| Analog Signals |
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| An analog or analogue signal is any time continuous signal where some time varying feature of the signal is a representation of some other time varying quantity. 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. |
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| 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. |
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| 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. |
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| For example, in sound recording, fluctuations in air pressure (that is to say, sound) strike the diaphragm of a microphone which 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. |
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| Since an analogue signal has a theoretically infinite resolution, it will always have a higher resolution than any digital system where the resolution is in discrete steps. In practice, as analogue systems become more complex, effects such as non linearity and noise ultimately degrade analogue resolution such that digital systems surpass it. In analogue systems it is difficult to detect when such degradation occurs, but in digital systems, degradation can not only be detected, but corrected as well. |
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| The real world is analog. What does that mean? Well, an analog value is equivalent to a floating-point number with an infinite number of places to the right of the decimal point. For example, temperatures do not take on distinct values such as 75°, 76°, 77°, 78°, etc. They take values like 75.434535… In fact, between the temperatures 75.435° and 75.436°, there are an infinite number of possible values. |
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| A man doesn’t weigh exactly 178 pounds. Add an atom, and his weight changes. When values such as temperature or weight change over time, they follow what is called a continuous curve. Between any two values on the curve, an infinite number of values take place over an infinite number of points in time. |
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| Digital Signals |
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| Digital signals are digital representations of discrete-time signals, which are often derived from analog signals. |
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| An analog signal is a datum that changes over time—say, the temperature at a given location; the depth of a certain point in a pond; or the amplitude of the voltage at some node in a circuit—that can be represented as a mathematical function, with time as the free variable (abscissa) and the signal itself as the dependent variable (ordinate). |
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| A discrete-time signal is a sampled version of an analog signal: the value of the datum is noted at fixed intervals (for example, every microsecond) rather than continuously. |
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| If individual time values of the discrete-time signal, instead of being measured precisely (which would require an infinite number of digits), are approximated to a certain precision—which, therefore, only requires a specific number of digits—then the resultant data stream is termed a digital signal. The process of approximating the precise value within a fixed number of digits, or bits, is called quantization. |
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| In conceptual summary, a digital signal is a quantized discrete-time signal; a discrete-time signal is a sampled analog signal. |
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| In the Digital Revolution, the usage of digital signals has increased significantly. Many modern media devices, especially the ones that connect with computers use digital signals to represent signals that were traditionally represented as continuous-time signals; cell phones, music and video players, personal video recorders, and digital cameras are examples. |
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| In most applications, digital signals are represented as binary numbers, so their precision of quantization is measured in bits. Suppose, for example, that we wish to measure a signal to two significant decimal digits. Since seven bits, or binary digits, can record 128 discrete values (viz., from 0 to 127), those seven bits are more than sufficient to express a range of one hundred values. |
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| There is such a thing as an analog computer, a computer that processes information using analog levels of electricity or the positions of mechanical devices. The overwhelming majority of today’s computers do not do this, however. |
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| Instead, they represent an analog value by converting it to a number with a fixed resolution, i.e., a fixed number of digits to the right of the decimal point. This measurement is referred to as a digital value. If the value is changing with respect to time, then a sequence of measurements can be taken, the period between the measurements typically remaining fixed. |
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