Digital data can also be represented by(

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问题 Digital data can also be represented by(  )signals by use of a modem. The modem converts a series of binary voltage (  ) into an analog signal by encoding the digital data onto a carrier frequency. The resulting signal occupies a certain spectrum of (  ) centered about the carrier and may be propagated across a' medium suitable for that carrier. The most common modems represent digital data in the voice (  ) and hence allow those data to be propagated over ordinary voice-grade (  ) lines. At the other end of the line, another modem demodulates the signal to recover the original data.A.waveB.frequencyC.codeD.spectrum

选项 A.wave
B.frequency
C.code
D.spectrum

答案 D

解析 数字数据也可以通过调制解调器用模拟信号来表示。调制解调器把数字数据编码为一种载波频率,从而可以把二进制电压序列转换为模拟信号。产生的信号占用了以某一频率为中心的载波频带,可以通过适合于该载波的介质进行传播。大多数通用的调制解调器表示话音频带的数字数据,因而允许这些数据在通常的话音级电话线路上传播。在线路的另一端,另外一个调制解调器对信号进行解调并恢复原来的数据。
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