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．