An even more complex mode of communication occurs when the originator's written text is transmitted to the receptor orally by another person, a reader. To speak the text the originator intended, the reader must first understand its meaning. A complete text-to-speech system operates in this, most difficult, mode of communication -- in which the computer reads a text written by a third party.

The computer can also, of course, be a generator of ideas -- as HAL is -- and have complete information about the content of the message it is conveying. Consider, for example, HAL's first utterance in 2001: "Good afternoon, Mr. Amer. Everything is going extremely well." To say HAL's speech, a computer would have to have all the information about the text in its "brain." It would know that "Good afternoon" is a greeting, a phrase consisting of a compound word, afternoon, which is spoken with the stress on the syllable noon, not on the syllable after. It would be aware that Mr. is pronounced mister, that Amer is a person's name, and so on. Thus the computer formulating the speech would have a great deal of information about the message.

Reading Text

The situation is very different, however, when the computer is reading text, either printed text or a stored data base. An educated person can read text of a familiar language without difficulty, unlike a reading machine, which is not familiar with the language. A machine does not understand what it is reading. The first problem a machine encounters is reading characters that are not words or nonalphabetic characters (e.g., Mr., 72, and AT&T). A person would have no difficulty with such items. Often, we rely on contextual cues to decide how to pronounce such characters or words as St. (saint or street), bass (a musical instrument or a fish) and 3/5 (March fifth or three-fifths). Reading numbers is always a problem: is 5 five or fifth? is 325-4321 a telephone number or an arithmetic problem? And certainly we would not pronounce $1.5 million as dollar sign one point five million. Thus, the first task of a reading machine is to normalize the text by expanding nonword characters into words and, in the case of bass or read (present or past tense), deciding which is the correct pronunciation.

When humans speak, they try to convey the structure of the message by segmenting the speech into a hierarchical structure of words, minor phrases, and sentences. Take the following sentence:

HAL: This sort of thing has cropped up before and it has always been due to human error.

In English, luckily, white spaces in the text mark word boundaries; this is not true in, for example, Chinese and Japanese. The ends of sentences are also well marked in written English, with a period. (However, we probably wouldn't mistake the period at the end of an abbreviation as a mark for the end of a sentence.) Minor phrases are often indicated by the use of commas (not to be mistaken for the use of commas in a list). Moreover, a comma is sometimes omitted from text, as it is in HAL's statement, which has no comma after the word before. Conjunctions, such as and are often a cue for a minor phrase break, but not always. When HAL mentions "putting Drs. Hunter, Kimball and Kamisky aboard," neither the comma nor the conjunction indicate a minor phrase boundary. Speaking the sentence with a minor boundary in either place alters its meaning.

Finally, the words in a sentence have to be grouped into minor phrases. Going back to our original example, (this sort of thing) (has cropped up) (before) are the proper groupings in the first minor phrase. If the speaker does not speak the phrase in such groupings, he or she can either say it as a unit or pronounce each word with equal emphasis. In either case, the listeners will have difficulties comprehending the message, for they have no way to identify the important parts of the message.