A talking computer also needs to determine the focus of each phrase.

HAL: I enjoy working with people.

He could stress any word in the sentence and change its meaning. If he stresses I he contrasts the meaning with "you enjoy ..."If he stresses enjoy, he implies a contrast with "I hate ..."When working is stressed, it means "rather than playing." To convey the meaning of a message the computer must assign a prominent stress to the correct word.

Of course, for educated human readers familiar with the language the numerous steps needed to speak a written or printed text are natural, because they understand what they are reading. Today, alas, we do not have machines that understand text, although their analysis of a text can help them sound as if they do. The Bell Labs synthesizer does paragraph-length analyses of texts. Using discourse information, statistics about word relations, and assigning words the proper part of speech (nouns, verbs, etc.), the synthesizer expands the input, segments the text, and assigns sentence-level stresses. This process, though not perfect, works well enough to enable the machine to read very long sentences with only a minimal loss of intelligibility.

Generating Linguistic Units

A number of issues are common to the task of reading text and generating computer speech. First, we assumed that when a computer generates the speech, it "knows" what it is trying to say (as hard a problem as that might be). HAL knows that he is trying to say "Dr. Poole" and not "drive Poole," just as he knows where the break for the phrasal hierarchies belongs and which word he needs to stress.

Next, the computer performing either task needs to know how to pronounce each word. As the English language employs a limited alphabet, there are many ways to pronounce certain letter sequences. For example, only six letters (a, e, i, o, u and y) are used to describe vowel sounds in English, but there are thirteen different vowel phonemes in the language; for example, the vowel in the word book is quite different from that in boot.In the preceding section we touched on homograph disambiguation (i.e., distinguishing among the various meanings and sounds of words like (bass, live, read, etc.), but we, and the computer, also need to know how to correctly pronounce the letters /sch/ in the words school, schedule, and mischief. Phoneticians have an alphabet that corresponds to pronunciation rather than to the spelling of words. Most dictionaries use this alphabet to indicate pronunciation.

Another important aspect of pronunciation is lexical or word stress. When HAL says, "my mission responsibility,"he puts the stress on the syllable bi. If he were to say responsibility or responsibility, the listener might not understand the word, or might even hear two words instead of one.

By storing a pronunciation dictionary in the computer, we can tell the computer how to pronounce many words. Still, because of prefixes and suffixes and the constant addition of new words to the language, it is impossible to store all the words and their variations. We therefore need to supplement the dictionary with a morphological analyzer and a set of letter-to-sound rules. Movement of the stressed syllable makes writing a morphological program a complicated task. For example, when HAL says melodramatic, with the stress on the fourth syllable, ma, he compounds the morphemes melo and drama, both of which stress the first syllable. When combined to form melodrama, the morpheme melo maintains its first-syllable stress. Addition of the suffix tic, however, shifts the stress to the penultimate syllable. Moreover, the moving stress does not always fall on the same syllable; the shifts of act, active, activity, and activation demonstrate the variety of stress options a computer's analyzer has to recognize.