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phonograph,and the British musician David Edward Hughes invented a microphone in which the pressure from sound waves altered the electrical properties of grains of carbon. Later innovations included magnetic recording, first proposed in 1920, and high-quality sound reproduction along with stereophonic sound, which began in the late 1950s and early 1960s.

Even early in the history of computers, it was music that drove the further development of digital sound techniques.The first music synthesizer program was written at Bell Labs in 1960, and by 1984, a set of standards had been created to transmit musical information in digital form between electronic synthesizers and computers. Now digital recording and playback of music, and word recognition and synthesis, are routine functions even on small desktop computers. But as with vision, the mere ability to register or produce sounds or words under computer control does not give them meaning. In humans, the linguistic analysis that the brain performs as we speak and listen is one of our most demanding intellectual functions.

Whereas the mechanics of artificial vision and hearing have been refined by decades of development, synthetic touch, taste, and smell are less highly evolved, partly because their nature in humans and animals is not so well understood.Touch and taste have the complica- tion of operating over large areas with many sensors, taste and smell involve varied chemical interactions, and taste also seems to depend on texture. Still, artificial versions of all these senses exist and are being steadily improved.Touch has been implemented by sensors that produce an electrical effect when they are deflected or change posi- tion in space; these serve as collision-avoiding devices and enable an artificial being to judge its bodily orientation. Finer tactile sensing, like that of the human fingertips, is also under development, as are analogues for smell and taste.

In practice, these last two senses are probably the least essential for an artificial being, which could be highly functional with only vision, hearing, and a limited sense of touch. But the sense of smell carries a special meaning for us and illustrates the complexities of simulating human behavior. Although smell does not play the central role for

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