Analog Signals and Digital Signals
An analog signal varies in step some other physical phenomenon, acting as an analog to or model of it. For example, the electrical signal produced by a microphone is an analog of the sound waves impinging on the mike.
The term analog is also commonly used to denote any smoothly varying waveform, even one (e.g., the voltage available from an AC power outlet) that does not convey information. Any waveform that is continuous in both time and amplitude is an analog waveform, while an analog waveform that happens to convey information is an analog signal.
The elemental or archetypal analog signal is a sinusoidal wave or sinusoid, because any analog signal can be viewed as a sum of sinusoids of different frequencies that have been variously shifted in time and magnified in amplitude. The rapidity with which a sinusoid repeats its cycle (one crest plus one dip) is termed its frequency. A plot of the frequencies and amplitudes of all the sinusoids that would be needed to build up a given analog waveform depicts its frequency content, or spectrum. Processing of analog signals consists largely of altering their spectra. For example, turning up the treble on a stereo system selectively amplifies the high-frequency part of the music's spectrum.
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