ÃÛÑ¿´«Ã½

This week we began the first phase of a trial of low latency live streaming on ÃÛÑ¿´«Ã½ iPlayer. For the next few weeks, viewers using selected devices who have opted into iPlayer’s ‘beta’ mode may see an experimental low latency stream of ÃÛÑ¿´«Ã½ Two when watching live on iPlayer. The trial will help us assess how well low latency streaming approaches perform over today’s internet into our viewers’ homes.

In ÃÛÑ¿´«Ã½ Research & Development we are working to improve the quality and reliability of video delivery over the internet. Currently viewers watching streams over the internet see things happening later than those watching on traditional broadcast. This happens for a variety of reasons that I have described before. The delay on iPlayer has reduced significantly in recent times but is still around 40 seconds, though it is not the same for all viewers. Many streaming services today have similar delays. On our broadcast services the end to end delay is just 8-10 seconds - a significant difference. For some content, viewers may not notice, but for live sport and for live events with a social media following, delays can detract from the viewing experience.  We are aiming to address these delays whilst maintaining a high quality of experience through our work on ‘low latency streaming’.

Reducing internet delivery delays to match traditional broadcast means less time for the streaming client to adapt to changes in network conditions. This in turn makes it harder to avoid ‘stalling’ where video playback stops and viewers have to wait for a period of ‘rebuffering’ before the stream continues. We have worked on ways to solve this and have modelled, tested and analysed the performance.  That work suggests that we should be able to achieve delays comparable to broadcast. But, since everyone’s internet connection is different, the only way to be sure how well it works in the real world is to try it in the real world.

Working with the teams responsible for our TV iPlayer product, we have arranged for some viewers of ÃÛÑ¿´«Ã½ Two on iPlayer to see a low latency stream. We’re using a technology called Low Latency DASH with chunked CMAF segments. We will use performance data from these streaming sessions to get a detailed understanding of how the quality and reliability compares with our traditional live streaming in a wide range of real homes.  If this goes well, we will then move on to fine tuning the stream and testing some different variants, aiming to maximise reliability.

With low latency streaming, we also have to think about what should happen if users do experience some rebuffering. Should the stream just resume, leaving them further behind for the rest of the programme? Should the stream catch up, skipping a few seconds in the process? Neither is ideal. Instead, our trial stream will play slightly faster after a stall, aiming to maintain low latency but without the viewer missing any of the action. Doing this without the viewer being aware requires some functionality that is not available on all TVs. To get the best quality data on the reliability we can achieve at delays comparable to broadcast, we are starting our trial using only devices that support this variable speed playback capability and will maintain the target latency we have set. Later on, we may extend the trial to other devices too.

The trial is limited to certain television and streaming devices. We are starting with a very small number of device models but targeting ones that have significant numbers of viewers watching live streams. The first device models added are 3rd generation Amazon FireTV Stick, 2nd generation Amazon FireTV Stick 4K, and Samsung CU8000 and CU8500 TVs. We will add more devices during the trial. Initially the trial will run on ÃÛÑ¿´«Ã½ Two between 9am and 5pm for viewers who have enabled ‘iPlayer beta’ in settings and have set their location to England or Scotland. Apart from the reduced delay, viewers should not notice any major differences when watching this stream. Anyone experiencing any problems with the stream can return to the conventional version by turning off ‘iPlayer beta’ mode.

This trial is to help us understand how well low latency streaming can cope with the real world performance of our viewers’ internet connections. We are starting small but hope to expand the trial as we gain confidence in its performance. We also hope to gain a better understanding of the scenarios under which low latency does not work so well, and whether we could avoid offering a low latency stream to those less likely to benefit.

This trial is only our first step in trialling low latency streaming to the public: viewers should not expect that all live internet viewing will match the delays on broadcast at the end of the trial. Gradual improvements are more likely. This trial is aiming to assess the reliability of the delivery of low latency media over the internet. More work is needed to build a fully resilient, fault tolerant system that can scale to the size of audience that the ÃÛÑ¿´«Ã½ serves for major sporting events. Since common ways to build in resilience to failure themselves add delay, achieving this for low latency streams remains a challenge.