Speck Electronics ASC - Manual (Page 26)

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 to get from “Point A” to “Point B” requires
a cable with 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.).
If a patchbay is utilized in your mixing system, the rules for shielding
change. With a patchbay, normally all shields are connected at the
patchbay jacks, and not connected at the mixer or external audio
equipment.
The occurrence of EMI (Electro Magnetic Interference) and RFI
(Radio Frequency Interference) in a contemporary studio system should
be of great concern and not overlooked when installing the Model ASC.
EMI is defined as any unwanted signal which adversely affects the
operation of the ASC or your audio system.
Stated simply, the undesirable effects of EMI may be perceived as a low
frequency smooth sounding 60Hz hum; a low frequency "edgy"
sounding 120Hz buzz; or a higher frequency "whine" caused by the
timing circuits in microprocessor based devices.
22
Electronics earth
Chapter 4 Wiring and Other
Proper Audio Grounding
and Shielding
EMI and RFI