Transmitting information from one place to another has always been of great interest to humankind. Before the discovery of electricity and the invention of electronic devices, the principal means for transmitting information were limited to sound, writing and printing, and light. Since the discovery of electricity, transmitting information with electrical signals through wires and with electromagnetic waves through the air have come to dominate information technology.

Most electronic transmission of information makes use of a very limited symbol set—usually  binary numbers  consisting of groups of  binary digits (0s and 1s). An electrical current or a magnetic field can very easily be interpreted as being either on or off, or in one direction or the other, and so is well suited to representing binary digits.

Binary numbers are the building blocks of digital technology. To put digital signals into context, we must first understand how information messages can be transmitted.

Transmitting Messages

Every message transmitted contains information in a coded form. What is actually transmitted is a signal that conveys the information by some physical means. A voice message is sent using sound waves, which are pressure variations in a fluid (such as air) or a solid. A written or printed message involves ink marks on paper or dark and light spots on a computer monitor or television screen. A picture message uses paint or chalk on a canvas. An electronic message involves movement of electrons.

No matter how the message is transmitted, understanding it requires interpretation and reconstruction of what is received to reveal what the sender intended to convey; namely, the information. Until the message is interpreted by the receiver, the information remains hidden. It is coded in and represented by the physical changes—variations and arrangements of pressure, color, inkspots, electrical voltage, electromagnetic waves, light

patterns, and so forth—that are involved in the actual transmission of the message.

Analog and Digital Messages.

Whether a message is classified as analog or digital depends on the type of variation in the physical changes. If the variation is continuous, then the message is said to be analog. If the infor mation in the message is represented by variations that go in distinct steps,  then the message is digital. For example, a digital clock uses digits to show each distinct minute. An analog clock shows the time by an analogy (comparison) between the time and the position of the clock hands.

An ordinary voice message, conveyed directly from a speaker to a hearer by pressure changes in the air, is an analog message because the pressure changes are continuous: the pressure varies smoothly up and down to create sound. An ordinary photograph is an analog message—colors vary continuously from one part of the picture to another.

A message conveyed using an alphabet is digital. Each unit in the message is a particular letter. If the signal is clear, then each symbol in the message is immediately recognizable as one letter or another.

Advantages of Digital Information

The power of digital signals comes from the fact that the receiver knows that each symbol is supposed to be a member of the symbol set being used.

For example, when one is reading a message written in particularly bad hand writing, although it may be difficult to decide what some of the letters are,  one knows that there are only twenty-six possibilities (in the Roman alphabet) for each letter of the message. This often makes it possible to read successfully a message written in quite a scrawl.

The smaller the number of possible symbols that can be transmitted, the easier it is to read a message in which the signal has been degraded. For example, if one receives a message that is known to be in the base-10 number system, then each symbol must be one of the ten decimal digits. Even very hastily written numbers can be read. However, using even this limited set, it is sometimes hard to distinguish between 5s and 6s or between 7s and 9s. In contrast, if the set of allowable symbols is limited to 0 and 1 (the binary or base-2 digits), then the chance of successfully interpreting even a poor message increases.

Combining binary digits (or bits) in large groups can be used to represent information of any desired complexity. A string of eight bits is known as a byte. With eight bits in a byte, there are 256 possible different arrangements for a string of eight 0s and 1s, ranging from 0 to 255. Hence, one byte can represent any one of 256 colors at a particular point on a cathoderay tube, or it can represent any one of 256 characters in an “alphabet.”

Such an alphabet can represent all the lower-case letters, all upper-case letters, all decimal digits, many punctuation characters, and many special symbols such as a smiley face, a frowny face, arrows, and icons.

By digitally representing the rich alphabet of a written language, using groups of bits and bytes, messages can be transmitted that can be interpreted much more reliably than if the same messages were transmitted in analog form. One can easily see the difference by comparing a fax message with an e-mail message. The image of the printed page is conveyed in a fax by continuous variations of electrical current that are reassembled at the receiver to make another image of the page. Noise on the line degrades the transmitted signal and makes the received image fuzzy. In contrast, the information content of an e-mail message is conveyed through electrical signals that represent binary digits. The receiver has only to decide whether each bit received is a 0 or a 1, which is a relatively easy choice.

Cellular telephones, digital versatile disc (DVD) recordings, compact disc recordings, and satellite TV all use digital signals to increase the fidelity of the information received. Complex pictures and sounds are divided into very many small pieces by sampling; that is, by looking at the sound or light individually at each of very many points or times in the picture or sound. Information about each of these points is converted to a series of binary digits,  which is transmitted. The receiver then converts this digital signal back into the individual tiny pieces of the picture or sound and assembles them again to make an accurate representation of the original. By doing a large amount of processing, a message can be transmitted very accurately using digital signal methods. SEE ALSO  Bases; Communication Methods; Compact Disc, DVD, and MP3 Technology; Computers and the Binary System.

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Reference

Lathi, Bhagwandas P. Modern Digital and Analog Communication Systems, 3rd ed. New York: Oxford University Press, 1998.