Speech Recognition

Speech Recognition is one of the main elements of natural language processing, or computer speech technology. Speech recognition is equivalent to taking dictation: converting speech into comprehensible data. This is a skill that is done seemingly without effort by humans, but requires formidable processing and algorithmic resources from computers.

History of Speech Recognition

Writing systems are ancient, going back as far as the Sumerians of 6,000 years ago. The phonograph, which allowed the analog recording and playback of speech, dates to 1877. Speech recognition had to await the development of computer, however, due to multifarious problems with the recognition of speech.

First, speech is not simply spoken text--in the same way that Miles Davis playing So What can hardly be captured by a note-for-note rendition as sheet music. What humans understand as discrete words with clear boundaries are actually delivered as a continuous stream of sounds. Iwenttothestoreyesterday, rather than I went to the store yesterday. Words can also blend, with Whaddayawa? representing What do you want?

Second, there is no one-to-one correlation between the sounds and letters. In English, there are slightly more than five vowels--a, e, i, o, u, and sometimes y. There are more than twenty different vowel sounds, though, and the exact count can vary. The reverse problem also occurs, where more than one letter can represent a given sound. The letter c can have the same sound as the letter k or as the letter s.

In addition, people who speak the same language do not make the same sounds. There are different dialects--the word 'water' could be pronounced watter, wadder, woader, wattah, and so on. Each person has a distinctive pitch when they speak--men typically having the lowest pitch, women and children have a higher pitch (though there is wide variation and overlap within each group.) Pronunciation is also colored by adjacent sounds, the speed at which the user is talking, and even by the user's health. Consider how pronunciation changes when a person has a cold.

Lastly, consider that not all sounds are meaningful speech. Regular speech is filled with interjections that do not have meaning: Oh, like, you know, well. There are also sounds that are a part of speech that are not considered words: er, um, uh. Coughing, sneezing, laughing, sobbing, even hiccupping can be a part of what is spoken. And the environment adds its own noises; speech recognition is difficult even for humans in noisy places.