Analog signals and digital signals
Analog signals and digital signals
A signal is any time-varying physical quantity— voltage, light beam, sound wave, or other—that is used to convey information. Analog signals are so-called because they convey information by analogy (i.e., by mimicking the behavior of some other quantity). Digital signals convey information by assuming a series of distinct states symbolizing numbers (digits). Both analog and digital signals are essential to modern communications and computing, but the greater simplicity and generality of digital signaling has encouraged an increasing reliance on digital devices in recent decades.
An analog signal varies in step with some other physical phenomenon, acting as an analog to or model of it. For example, the electrical signal produced by a microphone is an analog of the sound waves impinging on the mike.
The term analog is also commonly used to denote any smoothly varying waveform, even one (e.g., the voltage available from an AC power outlet) that does not convey information. Any waveform that is continuous in both time and amplitude is an analog waveform, while an analog waveform that happens to convey information is an analog signal.
The elemental or archetypal analog signal is a sinusoidal wave or sinusoid, because any analog signal can be viewed as a sum of sinusoids of different frequencies that have been variously shifted in time and magnified in amplitude. The rapidity with which a sinusoid repeats its cycle (one crest plus one dip) is termed its frequency. A plot of the frequencies and amplitudes of all the sinusoids that would be needed to build up a given analog waveform depicts its frequency content, or spectrum. Processing of analog signals consists largely of altering their spectra. For example, turning up the treble on a stereo system selectively amplifies the high-frequency part of the music’s spectrum.
Digital signals convey discrete symbols that are usually interpreted as digits. For example, a voltage that signals the numbers 1 through N by shifting between N distinct levels is a digital signal, and so is a sinusoid that signals N digits by shifting between N distinct frequencies or amplitudes. (The latter would be analog as regards its waveform, but digital as regards its signaling strategy.)
Most digital signals are binary; that is, they signal the digits 0 and 1 by shifting between two distinct physical states (e.g., a high voltage and a low voltage). Each 0 or 1 is a bit (binary digit). Other numbers are communicated by transmitting “words,” bundles of 0s and 1s, either bit by bit along a single channel or in parallel (as by N wires all signaling at the same time). Typical word lengths are: 24 =16, 25 =32, and 26 = 64 bits.
Although the term digital emphasizes the use of a finite number of signal states to communicate digits, it is really the use of such signals to convey symbols that makes digital signals uniquely useful. Whether the two states of a binary signal represent 0 and 1, Yes and No, “first half of alphabet” and “second half of alphabet,” or any other pair of meanings is entirely up to the human designer. In principle, it is possible to use analog signals the same way, but in practice it is quite awkward to do so.
Another feature of virtually all digital systems is that all signals in the system change state (low to high, high to low) at frequent, regularly spaced instants. These system-wide changes of state are governed by a central timing device, or system clock. The system clock in a modern digital device may change state millions or billions of times per second.
Because most physical quantities can be described by measurements, and because any measurement can be represented by a sufficiently long series of 0s and 1s, it is possible to transfer some of the information in any analog signal to a digital signal. In fact, according to the Nyquist sampling theorem, sufficiently precise measurements of an analog waveform made at twice or more the maximum frequency present in that waveform will preserve all its information. Digital signals can, conversely, be converted to analog signals by using them as inputs to a device whose output shifts smoothly between a series of voltages (or other physical states) as directed by a changing set of input bits. Analog-to-digital conversion is performed, for example, when a digital compact disc (CD) is recorded from a live audio source, and digital-to-analog conversion is performed when a CD is played back over an audio system.
Couch, Leon W. Digital & Analog Communication Systems. 7th ed. Upper Saddle River, NJ: Prentice Hall, 2006.
Haykin, Simon. An Introduction to Analog and Digital Communication. New York: John Wiley & Sons, 2006.
Sklar, Bernard. Digital Communications: Fundamentals and Applications. 2nd ed. Englewood Cliffs, NJ: Prentice-Hall, Inc., 2001.