If you've tried speech-recognition software in the past, you may be skeptical of Android's capabilities. Older speech software required you to talk in a stilted manner, and it was so prone to error that it was usually easier just to give up and type.
Today's top-of-the-line systems—like software made by Dragon—don't ask you to talk funny, but they tend to be slow and use up a lot of your computer's power when deciphering your words. Google's system, on the other hand, offloads its processing to the Internet cloud.
Everything you say to Android goes back to Google's data centres, where powerful servers apply statistical modeling to determine what you're saying. The process is fast, can be done from anywhere, and is uncannily accurate.
You can speak normally but if you want punctuation in your email, you've got to say "period" and "comma", you can speak for as long as you like, and you can use the biggest words you can think of. It even works if you've got a regional accent.
How does Android's speech system work so well? The magic of data. Speech recognition is one of a handful of Google's artificial intelligence programs—the others are language translation and image search—that get their power by analysing impossibly huge troves of information.
For the speech system, the data accesses a large number of voice recordings. If you've used Android's speech recognition system, Google Voice's e-mail transcription service, Goog411 (a now-defunct information service), or some other Google speech-related service, there's a good chance that the company has your voice somewhere on its servers and it's only because Google has your voice, and millions of others, that it can recognise mine.
Read more here: Google speech recognition software for your cellphone actually works. - By Farhad Manjoo - Slate Magazine