Equipment Interface
The Difference between Grounds and Shields
While the usual scenario in hooking up equipment is that one plugs in the cables and
starts to work, the more complicated a system, the more likely it is that something will
not work correctly as far as hum and noise performance is concerned. While some would
blame the equipment, this is the equivalent of blaming the eggs for a bad soufflé. Usually,
hum and noise problems (and jitter or clock troubles in digital interfaces) can be traced to
poor planning and implementation of the studio’s
grounding situation
.
It is illuminating to realize that the engineers of yore in the recording, broadcast,
and communications industries have been through these troubles and figured out the
solutions. History can teach us a lot about how to avoid ground loops and their associated
problems. The manuals of many test instrument and recording equipment manufacturers
from the ‘50’s to the 80’s had chapters on how to fix hum and noise problems and it is
from this wealth of information that this writer draws ideas from for trouble free
grounding schemes.
To comply with international standards and wiring practices, recording equipment
manufacturers are required to connect all the shield pins of audio and data connectors to
the chassis grounds of their gear. Sometimes, this can cause noise problems in large
systems where pieces of equipment are spread out around a facility because two
‘grounds’ are never quite at the same potential. This can cause ground loops (hums or
buzzing in the speakers) if the cable shields are allowed to connect two chassis that are at
different potentials due to location, circuit wiring, or induction.
If the audio cables between the racks connect the equipment grounds together via
the shields and the racks are at even slightly different potentials (on different circuits with
different loads, long distance, etc) the shields will try to equalize the potential difference.
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