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What drove man to the invention of speech was, as I imagine, not so much the need of expressing his thoughts (for that might have been done quite satisfactorily by bodily gesture) as the difficulty of ''talking with his hands full."

Indeed, speech is a singularly efficient way for humans to express ideas and desires. Therefore, it is not surprising that we have always wanted to communicate with and command our machines by voice. What may be surprising is that a paradigm for this has been around for centuries. When machines began to be powered by draft animals, humans discovered that the same animals that provided the power for the machine also could provide enough intelligence to understand and act appropriately on voice commands. For example, the simple vocabulary of gee, haw, back, giddap, and whoa served nicely to allow a single human to control the movement of a large farm machine. Of course, voice commands were not the only means of controlling these horse- or mule-powered machines. Another system of more direct commands was also available through the reins attached to the bit in the animal's mouth. However, in many cases, voice commands offered clear advantages over the alternative. For example, the human was left completely free to do other things, such as walking alongside a wagon while picking corn and throwing it into the wagon. This eliminated the need for an extra person to drive the machine, and the convenience of not having to return to the machine to issue commands greatly improved the efficiency of the operation. (Of course, the reins were always tied in a conveniently accessible place just in case the voice control system failed to function properly!)

Clearly, this reliance on the modest intelligence of the animal source of power was severely limiting, and even that limited voice control capability disappeared as animal power was replaced by fossil fuel power. However, the allure of voice interaction with machines remained and became stronger as technology became more advanced and complex. The obvious advantages include the following:

• Speech is the natural mode of communication for humans.

• Voice control is particularly appealing when the human's hands or eyes are otherwise occupied.

• Voice communication with machines is potentially very helpful to handicapped persons.

• The ubiquitous telephone can be an effective remote terminal for two-way voice communication with machines that can also speak, listen, and understand.

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