Dolby
®
Model CP650 Installation Manual
Installation
2-10
Professional Interface Standards for Digital Audio
There are two professional interface formats used for digital audio:
AES/EBU (also
known as AES3)
and
AES3id
. These stream the same digital data and professional
audio header information over copper conductor links, but use different types of
conductors and connectors.
AES/EBU uses a balanced connection (two conductors plus shield) with a
characteristic input impedance of 110
Ω
, nominal peak-to-peak signal level of 5 V,
and most commonly, XLR connectors. The typical maximum transmission distance is
100 meters. AES3id uses an unbalanced connection (one signal conductor plus shield)
with a characteristic input impedance of 75
Ω
, peak-to-peak signal level of 1 V, and
BNC (“push and twist”) connectors. The typical maximum transmission distance is
1000 meters.
Professional digital audio
equipment usually uses the AES/EBU format because
balanced operation yields superior noise immunity, as it does with analog audio
signals, and because XLR connectors have been standard on analog professional
audio equipment.
Professional video
equipment usually uses the AES3 variation of this interface, with
BNC connectors. Like the use of XLR connectors on pro audio equipment, the
adoption of BNC connectors for the audio on professional video equipment stems
from their existing use for the video signal. Also, the unbalanced AES3 signal can
connect to more than one piece of equipment by using the loop-through connectors
that are available on some devices, and is robust for long cable runs.
Consumer Interface Standards for Digital Audio
The consumer interface standard for digital audio is
S/PDIF
(IEC61937). S/PDIF is
found using either coaxial unbalanced connections (one signal conductor plus shield)
with a characteristic input impedance of 75
Ω
with RCA (phono) connectors, or a
fiber-optic cable with Toslink
TM
connectors. The unbalanced coaxial connection has a
peak-to-peak signal level of 0.5 V. The typical maximum transmission distance is
10 meters. Although S/PDIF-specific cables with suitable connectors can be
purchased, good results can also be obtained using high-quality 75
Ω
video cable with
the appropriate connectors and/or adapters.
Cable Issues
Even in digital audio, noise-free signals are still very important. The cable used for
digital signals is specifically designed for digital audio use, even though it appears to
be the same as that used for analog audio or video signals. Any professional audio
equipment or broadcast supply company can provide 110
Ω
cable with connectors (or
without, if you wish to terminate them yourself) for AES/EBU connections, and high-
quality 75
Ω
video cables with BNC connectors for AES3id connections. Use of
cables or connectors with incorrect impedance or not designed for digital
transmission compromises the integrity of the bitstream and may create an unreliable
link between pieces of equipment, particularly with long cable runs.
