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| Passbandmodulation |
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| Analog modulation |
| Digital modulation |
| Hierarchical modulation |
| Spread spectrum |
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Demodulation is the process of extracting the original information-bearingsignal from acarrier wave. Ademodulator is anelectronic circuit (orcomputer program in asoftware-defined radio) that is used to recover the information content from the modulated carrier wave.[1] There are many types ofsignal modulation, and there are many types of demodulators. The signal output from a demodulator may represent sound (ananalogaudio signal), images (an analogvideo signal) orbinary data (adigital signal).
These terms are traditionally used in connection withradio receivers, but many other systems use many kinds of demodulators. For example, in amodem, which is a contraction of the terms modulator/demodulator, a demodulator is used to extract a serial digital data stream from acarrier signal which is used to carry it through atelephone line,coaxial cable, oroptical fiber.
Demodulation was first used inradio receivers. In thewireless telegraphy radio systems used during the first 3 decades of radio (1884–1914) the transmitter did not communicate audio (sound) but transmitted information in the form of pulses of radio waves that represented text messages inMorse code.[citation needed] Therefore, the receiver merely had to detect the presence or absence of the radio signal, and produce a click sound. The device that did this was called adetector. The first detectors werecoherers, simple devices that acted as a switch. The termdetector stuck, was used for other types of demodulators and continues to be used to the present day for a demodulator in a radio receiver.
The first type ofmodulation used to transmit sound over radio waves wasamplitude modulation (AM), invented byReginald Fessenden around 1900. An AM radio signal can be demodulated byrectifying it to remove one side of the carrier, and then filtering to remove the radio-frequency component, leaving only the modulating audio component. This is equivalent to peak detection with a suitably long time constant. Theamplitude of the recovered audio frequency varies with the modulating audio signal, so it can drive an earphone or an audio amplifier. Fessendon invented the first AM demodulator in 1904 called theelectrolytic detector, consisting of a short needle dipping into a cup of dilute acid. The same yearJohn Ambrose Fleming invented theFleming valve orthermionic diode which could also rectify an AM signal.
There are several ways of demodulation depending on how parameters of the base-band signal such as amplitude, frequency or phase are transmitted in the carrier signal. For example, for a signal modulated with a linear modulation likeamplitude modulation (AM), we can use asynchronous detector. On the other hand, for a signal modulated with an angular modulation, we must use afrequency modulation (FM) demodulator or aphase modulation (PM) demodulator. Different kinds of circuits perform these functions.
Many techniques such ascarrier recovery,clock recovery,bit slip,frame synchronization,rake receiver,pulse compression,Received Signal Strength Indication,error detection and correction, etc., are only performed by demodulators, although any specific demodulator may perform only some or none of these techniques.
Many things can act as a demodulator, if they pass the radio waves onnonlinearly.[2]
AnAM signal encodes the information into the carrier wave by varying its amplitude in direct sympathy with theanalogue signal to be sent. There are two methods used todemodulate AM signals:
SSB is a form of AM in which thecarrier is reduced or suppressed entirely, which require coherent demodulation. For further reading, seesideband.
Frequency modulation (FM) has numerous advantages over AM such as better fidelity and noise immunity. However, it is much more complex to both modulate and demodulate a carrier wave with FM, and AM predates it by several decades.
There are several common types of FM demodulators:
QAM demodulation requires a coherent receiver. It uses two product detectors whose local reference signals are a quarter cycle apart in phase: one for the in-phase component and one for the quadrature component. The demodulator keeps these product detectors tuned to a continuous or intermittent pilot signal.
