YouTube is adding captions to millions of Internet videos, the company announced on Thursday. The feature expands upon speech-recognition technology that YouTube began using to make captions available on a limited number of videos last November. 'Auto-captioning' combines some of the speech-to-text algorithms found in Google's Voice Search to automatically generate video captions when requested by a viewer. The video owner can also download the auto-generated captions, improve them, and upload the new version. Viewers could even choose an option to translate those captions into any one of 50 different languages.

On Thursday, Youtube opened up auto-captions to all YouTube users. There will even be a "request processing" button for un-captioned videos that any video owner can click on if they want to speed up the availability of auto-captions. Youtube said that it would take some time to process all the available video and that currently, auto-captioning is only for videos where English is spoken.

Just like any speech recognition application, auto-captions require a clearly spoken audio track. Videos with background noise or a muffled voice can't be auto-captioned.

In addition, auto-captions aren't perfect and just like any other transcription, the owner of the video needs to check to make sure they're accurate. In other cases, the audio file may not be good enough to generate auto-captions.

Youtube said that itw was also working to provide auto-captions for all past user uploads that fit the above mentioned requirements.