t between 100 and 10,000 times per second?) Once Gretchen's vocal cords begin to vibrate, here are the three things that happen next: (1) the vibrations of her vocal cords cause vibrations in the air in her windpipe; (2) these vibrations in the air move up her windpipe to her mouth; and (3) as the vibrating air moves out through Gretchen's mouth, the shape of her mouth and lips, and the position of her tongue, work together to create the intended sound. The Channel In our example, the channel is simply the air between Gretchen and Carl. The shaped vibrations that leave Gretchen's mouth cause vibrations in the air, and these vibrations move through the air from Gretchen to Carl. The Receiver The receiver in this case is Carl's eardrum and brain. The vibrations in the air hit Carl's eardrum, causing it to vibrate in the same way. Carl's shaking eardrum sends electrical signals to his brain, which interprets the shaking as spoken sound. The human eardrum can actually pick up vibrations between 50 Hz and 16,500 Hz, allowing us to hear sounds beyond the range of what we can speak, including a variety of musical sounds. 1.2 Telecommunication Systems 1.2.1 Definition A telecommunication system is two things: (1) a communication system-that is, a system in which information is transmitted from one physical location, A, to a second physical location, B; and (2) a system which allows this information to be sent beyond the range of usual vocal or visual communications. Gretchen and Carl's lunchtime chat would not qualify as a telecommunication system, but the telephone system which they used later for an afternoon talk does qualify. Chapter One 1.2.2 Four Examples and an Erratic History Lesson Here are four examples of telecommunication systems, ordered chronologically to create what we'll optimistically call "a brief history of telecommunications." Smoking Up In the B.C.'s, smoke signals were sent out using fire and some smoke signal equipment (such as a blanket). The smoke, carried upward by the air, was seen by people fa