The Economist writes:
Searching video clips or streams is…much harder than searching text. There are three main approaches. The first, and simplest, is to search the closed captioning, or subtitles, that are broadcast alongside television programmes. In America, most television programmes already include captions; by law, all will have to do so starting in 2006. In Europe, captions should be broadly available by 2010. Such captions are not perfect, however, especially for live television, so searching using captions can be a hit-and-miss affair. The second approach uses software to listen to the video’s soundtrack. Turning spoken dialogue into text requires fancy algorithms and is not always reliable, as anyone who has ever used a speech-recognition system can testify. But the resulting text is then simple to search. The third approach, called semantic tagging, involves applying tags to video clips, either manually or automatically. Tags may describe the genre of a clip, which actors appear in it, and so on. The tagged video can then be easily searched.