Digital data can also be represented by（71）signals by use of a modem. The modem converts a series of binary voltage （72） into an analog signal by encoding the digital data onto a carrier frequency. The resulting signal occupies a certain spectrum of （73） 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 （74） and hence allow those data to be propagated over ordinary voice-grade （75） lines. At the other end of the line, another modem demodulates the signal to recover the original data．
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