Google is to add automatic captions to the tens of millions of English language videos it hosts on YouTube, the web search giant said Friday. The move will make the videos more accessible to deaf viewers but will also help Google index the content and supply relevant ads alongside it, analysts said.

Google has been experimenting with the automated captions for several months with a handful of high profile partners like the University of California, Berkeley, Yale University and National Geographic. All other captions on YouTube videos were provided by the videos’ producers.

Google has been working on speech recognition technology for some five years, and uses the technology to transcribe audio voice mails through its Google Voice service, and to provide spoken Web searches from smart phones.

However, engineers warned that the technology is far from perfect and that the machine translations are sure to contain mistakes. “We know it’s not perfect, and sometimes it will be funny,” said Google engineer Ken Harrenstien, who is deaf. “But it’s better than nothing.”

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