YouTube introduced a system Thursday that will automatically create subtitles for all its English language videos. The technology, called “auto-captioning”, uses complex algorithms to work out what is being said in a video and convert it into text. Once the feature is turned on, the words will appear on the bottom of the video screen just like subtitles in a film. The system was previously available as part of its “Voice Search” feature and on a small sample of videos made by selected partners. Now, Google is opening the system to any video with a spoken soundtrack in the English language. Auto-captioning began to be rolled out this afternoon. Google said it would take time to “fully process” the many millions of YouTube clips in English. But YouTube wants its users to help speed up the process, creating a button for people to request subtitles, which YouTube will then prioritise for captioning. Though the feature is currently only available in English, it will include more languages in the next few months. Google, which owns the video-sharing website, admitted the system was not perfect. It only works where there is a clearly spoken audio track. Videos with background noise of muffled voices will not be able to be auto-captioned. Instead, the internet company is asking the video’s owners to improve the subtitles by uploading their own, fully checked captions.