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Analogue electronics (or analog in American English) are those electronic systems with a continuously variable signal. In contrast, in digital electronics signals usually take only two different levels. The term "analogue" describes the proportional relationship between a signal and a voltage or current that represented the signal.

Main article: Analog signal

An analogue signal uses some property of the medium to convey the signal's information. For example, an aneroid barometer uses angular position as the signal to convey pressure information. Electrical signals may represent information by changing their voltage, current, frequency, or total charge. Information is converted from some other physical form ( such as sound, light, temperature, pressure, position) to an electrical signal by a transducer.

The signals take any value from a given range, and each unique signal value represents different information. Any change in the signal is meaningful, and each level of the signal represents a different level of the phenomenon that it represents. For example, suppose the signal is being used to represent temperature, with one volt representing one degree Celsius. In such a system 10 volts would represent 10 degrees, and 10.1 volts would represent 10.1 degrees.

Another method of conveying an analogue signal is to use modulation. In this, some base carrier signal has one of its properties altered: amplitude modulation (AM) involves altering the amplitude of a sinusoidal voltage waveform by the source information, frequency modulation (FM) changes the frequency. Other techniques, such as changing the phase of the carrier signal are also used.

In an analogue sound recording, the variation in pressure of a sound striking a microphone creates a corresponding variation in the current passing through it or voltage across it. An increase in the volume of the sound causes the fluctuation of the current or voltage to increase proportionally while keeping the same waveform or shape.

Mechanical, pneumatic, hydraulic and other systems may also use analogue signals.

Analogue systems invariably include noise; that is, random disturbances or variations, some caused by the random thermal vibrations of atomic particles. Since all variations of an analogue signal are significant, any disturbance is equivalent to a change in the original signal and so appears as noise. As the signal is copied and re-copied, or transmitted over long distances, these random variations become more significant and lead to signal degradation. Other sources of noise may come from external electrical signals, or poorly designed components. These disturbances are reduced by shielding, and using low noise amplifiers.

Since the information is encoded differently in analogue and digital electronics, the way they process a signal is consequently different. All operations that can be performed on an analogue signal such as amplification, filtering, limiting, and others, can also be duplicated in the digital domain.

The first electronic devices invented and mass produced were analogue. The use of microelectronics has reduced the cost of digital techniques and now make digital methods feasible and cost-effective.

The main differences between analogue and digital electronics are listed below:

Noise 

Because of the way information is encoded in analogue circuits, they are much more susceptible to noise than digital circuits, since a small change in the signal can represent a significant change in the information present in the signal and can cause the information present to be lost. Since digital signals take on one of only two different values, a disturbance would have to be about one-half the magnitude of the digital signal to cause an error; this property of digital circuits can be exploited to make signal processing noise-resistant. In digital electronics, because the information is quantized, as long as the signal stays inside a range of values, it represents the same information. Digital circuits use this principle to regenerate the signal at each logic gate, lessening or removing noise.

Precision 

A number of factors affect how precise a signal is, mainly the noise present in the original signal and the noise added by processing. See signal-to-noise ratio. Fundamental physical limits such as the shot noise in components limits the resolution of analogue signals. In digital electronics additional precision is obtained by using additional digits to represent the signal; the practical limit in the number of digits is determined by the performance of the analogue to digital converters, since digital operations can usually be performed without loss of precision.

Design Difficulty 

Digital systems are much easier and smaller to design than comparable analogue circuits. This is one of the main reasons why digital systems are more common than analog. An analogue circuit must be designed by hand, and the process is much less automated than for digital systems. Also, because the smaller the integrated circuit (chip) the cheaper it is, and digital systems are much smaller than analog, digital is cheaper to manufacture.

• Analog computer
• Analog signal
• Digital - See here for a discussion of digital vs. analog.
• Analog sound vs. digital sound
• Analog chip

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