Modulation

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In electronics and physics, modulation refers to methods of communications that change the characteristics of a periodic waveform, such that the changes represent information to be transmitted over the electromagnetic carrier wave of the basic waveform. While the term is most often used with radio frequency carrier waves, it can apply to sound waves as well, such as when a basic steady tone from a musical instrument is varied to express a musical theme.

There are several basic forms of modulation, which can be divided into basic analog, signal-processed analog, and digital. The basic analog types impose the information change by:

• Amplitude modulation (AM): Varying the strength of the signal in proportion to the information carried. Pure amplitude modulation transmits a carrier, plus a high and low sideband as an effect of superheterodyne|heterodyning
  • Double sideband suppressed carrier: the carrier wave, which does not carry information, is suppressed and both identical sidebands transmitted. Rarely used.
  • Single sideband (SSB): Only one of the sidebands is transmitted; the carrier and other sideband is suppressed. Very common technique for non-entertainment communications radio due to its power efficiency.
• Frequency modulation:Changing the frequency (i.e., the time between peaks of the basic frequency)
• Phase modulation: Shifting the phase of the signal by changing the relative starting point of individual waves with respect to a reference carrier wave.

Digital modulation is somewhat more indirect than the analog methods. In digital modulation, the information is first encoded into a set of nonperiodic pulse, such as zero voltage to convey a zero bit and a nonzero voltage to convey a nonzero bit. The pulses are then used to amplitude, frequency, or phase modulate a carrier wave.