AUDIO I/O AND SERIAL PORT WIRING
ENG
Page 33
15.2 DIGITAL AUDIO INPUT *
The digital input (available only as an option - DG-IN) features two connectors: coaxial and optic. Only one
connector may be connected at a time to avoid malfunctions (TOSLINK optical connector should be kept
covered while not used).
XLR female connector is designed for AES/EBU sources (“professional standard” interface), while TOSLINK
connector is designed to carry S/P DIF signals (“consumer standard” interface).
The digital input automatically recognizes the digital format (AES, EBU, SPDIF, etc.), the type of connection
(optic or coaxial) and can seamlessly handle 32KHz, 44.1KHz, 48KHz and 96KHz sample rates.
Coaxial input (on XLR) is transformer balanced, and so highly immune from external disturbances and noise
(EMI).
Digital Input
1
3
2
Pin 1
Gnd
Pin 2
Signal
Pin 3
Return
To enable the digital input level, see chapter 19.1. The Digital Input LED on the front panel indicates while
the digital input is active.
Attention: while enabled, the digital input excludes the analog input.
8.2.1 Converting between AES/EBU and S/PDIF interfaces
There are a number of differences in the electrical characteristics of AES/EBU and S/PDIF interfaces
which in some cases can render them completely incompatible. Although the audio data is the same in
both AES/EBU and S/ PDIF interfaces, they are indeed different formats, at least in their subcode.
AES converted to coax is NOT S/PDIF, and S/PDIF converted to XLR balanced is NOT AES.
They are still their native format, just the transmission medium has changed. Whether they will work in
your application depends on the equipment chosen and, for proper performance, XLR input should
therefore not be used for connection to consumer equipment.
AES/EBU uses a balanced differential line based on XLR connectors and the signal
levels are 5 volts. S/P-DIF uses a coaxial unbalanced line with RCA connectors and
the signal levels are around 0.5 volts. The protocol used in AES/EBU and S/PDIF is
not exactly the same and that can cause problems sometimes. The basic data format
of AES and S/P-DIF are identical. There is a bit in the channel status frame that tells
which is which. Depending upon the setting of that bit, some bits have different
meanings. For example, the bits used to describe de-emphasis in the AES/EBU
protocol overlap the bits used to implement the SCMS (serial copy management
system) protocol in S/P-DIF land.
Furthermore, S/PDIF usually uses 75 ohm coaxial cable and RCA connectors. 75 ohm
coaxial cable is inexpensive, because it is the same cable as used in video
transmission. AES/EBU-interface uses the well known symmetrical connections with
transformer isolation and an output impedance of 110 ohm.
AES/EBU digital audio signals are transmitted at high, video-like frequencies (at around 6MHz) and
should be handled very differently than standard analog audio lines. Commonly used XLR-3
microphone cables have various impedance ratings (30 ohm to 90 ohm typical) and exhibit poor digital
transmission performance. The result is signal drop out and reduced cable lengths due to severe
impedance mis-matching (VSWR) between AES/EBU 110 ohm equipment. It is important to use a
balanced cable with an impedance rating of 110 ohms designed for transmission of digital audio.
* available as an option on ST & FM versions
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