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Transporting high-dynamic-range analog signals from one piece of equipment to another is not a trivial task. Even subtle design variations can make huge differences in the equipment's ability to reject interference from the ac power line and other sources when the equipment connects to a real-world system.
Noise is pervasive
The fundamental interface problem stems from the fact that once noise contaminates a signal, it's nearly impossible to remove the noise. Dynamic range quantifies the ratio of the maximum undistorted signal to the noise floor, whereas SNR quantifies the ratio of the reference signal to the noise floor. Dynamic range equals SNR plus "head room"--the ratio of the maximum undistorted signal to the reference signal. These values are generally ex-pressed in decibels.
System-dynamic-range requirements depend on the application and on user expectations. The human ear has about 140 dB of dynamic range, whereas a high-performance audio-reproduction system in a typical home listening environment may require as much as 120 dB . Video systems generally accept 50 dB of dynamic range as the limit beyond which expert viewers perceive no further improvement.
Both basic types of interfaces--unbalanced and balanced--use a pair of wires to carry the signal; the impedances of these wires--with respect to a reference point, usually ground--define them. In an ideal unbalanced interface, one wire has zero impedance, and the other signal-carrying wire has nonzero impedance to ground. In the ideal balanced interface, both wires have equal and nonzero impedances to ground.
When you are dealing with any ac-line-powered system, you must accept the existence of significant ground-voltage differences between system components. Although you can sometimes reduce these voltages by carefully designing and executing system-grounding schemes, they are virtually impossible to eliminate. In most systems, these voltages are the dominant noise source, entering unbalanced signal paths through common-impedance coupling and balanced paths through common-mode conversion. Common symptoms are hums, buzzes, pops, clicks, and other noises in audio systems; hum bars or bands of "sparkles" in video systems; and unexplained data errors or crashes in data systems.
http://www.edn.com/archives/1998/060498/12df_03.htm
