Even without defining information formally, we intuitively understand that speech, audio, and video signals contain information. We use the term message signals for such signals, since these are the messages we wish to convey over a communication system. In their original form–both during generation and consumption–these message signals are analog: they are continuous time signals, with the signal values also lying in a continuum. When someone plays the violin, an analog acoustic signal is generated (often translated to an analog electrical signal using a microphone). Even when this music is recorded onto a digital storage medium such as a CD, when we ultimately listen to the CD being played on an audio system, we hear an analog acoustic signal. The transmitted signals corresponding to physical communication media are also analog. For example, in both wireless and optical communication, we employ electromagnetic waves, which correspond to continuous time electric and magnetic fields taking values in a continuum.
Analog Communication
Block diagram for an analog communication system. The modulator transforms the message signal into the transmitted signal. The channel distorts and adds noise to the transmitted signal. The demodulator extracts an estimate of the message signal from the received signal arriving from the channel.
Given the analog nature of both the message signal and the communication medium, a natural design choice is to map the analog message signal (e.g., an audio signal, translated from the acoustic to electrical domain using a microphone) to an analog transmitted signal (e.g., a radio wave carrying the audio signal) that is compatible with the physical medium over which we wish to communicate (e.g., broadcasting audio over the air from an FM radio station). This approach to communication system design, depicted in Figure 1.1, is termed analog communication. Early communication systems were all analog: examples include AM (amplitude modulation) and FM (frequency modulation) radio, analog television, first generation cellular phone technology (based on FM), vinyl records, audio cassettes, and VHS or beta videocassettes While analog communication might seem like the most natural option, it is in fact obsolete. Cellular phone technologies from the second generation onwards are digital, vinyl records and audio cassettes have been supplanted by CDs, and videocassettes by DVDs. Broadcast technologies such as radio and television are often slower to upgrade because of economic and political factors, but digital broadcast radio and television technologies are either replacing or sidestepping (e.g., via satellite) analog FM/AM radio and television broadcast. Let us now define what we mean by digital communication, before discussing the reasons for the inexorable trend away from analog and towards digital communication.
Digital Communication
The conceptual basis for digital communication was established in 1948 by Claude Shannon, when he founded the field of information theory. There are two main threads to this theory:
Source coding and compression: Any information-bearing signal can be represented efficiently, to within a desired accuracy of reproduction, by a digital signal (i.e., a discrete time signal taking values from a discrete set), which in its simplest form is just a sequence of binary digits (zeros or ones), or bits. This is true whether the information source is text, speech, audio or video. Techniques for performing the mapping from the original source signal to a bit sequence are generically termed source coding. They often involve compression, or removal of redundancy, in a manner that exploits the properties of the source signal (e.g., the heavy spatial correlation among adjacent pixels in an image can be exploited to represent it more efficiently than a pixel-by-pixel representation).
Digital information transfer: Once the source encoding is done, our communication task reduces to reliably transferring the bit sequence at the output of the source encoder across space or time, without worrying about the original source and the sophisticated tricks that have been used to encode it. The performance of any communication system depends on the relative strengths of the signal and noise or interference, and the distortions imposed by the channel. Shannon showed that, once we fix these operational parameters for any communication channel, there exists a maximum possible rate of reliable communication, termed the channel capacity. Thus, given the information bits at the output of the source encoder, in principle, we can transmit them reliably over a given link as long as the information rate is smaller than the channel capacity, and we cannot transmit them reliably if the information rate is larger than the channel capacity. This sharp transition between reliable and unreliable communication differs fundamentally from analog communication, where the quality of the reproduced source signal typically degrades gradually as the channel conditions get worse.
A block diagram for a typical digital communication system based on these two threads is shown in Figure 1.2. We now briefly describe the role of each component, together with simplified examples of its function.
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