Low latency protocols like CMAF are wreaking havoc with traditional ABR algorithms. We’re having to come up with new ways of assessing if we’re running out of bandwidth. Traditionally, this is done by looking at how long a video chunk takes to download and comparing that with its playback duration. If you’re downloading at the same speed it’s playing, it’s time consider changing stream to a lower-bandwidth one.
As latencies have come down, servers will now start sending data from the beginning of a chunk as it’s being written which means it’s can’t be downloaded any quicker. To learn more about this, look at our article on ISO BMFF and this streaming primer. Since the file can’t be downloaded any quicker, we can’t ascertain if we should move up in bitrate to a better quality stream, so while we can switch down if we start running out of bandwidth, we can’t find a time to go up.
Ali’s algorithm uses the time the last chunk finished downloading in place of the missing timestamp figuring that the new chunk is going to load pretty soon after the old. Now, looking at the data, we see that the gap between one chunk finishing and the next one starting does vary. This lead Ali’s team to move to a sliding window moving average taking the last 3 download durations into consideration. This is assumed to be enough to smooth out some of those variances and provides the data to allow them to predict future bandwidth and make a decision to change bitrate or not. There have been a number of alternative suggestions over the last year or so, all of which perform worse than this technique called ACTE.
In the last section of this talk, Ali explores the entry he was part of into a Twitch-sponsored competition to keep playback latency close to a second in test conditions with varying bitrate. Playback speed is key to much work in low-latency streaming as it’s the best way to trim off a little bit of latency when things are going well and allows you to buy time if you’re waiting for data; the big challenge is doing it without the viewer noticing. The entry used a heuristics and a machine learning approach which worked so well, they were runners up in the contest.
A bumper video here with 7 short talks from VideoLAN, Will Law and Hulu among others, all exploring the state of MPEG DASH today, the latest developments and the hot topics such as low latency, ad insertion, bandwidth prediction and one red letter feature of DASH – multi-DRM.
The first 10 minutes sets the scene introducing the DASH Industry Forum (DASH IF) and explaining who takes part and what it does. Thomas Stockhammer, who is chair of the Interoperability Working Group explains that DASH IF is made of companies, headline members including Google, Ericsson, Comcast and Thomas’ employer Qualcomm who are working to promote the adoption MPEG-DASH by working to imrove the specification, advise on how to put it into practice in real life, promote interoperability, and being a liaison point for other standards bodies. The remaining talks in this video exemplify the work which is being done by the group to push the technology forward.
Meeting Live Broadcast Requirements – the latest on DASH low latency!
Akamai’s Will Law takes to the mic next to look at the continuing push to make low-latency streaming available as a mainstream option for services to use. Will Law has spoken about about low latency at Demuxed 2019 when he discussed the three main file-based to deliver low latency DASH, LHLS and LL-HLS as well as his famous ‘Chunky Monkey’ talk where he explains how CMAF, an implementation of MPEG-DASH, works in light-hearted detail.
In today’s talk, Will sets out what ‘low latency’ is and revises how CMAF allows latencies of below 10 seconds to be achieved. A lot of people focus on the duration of the chunks in reducing latency and while it’s true that it’s hard to get low latency with 10 second chunk sizes, Will puts much more emphasis on the player buffer rather than the chunk size themselves in producing a low-latency stream. This is because even when you have very small chunk sizes, choosing when to start playing (immediately or waiting for the next chunk) can be an important part of keeping the latency down between live and your playback position. A common technique to manage that latency is to slightly increase and decrease playback speed in order to manage the gap without, hopefully, without the viewer noticing.
Chunk-based streaming protocols like HLS make Adaptive Bitrate (ABR) relatively easy whereby the player monitors the download of each chunk. If the, say, 5 second chunk arrives within 0.25 seconds, it knows it could safely choose a higher-bitrate chunk next time. If, however the chunk arrives in 4.8 seconds, it can choose to the next chunk to be lower-bitrate so as to receive the chunk with more headroom. With CMAF this is not easy to do since the segments all arrive in near real-time since the transferred files represent very small sections and are sent as soon as they are created. This problem is addressed in a later talk in this talk.
To finish off, Will talks about ‘Resync Elements’ which are a way of signalling mid-chunk IDRs. These help players find all the points which they can join a stream or switch bitrate which is important when some are not at the start of chunks. For live streams these are noted in the manifest file which Will walks through on screen.
Ad Insertion in Live Content:Pre-, Mid- and Post-rolling
Whilst not always a hit with viewers, ads are important to many services in terms of generating the revenue needed to continue delivering content to viewers. In order to provide targeted ads, to ensure they are available and to ensure that there is a record of which ads were played when, the ad-serving infrastructure is complex. Hulu’s Zachary Cava walks us through the parts of the infrastructure that are defined within DASH such as exchanging information on ‘Ad Decision Parameters’ and ad metadata.
In chunked streams, ads are inserted at chunk boundaries. This presents challenges in terms of making sure that certain parameters are maintained during this swap which is given the general name of ‘Content Splice Conditioning.’ This conditioning can align the first segment aligned with the period start time, for example. Zachary lays out the three options provided for this splice conditioning before finishing his talk covering prepared content recommendations, ad metadata and tracking.
Bandwidth Prediction for Multi-bitrate Streaming at Low Latency
Next up is Comcast’s Ali C. Begen who follows on from Will Law’s talk to cover bandwidth prediction when operating at low-latency. As an example of the problem, let’s look at HTTP/1.1 which allows us to download a file before it’s finished being written. This allows us to receive a 10-second chunk as it’s being written which means we’ll receive it at the same rate the live video is being encoded. As a consequence the time each chunk takes to arrive will be the same as the real-time chunk duration (in this example, 10 seconds.) When you are dealing with already-written chunks, your download time will be dependent on your bandwidth and therefore the time can be an indicator of whether your player should increase or decrease the bitrate of the stream it’s pulling. Getting back this indicator for low-latency streams is what Ali presents in this talk.
Based on this paper Ali co-authored with Christian Timmerer, he explains a way of looking at the idle time between consecutive chunks and using a sliding window to generate a bandwidth prediction.
Implementing DASH low latency in FFmpeg
Open-source developer Jean-Baptiste Kempf who is well known for his work on VLC discusses his work writing an MPEG-DASH implementation for FFmpeg called the DASH-LL. He explains how it works and who to use it with examples. You can copy and paste the examples from the pdf of his talk.
Managing multi-DRM with DASH
The final talk, ahead of Q&A is from NAGRA discussing the use of DRM within MPEG-DASH. MPEG-DASH uses Common Encryption (CENC) which allows the DASH protocol to use more than one DRM scheme and is typically seen to allow the use of ‘FairPlay’, ‘Widevine’ and ‘PlayReady’ encryption schemes on a single stream dependent on the OS of the receiver. There is complexity in having a single server which can talk to and negotiate signing licences with multiple DRM services which is the difficulty that Lauren Piron discusses in this final talk before the Q&A led by Ericsson’s VP of international standards, Per Fröjdh.
CMAF is often seen as the best hope for streaming to match the latency of broadcast. Fully standards based, many see this as the best route over Apple’s LL-HLS. Another benefit of it over LL-HLS is that it’s already a completed standard with growing support.
This talk from Tomas Bacik starts by explaining CMAF to us. Standing for the Common Media Application Format, it’s based on the standardised ISOBMFF container format and whilst CMAF isn’t by default low-latency, it is flexible enough to deliver just that. However, as Tomas from CDN77 points out, there are other major benefits in terms of its use of the Common Encryption format, reduces storage fees and more.
MPEG DASH is a commonly found streaming format based on ISO BMFF. It has always had the benefit of supporting other codecs such as HEVC and AV1 over HLS which is an AVC-only specification. CMAF is an extension of MPEG DASH which goes one step further in that it can deal with both HLS-style manifest files (.hls) as well as MPEG DASH format (.mpd) inheriting, of course, the multi-codec ability of DASH itself.
Next is central theme of the talk, looking at VoD workflows showing how CMAF fits in and, indeed, changes workflows for the better. CMAF directly impacts packaging, storage and CDN which is where we focus now. Given that some devices can play HLS and some can play DASH, if you try to serve both, you will double your requirements of packaging, storage etc. Dynamic packaging allows for immediately repackaging your chunks into either HLS or DASH as needed. Whilst this reduces the storage requirements, it increases processing and also increases the time to first byte. As you might expect, using CMAF throughout, Tomas explains in this talk, allows you to package once and store once which solves these problems.
Tomas continues by explaining the DRM abilities of CMAF including AES-CBC and finishes by taking questions from the audience.
A trip down memory lane for some, a great intro to the basics of streaming for others, this video from IET Media looks at the history of broadcasting and how that has moved over the years to online streaming posing the question whether, with so many people watching online, is that broad enough to now be considered broadcast?
The first of a series of talks from IET Media, the video starts by highlighting that the recording of video was only practical 20 years after the first television broadcasts then talks about how television has moved on to add colour, resolution and move to digital. The ability to record video is critical to almost all of our use of media today. Whilst film worked well as an archival medium, it didn’t work well, at scale, for recording of live broadcasts. So in the beginning, broad casting from one, or a few, transmitters was all there was.
Russell Trafford-Jones, from IET Media, then discusses the advent of streaming from its predecessor as file-based music in portable players, through the rise of online radio and how this naturally evolved into the urge to stream video in much the same way.
Being a video from the IET video, Russell then looks at the technology behind getting video onto a network and over the internet. He talks about cutting the stream into chunks, i.e. small files, and how sending files can create a seamless stream of data. One key advantage of this method is Adaptive BitRate (ABR) meaning being able to change from one quality level, to another which typically means changing bitrate to adapt to changing network conditions.
Finishing by talking about the standards available for online streaming, this talk is a great introduction to streaming and an important part of anyone’s foundational understanding of broadcast and streaming.
This video was produced by IET Media, a technical network within the IET which runs events, talks and webinars for networking and education within the broadcast industry. More information
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