Why Computer Voices Still Don't Sound Human

"Modern text-to-speech systems are incredibly complex, and they're improving rapidly. But reading a book with anything near the expressiveness of an actual human voice is an enormously difficult computational task—the pinnacle of speech synthesis research. At the moment, text-to-speech programs are found in much simpler applications—customer-service phone lines and GPS navigators, for example. In these situations, you hear the computer's voice in short bursts, so it's easy to forgive its odd intonations and suspicious speech rhythms. But when listening to long passages, you can't help but compare the computer's voice with a human's—and the computer shrinks in the comparison."

Read Me a Story, Mr. Roboto
by Farhad Manjoo
http://www.slate.com/id/2212800/