1.3.6 Modulation And Frequency Translation

Baseband signals produced by the preceding stages are typically not suitable for direct transmission across most physical channels. Modulation translates the signal to a frequency range appropriate for the chosen channel—whether that channel is a pair of wires, an optical fiber, or free space.

In modulation, the information signal controls one or more parameters of a carrier waveform, such as its amplitude, frequency, or phase. This process enables efficient radiation from antennas, compatibility with frequency-selective channels, and the coexistence of multiple systems within the electromagnetic spectrum.

Modulation is required for several practical reasons. Many information signals occupy similar low-frequency ranges—for example, speech, music, and video—and if they were transmitted directly over the same medium, they would interfere with one another. By shifting each signal to a different frequency band, modulation allows multiple signals to be transmitted simultaneously without mutual interference. Modulation also helps reduce the impact of low-frequency noise, which is common in natural and man-made environments. Finally, for wireless transmission, modulation is essential because efficient antennas must be comparable in size to the signal wavelength; baseband frequencies correspond to wavelengths that are far too large to be practical. For these reasons, information signals are almost always modulated before transmission.