Dennis Roddy- Satellite communication - 283-313 - Chapter.1...

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~ Chapter .1 0 Digital Signals 10.1 Introduction As already mentioned in connection with analog signals, baseband sig- nals are those signals which occupy the lowest, or base, frequency band in the frequency spectrum used by the telecommunications network. A baseband signal may consist of one or more information signals. For example, a number of telephony signals in digital form may be combined into one baseband signal by the process known as time- division multiplexing. . Analog signals may be converted into digital signals for transmis- sion. Digital signals also originate in the form of computer and other data. In general, a digital signal is a coded version of the original data or analog signal. In this chapter, the characteristics of the more common types of digital baseband signals are described, along with representative methods of digital modulation. ~I II 10.2 Digital Baseband Signals 1111 "II' Digital signals are coded representations of information. Keyboard characters, for example, are usually encoded in binary digital code. A binary code has two symbols, usually denoted as 0 and 1, and these are combined to form binary words to represent the characters. For example, a teleprinter code may use the combination 11000 to repre- sent the letter A. Analog signals such as speech and video may be converted to a digi- tal form through an analog-to-digital (AID) converter. A particular form - ofAID conversion is employed, known as pulse-code modulation, which will be described in detail later. Some of these sources are illustrated diagrammatically in Fig. 10.1. 283
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284 Chapter Ten I ~I PC ~g~ Typical binary digital rates ~ 4800 bps Medium rate data 64 kbps Digitized speech c[] 3-6 Mbps Digitized video Figure10.1 Examples of binary data sources. In digital terminology, a binary symbol is known as a binit from binary digit. The information carried by a binit is, in most practical situations, equal to a unit of information known as a bit. Thus it has become common practice to refer to binary symbols as bits rather than binits, and this practice will be followed here. The digital information is transmitted as a waveform, some of the more common waveforms used for binary encoding being shown in Fig. 10.2. These will be referred to as digital waveforms, although strictly speak- ing they are analog representations of the digital information being transmitted. The binary sequence shown in Fig. 10.2 is 1010111. Detailed reasons for the use of different waveforms will be found in most books on digital communications (see Bellamy, 1982). The duration of a bit is referred to as the bit period and is shown as Tb. The bit rate is given by 1 Rb = Tb (10.1) With Tb in seconds, the bit rate will be in bits per second, usually denoted by b/s. Figure 10.2a shows a unipolar waveform, meaning that the waveform excursions from zero are always in the same direction, either positive or negative. They are shown as positive A in Fig. 10.2a. Because it has a dc component, the unipolar waveform is unsuitable for use on tele- phone lines and radio networks, including satellite links.
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