Analog signal

An analog or analogue signal is any variable signal continuous in both time and amplitude. It differs from a digital signal in 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.

An analog signal uses some property of the medium to convey the signal's information. For example, an aneroid barometer uses rotary position as the signal to convey pressure information. Electrically, the property most commonly used is voltage followed closely by frequency, current, and charge.

Any information may be conveyed by an analog signal, often such a signal is a measured response to changes in physical phenomena, such as sound, light, temperature, position, or pressure, and is achieved using a transducer.

For example, in an analog sound recording, the variation in pressure of a sound striking a microphone creates a corresponding variation in the voltage amplitude of a current passing through it. An increase in the volume of the sound causes the fluctuation of the current's voltage amplitude to increase while keeping the same rhythm.

Disadvantage

The primary disadvantage of analog signaling is that any system has noise--i.e., random 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 twisted pair.

The effects of noise make signal loss and distortion impossible to recover, since amplifying the signal to recover attenuated parts of the signal amplifies the noise as well.

Modulation

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 modulation involves altering the amplitude of a sinusoidal voltage waveform by the source information, frequency modulation changes the frequency. Other techniques, such as changing the phase of the base signal also work.

Analog circuits do not involve quantisation of 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.

See digital for a discussion of digital vs. analog.

Sources: Some of an earlier version of this article was originally taken from Federal Standard 1037C in support of MIL-STD-188.

See also

• Analog computer
• Analog to digital converter
• Digital to analog converter
• Analog television
• Analog synthesizer
• Analog photocopier
• Analog fax machine