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search For other uses, see Modulation (disambiguation).   In
telecommunications, modulation is the process of conveying a
message signal, for example a digital bit stream or an analog
audio signal, inside another signal that can be physically
transmitted. Modulation of a sine waveform transforms a baseband
message signal into a passband signal.  A modulator is a device
that performs modulation. A demodulator (sometimes detector or
demod) is a device that performs demodulation, the inverse of
modulation. A modem (from modulator–demodulator) can perform
both operations.   The aim of digital modulation is to transfer
a digital bit stream over an analog bandpass channel, for
example over the public switched telephone network (where a
bandpass filter limits the frequency range to 300–3400 Hz) or
over a limited radio frequency band.  Analog and digital
modulation facilitate frequency division multiplexing (FDM),
where several low pass information signals ar