Installation manual

Model DMA
™
8 Installation Manual Installation
2-5
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 75Ω with RCA (phono) connectors, or a
fiber-optic cable with Toslink
®
connectors. The unbalanced coaxial connection has a
peak-to-peak signal level of 0.5 V. Although S/PDIF-specific cables with suitable
connectors can be purchased, good results can also be obtained using high-quality
75Ω 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 110Ω cable with connectors (or
without, if you wish to terminate them yourself) for AES/EBU connections and high-
quality 75Ω video cables with BNC connectors for AES3 connections. Use of cables
or connectors with incorrect impedance or those 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.
Multiple Sources—Conversion Between Interface Standards
Although some details of the bitstreams used in the AES and S/PDIF standards differ,
the audio information is exactly the same. As a consequence, most audio equipment
accepts either standard with no need to convert the bitstream itself; this is the case
with the DMA8. However, if you intend to connect sources across different types of
digital audio inputs, do not
attempt to convert a digital interface type by, for example,
directly wiring an XLR connector to a BNC or RCA plug. This causes an impedance
mismatch and signal reflections, resulting in degradation of the digital waveform. It
may seem to work, but the results are unreliable and dropouts occur.
For conversion between the AES3 and S/PDIF formats you can use high-quality RCA
(phono plug)-to-BNC adapters, since the cable and impedance are the same (75Ω).
For conversion between the AES/EBU and AES3 or AES/EBU and S/PDIF formats,
a simple and economical method is to use inline transformers. These devices perform
the necessary impedance and balanced/unbalanced conversion. The following table
shows some examples of suitable adapters. The unbalanced connector in these
examples is a BNC. BNC-to-RCA adapters can be added to connect to consumer
S/PDIF connections. The units listed use passive circuitry.