line coding, PAM, PWM, PCM
Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave. A demodulator is an electronic circuit (or computer program in a software defined radio) that is used to recover the information content from the modulated carrier wave.
These terms are traditionally used in connection with radio receivers, but many other systems use many kinds of demodulators. Another common one is in a modem, which is a contraction of the terms modulator/demodulator.
Since the early days of radio when all transmissions were in Morse Code, a demodulator has also been called a detector. Early demodulators had only to detect the presence (or absence) of a radio wave using a device such as a coherer, without necessarily making it audible. This alternate term has survived despite the greater sophistication of modern circuits.
There are several ways of demodulation depending on how parameters of the base-band signal are transmitted in the carrier signal, such as amplitude, frequency or phase. For example, for a signal modulated with a linear modulation, like AM (Amplitude Modulated), we can use a synchronous detector. On the other hand, for a signal modulated with an angular modulation, we must use an FM (Frequency Modulation) demodulator or a PM (Phase Modulation) demodulator. Different kinds of circuits perform these functions.
Many techniques—such as carrier 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.
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