`
`From "Hargrave's Communications Dictionary, Wiley"
`
` Copyright © 2001 by the Institute of Electrical and Electronics Engineers, Inc.
`
`Modulation is the process of varying some characteristic of one signal (the carrier) in accordance with
`another signal (the message signal). Some of the parameters of a carrier that may be modulated are:
`
`• Amplitude (yielding AM or ASK).
`• Frequency (yielding FM, FSK, MFSK, MSK, etc.).
`• Phase (giving rise to PM, PSK, DPSK, QPSK, 4PSK, 8PSK, etc.).
`• Both amplitude and phase may be changed independently, giving rise to QAM (Quadrature Amplitude
`Modulation).
`
`AM (amplitude modulation), FM (frequency modulation), and PM (phase modulation) are the generic terms
`for the modulation method; that is, the information signal may be analog or digital in form. ASK (amplitude
`shift keying), FSK (frequency shift keying), and PSK (phase shift keying) apply only when the information
`signal is a digital signal. DPSK (differential phase shift key), 4PSK and 8PSK describe the number of phases
`possible in a PSK modulator. Three basic digitally modulated carrier waveforms are displayed in the
`diagram.
`
`
`
`The top trace is the information
`signal, and the second trace is the carrier. The three remaining traces show what happens to a carrier when
`it is modulated with a digital data stream using ASK, FSK, and PSK modulation techniques. Several forms of
`pulse modulation are illustrated in the following figure.
`
`
`
`The top trace is the modulating wave
`having values of 5, 8, 10, 8, 5, 2, 0, 2, 5 at the nine indicated sampling times. The modulation methods
`shown are pulse amplitude modulation (PAM), pulse width modulation (PWM) sometimes called pulse
`density modulation (PDM), pulse position modulation (PPM), and three forms of pulse code
`modulation (PCM). The three PCM waveforms are pulse count, direct code word, and difference code word
`(called delta modulation—DM). See also amplitude modulation (AM), frequency modulation (FM),
`gaussian minimum shift key (GMSK), minimum shift key (MSK), phase modulation (PM), pulse
`amplitude modulation (PAM), pulse code modulation (PCM), pulse position modulation (PPM), and
`pulse width modulation (PWM).
`
`
`
` Copyright © 2001 by the Institute of Electrical and Electronics Engineers, Inc.
`
`Persistent URL to the Entry: http://search.credoreference.com/content/entry/hargravecomms/modulation/0
`
`APA
`
`Modulation. (2001). In Hargrave's communications dictionary, Wiley. Retrieved from
`http://search.credoreference.com/content/entry/hargravecomms/modulation/0
`
`MLA
`
`"Modulation." Hargrave's Communications Dictionary, Wiley. Hoboken: Wiley, 2001. Credo Reference.
`Web. 19 June 2014.
`
`Chicago
`
`"Modulation." In Hargrave's Communications Dictionary, Wiley. Hoboken: Wiley, 2001.
`http://search.credoreference.com/content/entry/hargravecomms/modulation/0 (accessed June 19,
`2014.)
`
`Harvard
`
`2001 'Modulation' in Hargrave's communications dictionary, Wiley, Wiley, Hoboken, USA. Accessed: 19
`June 2014, from Credo Reference
`
`
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