Speck XTRAMIXcxi, Reference Manual

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Clock noise is one of the greatest enemies of the audio racks AC system.
If a synth or any microprocessor based device emits or somehow
couples its clock signal with the neutral or earth of its own power cable,
it will contaminate your AC system and carry the clock noise into other
equipment; almost always with undesirable results.
The earth connection exists to protect you, your equipment and possibly
your building from an electrical disaster. In a properly wired system, if
a 120 volt AC wire were to break within your equipment's chassis, it
should make contact with the Safety Earth Wire that is connected to the
chassis, and blow the fuse or trip the circuit breaker until the problem
has been corrected. Given the same circumstances, if the AC safety
ground has been defeated with a ground lift or the AC service is
incorrectly wired, the equipment's chassis and quite possibly everything
attached in that rack would be "live" with 120 volts.
In an electronics context, an earth provides a path for unwanted EMI
noise to be carried away from your audio equipment. If you disable
your earth with a ground lift or do not have a reliable earth connection,
the unwanted noise (EMI or RFI), will find an electrical path of least
resistance. That will most likely be your audio equipment and would
result in unwanted buzzes or hums.
In order for any audio signal, such as a synth signal to get from the synth
to the mixer, it requires a minimum of 2 conductors. One conductor is
the hot, or high, or whatever you are familiar with; the other conductor
is the ground or common. Additionally, all audio wires must be
protected from environmental occurrences such as EMI (Electro
Magnetic Interference) and RFI (Radio Frequency Interference) with an
outer shield. An outer shield protects the 2 inner conductors from
outside interference, and prevents that cable from inducing its signal
onto adjacent audio cables.
One common misconception is that the shield of a cable should act as
the common. This may be acceptable for guitar cords or semi-
professional applications, but not for professional applications. The
audio signals must be carried only by the 2 inner conductors and the
shield must act only to cover these 2 conductors without transmitting the
signal from one location to another. It is recommended that the shield
be attached to the common (ground) at one connector's end, and the
shield not be connected at the other connector's end. It is recommended
that all shields be connected at the mixer end, and the shields not be
connected at the other ends (synths, effects, power amps, etc.).
Clock Noise and AC
Proper Audio Grounding
and Shielding
47
Safety Earth Connection
Electronics Earth
Chapter 5 Wiring and Other