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Ofcom says live subtitles "very good"; concerns about latency

Ofcom has given TV subtitles a clean bill of health in its fourth and final report measuring the quality of live TV subtitles in UK programmes.

Over the past two years, Ofcom has required the BBC, ITV, Channel 4, Channel 5 and Sky to measure the quality of their live TV subtitles, helping to identify areas for improvement that Ofcom says could benefit the millions of viewers who use subtitles.

The report showed that the overall accuracy of live subtitling remained at what Ofcom called "very good" levels of over 98%.

However, the average ‘latency’ of subtitles – the time lag between words being spoken and the corresponding subtitles appearing on screen – was 5.6 seconds, well above Ofcom’s recommended level of three seconds. The majority of samples measured, reviewed by experts at the University of Roehampton, also had instances when subtitling was likely to be too quick for viewers to follow, often as a result of pre-recorded subtitles being speeded up to reduce latency. 

Since Ofcom began publishing these reports in 2014, broadcasters have engaged in discussions about improving the quality of live TV subtitles on their channels.

The BBC and Channel 4 are exploring a new approach that would take advantage of delays inherent in the processes for video, audio and subtitling – so-called ‘switchable delays’ – to reduce the latency of subtitles significantly. The BBC intends to start implementing this solution on some of its channels next year, while Channel 4 has committed to trialling this method. Both will share their results with other broadcasters.

Ofcom’s reports have shown that trade-offs sometimes need to be made between different aspects of subtitling quality, such as latency and speed. Ofcom intends to commission further research to understand how subtitle users would prefer these trade-offs to be struck, which is due to be published in early 2016.