Maintaining Audio Quality in the Broadcast Facility 2011 - Orban

in the Broadcast/Netcast Facility 21
not carry signal and is used only for shielding. It should be connected at the input
only to prevent ground loop hum
Digital Interconnection
Per the AES3 standard, each digital input or output line carries two audio channels.
The connection is 110� balanced and is transformer-coupled in high-quality equipment.
The AES3 standard specifies a maximum cable length of 100 meters. While almost
any balanced, shielded cable will work for relatively short runs (5 meters or less),
longer runs require used of 110� balanced cable like Belden 1800B, 1801B (plenum
rated), multi-pair 180xF, 185xF, or 78xxA. Single-pair Category 5, 5e, and 6 Ethernet
cable will also work well if you do not require shielding. (In most cases, the tight
balance of Category 5/5e/6 cable makes shielding unnecessary.)
The AES3id standard is best for very long cable runs (up to 1000 meters). This specifies
75� unbalanced coaxial cable, terminated in 75� BNC connectors. A 110�/75�
balun transformer is required to interface an AES3id connection to an AES3 connection.
S/PDIF is a consumer digital standard closely related to the AES3 standard. However
S/PDIF is available in two different physical interfaces, coaxial and optical. Coaxial is
is 75� unbalanced and optical is TOSLINK. Both interfaces offer excellent quality and
are good for short distances. Format converters are available to go between either
coaxial or optical and/or AES3.
Grounding
Very often, grounding is approached in a “hit or miss” manner. Nevertheless, with
care it is possible to wire an audio studio so that it provides maximum protection
from power faults and is free from ground loops (which induce hum and can cause
oscillation).
In an ideal system:
� All units in the system should have balanced inputs. In a modern system with low
output impedances and high input impedances, a balanced input will provide
common-mode rejection and prevent ground loops—regardless of whether it is
driven from a balanced or unbalanced source.
� All equipment circuit grounds must be connected to each other; all equipment
chassis grounds must be connected together.
� In a low RF field, cable shields should be connected at one end only—preferably
the destination (input) end. This also prevents input noise pick-up when the output
is disconnected.
� In a high RF field, audio cable shields should be connected to a solid earth
ground at both ends to achieve best shielding against RFI.
� Whenever coaxial cable is used, shields are automatically grounded at both ends