BBC publishes research on improving live caption timing

Wednesday, 4 May 2016 12:55pm

The BBC has released details of experiments it has undertaken which prove that it’s practical to reduce the delay in live captions appearing on the screen.

Young woman watches the TV with captions

Live captions, which are created as a program goes to air, are often frustrating for viewers because there is a delay (on average around 5 seconds, but sometimes much longer) between the audio and the captions appearing.

In the BBC’s white paper, ‘Live subtitles re-timing: proof of concept’ (link is external) (note that captions are called subtitles in the UK), researchers Trevor Ware from the BBC and Matt Simpson from Ericsson discuss the experiment, conducted in 2015, which involved a sample of live programs broadcast on high definition. The experiment hinges on the fact that there is an inherent time delay when broadcasting a video signal of about five seconds. Captions are usually set up to match this, but if the time lag is reduced for live captions, they will appear on screen 3 seconds earlier. This would significantly enhance the viewer experience.

To be feasible, the system must be able to reduce the delay for live captions, but switch back to normal for pre-prepared captions. The research has proved that this can be achieved, and according to the white paper the BBC will “take a phased approach to deploying the live subtitle re-timing to its broadcast television services”. For technical reasons, the system can only be effectively used for high definition broadcasts, but not standard definition.

In September 2015, the Australian Communications and Media Authority (ACMA) held a seminar giving representatives from industry, government and consumers an opportunity to discuss the future of live captioning and research currently being undertaken to improve it. Issues like the delay on live captioning and techniques for measuring the quality of captions in a meaningful way are also discussed in Media Access Australia’s white paper Caption quality: International approaches to standards and measurement.


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