(第2空)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．