Themes

From its beginnings in the late nineteenth century, electrical engineering has blossomed from focusing on electrical circuitsfor power, telegraphy and telephony to focusing on a much broader range of disciplines. However, the underlying themesare relevant today: Power creation and transmission and information have been the underlying themes of electrical engineering for a century and a half. This courseconcentrates on the latter theme: the representation, manipulation, transmission, and reception of information byelectrical means . This course describes what information is, how engineers quantify information, and howelectrical signals represent information.

Information can take a variety of forms. When you speak to a friend, your thoughts are translated by your brain into motorcommands that cause various vocal tract components--the jaw, the tongue, the lips--to move in a coordinated fashion. Informationarises in your thoughts and is represented by speech, which must have a well defined, broadly known structure so that someoneelse can understand what you say. Utterances convey information in sound pressure waves, which propagate to your friend's ear.There, sound energy is converted back to neural activity, and, if what you say makes sense, she understands what you say. Yourwords could have been recorded on a compact disc (CD), mailed to your friend and listened to by her on her stereo. Informationcan take the form of a text file you type into your word processor. You might send the file via e-mail to a friend, whoreads it and understands it. From an information theoretic viewpoint, all of these scenarios are equivalent, although theforms of the information representation--sound waves, plastic and computer files--are very different.

Engineers, who don't care about information content , categorize information into two different forms: analog and digital . Analog information is continuous valued; examples are audio andvideo. Digital information is discrete valued; examples are text (like what you are reading now) and DNA sequences.

The conversion of information-bearing signals from one energy form into another is known as energy conversion or transduction . All conversion systems are inefficient since some input energy islost as heat, but this loss does not necessarily mean that the conveyed information is lost. Conceptually we could use any formof energy to represent information, but electric signals are uniquely well-suited for information representation,transmission (signals can be broadcast from antennas or sent through wires), and manipulation (circuits can be built toreduce noise and computers can be used to modify information). Thus, we will be concerned with how to

• represent all forms of information with electrical signals,
• encode information as voltages, currents, and electromagnetic waves,
• manipulate information-bearing electric signals with circuits and computers, and
• receive electric signals and convert the information expressed by electric signals back into a usefulform.

Telegraphy represents the earliest electrical information system, and it dates from 1837. At that time, electricalscience was largely empirical, and only those with experience and intuition could develop telegraph systems. Electricalscience came of age when James Clerk Maxwell proclaimed in 1864 a set of equations that he claimed governed all electrical phenomena. These equationspredicted that light was an electromagnetic wave, and that energy could propagate. Because of the complexity of Maxwell'spresentation, the development of the telephone in 1876 was due largely to empirical work. Once Heinrich Hertz confirmedMaxwell's prediction of what we now call radio waves in about 1882, Maxwell's equations were simplified by Oliver Heavisideand others, and were widely read. This understanding of fundamentals led to a quick succession of inventions--thewireless telegraph (1899), the vacuum tube (1905), and radio broadcasting--that marked the true emergence of thecommunications age.

During the first part of the twentieth century, circuit theory and electromagnetic theory were all an electrical engineerneeded to know to be qualified and produce first-rate designs. Consequently, circuit theory served as the foundation and theframework of all of electrical engineering education. At mid-century, three "inventions" changed the ground rules. Thesewere the first public demonstration of the first electronic computer (1946), the invention of the transistor (1947), and thepublication of A Mathematical Theory of Communication by Claude Shannon (1948). Although conceived separately, these creations gave birth to the information age, in which digitaland analog communication systems interact and compete for design preferences. About twenty years later, the laser was invented,which opened even more design possibilities. Thus, the primary focus shifted from how to build communication systems (the circuit theory era) to what communications systems were intended to accomplish. Only once the intended system is specified canan implementation be selected. Today's electrical engineer must be mindful of the system's ultimate goal, and understand thetradeoffs between digital and analog alternatives, and between hardware and software configurations in designing informationsystems.