For the musical use of "modulation", see modulation (music).
Modulation describes a range of techniques for encoding information on a carrier signal, typically a sine-wave signal. A device that performs modulation is known as a modulator.
When transmitting digitaldata, modulation normally involves shifting one or more properties of the carrier wave between a set of states, a process referred to as keying. This type of modulation includes:
"Data Encoding Techniques" (http://www.rhyshaden.com/encoding.htm) and "Specifications for Data Encoding" (http://www.wildpackets.com/compendium/FE/FE-Encod.html) discuss the various encoding techniques that have been used with various types of Ethernet.
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When the carrier is thus modulated, a fraction of the power is converted to sidebands extending above and below the carrier frequency by an amount equal to the highest modulatingfrequency.
The modulation might vary the amplitude (PAM or pulseamplitudemodulation), the duration (PDM or pulse duration modulation), or the presence of the pulses (PCM or pulse code modulation).
Modulation of the outputsignal, the intensity of light waves forming an image of the specimen, corresponds to the formation of image contrast in microscopy.
Modulation is typically less in the image than in the specimen and there is often a slight phase displacement of the image relative to the specimen.
The modulation transfer function is also related to the point spread function, which is the image of a point source of light (commonly referred to as the Airy disk) from the specimen projected by the microscope objective onto the intermediate image plane.
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