As we saw yesterday, there’s an increasingly buoyant market for video codecs and whilst this is a breath of fresh air after AVC’s multi-decade dominance, we will likely never again see a market which isn’t fragmented with several dominant players, say AV1, AVC, VVC and VP9, each sharing 85% market share relatively equally and then ‘the rest’ bringing up the rear. So multi-codec distribution to home viewers is going to have to deal with delivering different codecs to different people.
fuboTV do this today and Nick Krzemienski is here to tell us how. Starting with an overview of fuboTV primarily streams both live and on VOD. Nick shows us the workflow they use and then explains how their AVC & HEVC combined workflow is set up. Starting with the ideal case where a single fmp4 is encoded into both AVD and HEVC, he proposes you would simply package both into an HLS and DASH manifest and let players work out the rest. Depending on your players, you may have to split out your manifests into single-codec files.
DRM’s very important for a sports broadcaster so Nick looks at how this might be achieved. CMAF allows you to deliver m3u8 and mpd files using CENC (Common ENCryption). This promises a single DRM process ahead of packaging, but the reality, we hear from Nick, is that you’ll need two sets of media for HLS and DASH if you’re going to use CENC.
When you’re delivering multiple manifest and, hence, multiple sources, how do you manage this? Nick outlines, and shows the code, of how he achieves this at the edge. Using Lamda, he’s able to look at the incoming requests and existing files at the CDN to deliver the right asset with the logic done close to the viewer. Nick closes by with his thoughts on the future for streaming and answering questions from the audience.
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.
With his usual entertaining vigour, Will Law explains the differences to the three approaches to low-latency streaming: DASH, LHLS and LL-HLS from Apple. Likening them partly to religions that all get you to the same end, we see how they differ and some of the reasons for that.
Please note: Since this video was recorded, Apple has released a new draft of LL-HLS. As described in this great article from Mux, the update’s changes are
“Delivering shorter sub-segments of the video stream (Apple call these parts) more frequently (every 0.3 – 0.5s)
Using HTTP/2 PUSH to deliver these smaller parts, pushed in response to a blocking playlist request
Blocking playlist requests, eliminating the current speculative manifest request polling behaviour in HLS
Smaller, delta rendition playlists, which reduces playlist size, which is important since playlists are requested more frequently
Faster rendition switching, enabled by rendition reports, which allows clients to see what is happening in another playlist without requesting it in its entirety”
Read the full article for the details and implications, some of which address some points made in the talk.
Anyone who saw last year’s Chunky Monkey video, will recognise Will’s near-Oscar-winning animation style as he sets the scene explaining the contenders to the low-latency streaming crown.
We then look at a bullet list of features across each of the three low latency technologies (note Apple’s recent update) which leads on to a discussion on chunked transfer delivery and the challenges of line-rate delivery. A simple view of the universe would say that the ideal way to have a live stream, encoded at a constant bitrate, would be to stream it constantly at that bitrate to the receiver. Whilst this is, indeed, the best way to go, when we stream we’re also keeping one eye on whether we need to change the bitrate. If we get more bandwidth available it might be best to upgrade to a better quality and if we suddenly have contested, slow wifi, it might be time for an emergency drop down to the lowest bitrate stream.
When you are delivered a stream as individual files, you can measure how long they take to download to estimate your available bandwidth. If a file can be downloaded at 1Gbps, then it should always arrive at 1Gbps. Therefore if it arrives at less than 1Gbps we know that there is a bandwidth restriction and can make adjustments. Will explains that for streams delivered with chunked transfer or in real time such as in LL-HLS, this estimation no longer works as the files simply are never available at 1Gbps. He then explains some of the work that has been undertaken to develop more nuanced ways of estimating available bandwidth. It’s well worth noting that the smaller the files you transfer, the less accurate the bandwidth estimation as TCP takes time to speed up to line rate so small 320ms-length video segments are not ideal for maximising throughput.
Continuing to look at the differences, we next look at request rates with DASH at 20 requests per second compared to LL-HLS at 720. This leads naturally to an analysis of the benefits of HTTP/2 PUSH technology used in LL-HLS and the savings that can offer. Will explores the implications, and some of the problems, with last year’s version of the LL-HLS spec, some of which have been mitigated since.
The talk concludes with some work Akamai has done to try and establish a single, common workflow with examples and a GitHub repository. Will shows how this works and the limitations of the approach and finishes with a look at the commonalities in approaches.
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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