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Communications via electronic means consist of the generation of signals that contain or carry intelligent information and their processing at the receiver to extract this information.
A signal is any kind of physical quantity that conveys information. The examples of signals are audible speech (as it conveys the thoughts (information) of one person to another through the physical medium of sound) or hand gestures (conveying information by means of light). In the field of communications, a signal is generally a relationship of a spatial parameter, such as amplitude, to time. This relationship can be discrete or continuous. A continuous signal or a continuous-time signal is one that has a value defined at every instant. A discrete signal or a discrete-time signal is represented by a time series consisting of a sequence of quantities. Discrete signals are usually received by acquiring values of a continuous-time signal at constant or variable rate - this process is called sampling, or by recording the number of events of a given kind over finite time periods (for example, the ancient Egyptian records of the Nile's yearly floods or the modern histograms of daily currency fluctuations).
An analog signal typically varies continuously over time, while digital signals are present at discrete points in time. A well-known example of analog vs. digital is that of clocks: analog being the type with pointers that slowly rotate around a circular scale, and digital being the type with decimal number displays or a "second-hand" that jerks rather than smoothly rotates. An analog clock is capable of indicating every possible time of a day. In contrast, a digital clock is capable of representing only a finite number of times (every tenth of a second, for example).
Most signals in nature are analog. Sound, noise, light, heat, and electronic communication signals going through air (or space) are examples of analog signals. The most fundamental analog signal is the periodic sine wave. Its three main characteristics are its amplitude (the size of the waveform), frequency (the number of cycles per second that the wave undergoes) and phase (the position of a point in time (instant) on a waveform cycle, while a complete cycle is defined as 360 degrees of phase).
A common digital signal configuration is the binary waveform represented by a sequence of two types of pulses of known shape. The information contained in a digital signal is given by the particular sequence of the presence (a binary, or logic, one) or absence (a binary zero) of these pulses. These are known as bits (this word was derived from “binary digits”).
Fig. 2. Analog and digital signal
We commonly refer to the transmission of voice, music, video and data as information signals as these intended signals contain what we want to communicate. Information signals belong to baseband signals. A baseband signal, or lowpass signal, is a signal at frequencies that are very near zero. This refers to analog or digital data before they are merged with other signals (multiplexed) or modulated. Information signals are at low frequencies, often less than 50 kHz.
There is another type of signal which is called a pulse train. Here the signal is a train of impulses that last a long or a short time. The phase and amplitude of the impulse determines how it is decoded. EKG is a good example of this type of signal occurring in nature.
Two further common characteristics in communications are the frequency spectrum (f) and the bandwidth (B) of a signal. The spectrum of a signal is the range of frequencies that it contains. That is, the spectrum is the combination of all the individual sine waves of different frequencies which make up that signal. The bandwidth refers to the width of this spectrum. For example, if the spectrum of a signal ranges from 2 kHz to 22 kHz, its bandwidth is fhigh - flow = 20 kHz.