Analog signals are usually signals defined over
continuous independent variable(s).
Speech
is produced by your vocal cords exciting acoustic resonances
in your vocal tract. The result is pressure waves propagating
in the air, and the speech signal thus corresponds to a
function having independent variables of space and time and a
value corresponding to air pressure:
| Speech Example |
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Photographs are static, and are continuous-valued signals defined over space. Black-and-white images have only one value at each point in space, which amounts to its optical reflection properties. In Figure 2, an image is shown, demonstrating that it (and all other images as well) are functions of two independent spatial variables.
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Color images have values that express how reflectivity depends
on the optical spectrum. Painters long ago found that mixing
together combinations of the so-called primary colors--red,
yellow and blue--can produce very realistic color images.
Thus, images today are usually thought of as having three
values at every point in space, but a different set of colors
is used: How much of red, green and blue
is present. Mathematically, color pictures are
multivalued--vector-valued--signals:
Interesting cases abound where the analog signal depends not on a continuous variable, such as time, but on a discrete variable. For example, temperature readings taken every hour have continuous--analog--values, but the signal's independent variable is (essentially) the integers.
"Electrical Engineering Digital Processing Systems in Braille."