Sec 8.3 - Analog and Digital Communication 277 (Signal +1)...

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Analog and Digital Communication 277 Instead of broadcasting using AM techniques, radio can be broadcast using FM, as illustrated in Figure 8.8 . The instantaneous frequency of the carrier is varied, or mod- ulated, according to the value of the signal at each instant. Times labeled a , b , and c are times when the carrier frequency is smallest, making the carrier oscillations slower at those times. For FM radio, the carrier frequency is much higher (88–108 MHz) than for AM radio (520–1610 kHz). Broadcasting and receiving FM transmissions takes place as shown in Figure 8.4, although a different kind of recovery circuit must be used. A major difference is that AM is restricted by the Federal Communications Commission 1 to a frequency range with total bandwidth equal to 1090 kHz (i.e., 520 to 1610 kHz), whereas FM has a much wider range, 88 to 108 MHz, with a much larger total band- width, 20 MHz. We will see below that this larger bandwidth gives FM much better ± delity for sound reproduction. 8.3 BASICS OF DIGITAL RADIO The modern alternative to analog radio is digital radio, in which computers pass digital signals between one another. Computers “speak” to each other using a simple set of symbols, or alphabet. This alphabet, called binary, consists of only two characters, as we discussed in Chapter 2. We could call the two used characters A and B , or we could call them and . It really does not matter. Computers use as their two characters one (1) and zero (0). A nice illustration of the difference between analog and digital representations of music is shown in Figure 8.9 . The magni± ed photograph of the surface of a typical vinyl LP (long-playing) recording disc shows continuously varying tracks in which the playback needle moves, creating an analogy of the original sound wave. The magni± ed photograph of the surface of a typical compact disc, or CD, shows a binary representa- tion of the strength of the sound wave. How does digital radio work? As shown in Figure 8.10 , when a person, say Alice, speaks into a microphone, an analog voltage is produced in the wire. This voltage goes to Alice’s computer, where an electronic circuit converts it into a list of ones and zeros ; that is, binary data. A one is represented by a higher voltage (e.g., 9 V), whereas a zero is represented by a lower voltage (e.g., 0 V). These data are transmitted across a net- work of computers and are received by Bob’s computer, where the ones and zeros are converted back to a continuous analog voltage, which drives Bob’s loudspeaker. The process of converting a continuously varying voltage or signal into discrete values is called signal sampling. Say the voltage from the microphone varies in time 1 The Federal Communications Commission (FCC) is a United States government agency. The FCC is charged with regulating interstate and international communications by radio, television, wire, satellite, and cable.
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Sec 8.3 - Analog and Digital Communication 277 (Signal +1)...

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