Speck Electronics SSM - Manual (Page 60)

Chapter 5 Wiring and Other
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 synth and sampler based system
should be of great concern and not overlooked when installing the
Model SSM. EMI is defined as any unwanted signal which adversely
affects the operation of the mixer or mixing 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.
Almost every electronic device generates some amount of EMI
emissions. These emissions can be transmitted as electromagnetic
radiation or simply conducted though audio cables and power cords. In
the same respect, most electronic devices are also very susceptible to the
EMI emissions generated by other electronic devices.
There are natural and man made sources of EMI that you can't do
anything about. These sources include radio, TV, and radar transmitters,
as well as motors, lights, and computers. Even the Sun and atmospheric
conditions can be contributors to noise that you experience in your audio
system.
There are generally three elements that must be present for EMI to exist.
These include the source of the EMI (conducted or radiated), the
propagation medium by which EMI is transmitted (directly on the cables
or through the air), and the receptor that suffers the adverse affects of
EMI. If any of these three elements are eliminated or reduced, the EMI
interference will be eliminated or reduced.
The more electronic equipment operating within an equipment rack, the
higher the EMI emissions. The more audio cable and low level audio
equipment that exists within the same rack, the greater possibility of
unwanted noise. The result of EMI in an audio system manifests itself
as a buzz, hum, whine, or all three.
The most common EMI occurrence in an audio system is radiated
emissions from microprocessors in synths and samplers, and magnetic
field sources from transformers and power supplies.
EMI and RFI
Sources of EMI
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Reducing EMI