As mentioned earlier, “analog” refers to something that moves or varies in value with no fixed units of resolution. Sound waves are inherently analog, as are most things that move in the world. A guitar string doesn’t jump from side to side; it passes smoothly through all of the points in between. Another meaning of analog is something that is related in a direct way to something else—for example, the way the position of the needle on a car’s speedometer relates to how fast it is moving.
Recording a sound with an analog system means that a direct representation of the sound waves is stored in a recording medium. The first system to record and play back sound was Thomas Edison’s phonograph (see Figure 8-5). The recording medium was tinfoil wrapped around a cardboard cylinder. A horn, similar to the bell of a tuba, focused the sound waves onto a thin membrane, or diaphragm, made of parchment, which was attached to a needle. As the cylinder rotated, the needle cut a continuous groove into the tinfoil. The air pressure variations of the sound waves caused the diaphragm and the needle to move up and down, varying the depth of the groove and thus creating a recording. Modern vinyl records operate on much the same principle, except that the needle moves from side to side.
Figure 8-5. Edison’s phonograph
To play back the recording, the needle was placed ...