An analog or analogue signal is any continuous signal for which the time varying feature (variable) of the signal is a representation of some other time varying quantity, i.e analogous to another time varying signal. It differs from a
digital signalin that small fluctuations in the signal are meaningful. Analog is usually thought of in an electrical context; however, mechanical, pneumatic, hydraulic, and other systems may also convey analog signals.
Essentially an analogue signal can be thought of as a
simulationor duplication of one continuous time varying quantity in another, possibly different, time varying quantity. It is then a mapping of one time varying quantity to another, often with the intent of recording or transmitting information about the former within the medium of the latter.
An analog signal uses some property of the medium to convey the signal's information. For example, an
aneroid barometeruses rotary position as the signal to convey pressure information. Electrically, the property most commonly used is voltagefollowed closely by frequency, current, and charge.
Any information may be conveyed by an analog signal; often such a signal is a measured
responseto changes in physical phenomena, such as sound, light, temperature, position, or pressure, and is achieved using a transducer.
For example, in sound recording, fluctuations in air
pressure(that is to say, sound) strike the diaphragm of a microphonewhich causes corresponding fluctuations in a voltage or the current in an electric circuit. The voltage or the current is said to be an "analog" of the sound.
Data is more easily corrupted in analogue form due to noise but may also be of higher density and processed more quickly. A 3 hour Domestic VHS cassette could hold for example 16GB of data ["Concise Dictionary of Computing." Penguin Reference - Penguin Books - pages 11-12.] .
Any measured analog signal must theoretically have noise and a finite
slew rate. Therefore, both analog and digital systems are subject to limitations in resolution and bandwidth. In practice, as analog systems become more complex, effects such as non-linearity and noise ultimately degrade analog resolution to such extent that the performance of digital systems may surpass it. In analog systems, it is difficult to detect when such degradation occurs. However, in digital systems, degradation can not only be detected but corrected as well.
The primary disadvantage of analog signaling is that any system has noise – i.e., random unwanted variation. As the signal is copied and re-copied, or transmitted over long distances, these random variations become dominant. Electrically, these losses can be diminished by shielding, good connections, and several cable types such as coaxial or
The effects of noise creates signal loss and distortion, that is impossible to recover, since amplifying the signal to recover attenuated (distorted) parts of the signal amplifies the noise (distortion/interference) as well. Even if the resolution of an analog signal is higher than a comparable digital signal, in many cases, the difference is overshadowed by the noise in the signal.
Another method of conveying an analog signal is to use
modulation. In this, some base signal (e.g., a sinusoidal carrier wave) has one of its properties modulated: amplitude modulationinvolves altering the amplitude of a sinusoidal voltage waveform by the source information, frequency modulationchanges the frequency. Other techniques, such as changing the phase of the base signal also work. Analog circuits do not involve quantisationof information into digital format. The concept being measured over the circuit, whether sound, light, pressure, temperature, or an exceeded limit, remains from end to end. Clocks with hands are called analog; those that display digits are called digital. However, many analog clocks are actually digital since the hands do not move in a smooth continuous motion, but in small steps every second or half a second, or every minute.
digitalfor a discussion of "digital vs. analog".
Sources: Parts of an earlier version of this article were originally taken from
Federal Standard 1037Cin support of MIL-STD-188.
Analog sound vs. digital sound
Analog signal processing
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