For the majority of humankind, speech production and understanding are quite natural and unconsciously acquired processes performed quickly and effectively throughout our daily lives. By the year 2001, speech synthesis and recognition systems are expected to play important roles in advanced user-friendly human-machine interfaces (Wilpon, in this volume). Speech recognition systems include not only those that recognize messages but also those that recognize the identity of the speaker. Services using these systems will include database access and management, various order-made services, dictation and editing, electronic secretarial assistance, robots (e.g., the computer HAL in 2001A Space Odyssey), automatic interpreting (translating) telephony, security control, and aids for the handicapped (e.g., reading aids for the blind and speaking aids for the vocally handicapped) (Levitt, in this volume). Today, many people in developed countries are employed to sit at computer terminals wearing telephone headsets and transfer information from callers to computer systems (databases) and vice versa (information and transaction services). According to the basic idea that boring and repetitive tasks done by human beings should be taken over by machines, these information-transfer workers should be replaced by speech recognition and synthesis machines. Dictation or voice typewriting is expected to increase the speed of input to computers and to allow many operations to be carried out without hand or eye movements that distract attention from the task on the display.
Figure 1 shows a typical structure for task-specific voice control and dialogue systems. Although the speech recognizer, which converts spoken input into text, and the language analyzer, which extracts meaning from text, are separated into two boxes in the figure, it is desirable that they perform with tight mutual connection, since it is necessary to use semantic information efficiently in the recognizer to obtain correct texts. How to combine these two functions is a most important issue, especially in conversational speech recognition (understanding). Then, the meanings extracted by the language analyzer are used to drive an expert system to select the desired action, to issue commands to various systems, and to receive data from these systems. Replies from the expert system are transferred to a text generator that constructs reply texts. Finally, the text replies are converted into speech by a text-to-speech synthesizer. "Synthesis from concepts" is performed by the combination of the text generator and the text-to-speech synthesizer.
Figure 2 shows hierarchical relationships among the various types