of DRM.
Before the Q&A, John then moves on to how CMAF is implemented to deliver low-latency stream: what to expect in terms of latency and the future items which, when achieved, will deliver the full low-latency experience.
Squeezing streaming latency down to just a few seconds is possible with CMAF. Bitmovin guides us through what’s possible now and what’s yet to come.
CMAF represents an evolution of the tried and tested technologies HLS and DASH. With massive scalability and built upon the well-worn tenants of HTTP, Netflix and a whole industry was born and is thriving on these still-evolving technologies. But the push to reduce latency further and further has resulted in CMAF which can be used to deliver streams with five to ten times lower latencies.
Paul MacDougall is a Solutions Architect with Bitmovin so is well placed to explain the application of CMAF. Starting with a look at what we mean by low latency, he shows that it’s still quite possible to find HLS latencies of up to a minute but more common latencies now are closer to 30 seconds. But 5 seconds is the golden latency which matches many broadcast mechanisms including digital terrestrial, so it’s no surprise that this is where low latency CMAF is aimed.
CMAF itself is simply a format which unites HLS and DASH under one standard. It doesn’t, in and of itself, mean your stream will be low latency. In fact, CMAF was born out of MPEG’s MP4 standard – officially called ISO BMFF . But you can use CMAF in a low-latency mode which is what this talk focusses on.
Paul looks at what makes up the latency of a typical feed discussing encoding times, playback latency and the other key places. With this groundwork laid, it’s time to look at the way CMAF is chunked and formatted showing that the smaller chunk sizes allow the encoder and player to be more flexible reducing several types of latency down to only a few seconds.
In order to take full advantage of CMAF, the play needs to understand CMAF and Paul explains these adaptations before moving on to the limitations and challenges of using CMAF today. One important change, for instance, is that chunked streaming players (i.e. HLS) have always timed the download of each chunk to get a feel for whether bandwidth was plentiful (download was quicker than time taken to play the chunk) or bandwidth was constrained (the chunk arrived slower than real-time). Based on this, the player could choose to increase or decrease the bandwidth of the stream it was accessing which, in HLS, means requesting a chunk from a different playlist. Due to the improvements in downloading smaller chunks and using real-time transfer techniques such as HTTP/1.1 Chunked Transfer the chunks are all arriving at the download speed. This makes it very hard to make ABR work for LL-CMAF, though there are approaches being tested and trialed not mentioned in the talk.
Latency seems to be the new battleground for streaming services. While optimising bandwidth and quality are still highly important, they are becoming mature parts of the business of streaming where as latency, and technologies to minimise it – as Apple showed this month – are still developing and vying for position.
Here, the Streaming Video Alliance brings together people from large streaming services to explore this topic finding out what they’ve been doing to reduce it, the problems they’ve faced and the solutions which are on the table.
Low latency streaming is always a compromise, but what can be done to keep QOE high?
This on-demand webinar looks at CMAF and presents some real-world data on this low latency technique. The webinar starts by explaining that CMAF is a low-latency streaming technology similar to HLS and other streaming protocols where the idea is to deliver the video as small files. Olivier and Alain from Harmonic explain how this is done and look at some of the trade-offs and compromises that are needed and introduce techniques to keep QOE high. They also look at deployment in cloud vs. on premise.
Pieter-Jan Speelmans talks about play tradeoffs and optimisations within the player. CMAF allows the buffer to be reduced and whilst a bad network may mean you buffer is similar to ‘normal’, but in good networks, this buffer can be brought down significantly. He also talks about how ABR switching is impacted by GOP length even in CMAF.
Viaccess-Orca explains how DRM works with CMAF and looks at some of the challenges including licences acquisition time and overloading licence servers at the beginning of events. Akamai’s Will Law explains some benefits of CMAF and the near-real-time of chunk-based transfer (HTTP 1.1) and how downloading chunks at full speed leads to problems when the same broadband link is used by several clients.
There are lots of good talks on CMAF, but this is one of the few which talks about CMAF not as theory, but as is deployable today.