20
User's Guide ADI-4 DD © RME
11.2 AES/EBU - SPDIF
The most important electrical properties of 'AES' and 'SPDIF' can be seen in the below table.
AES/EBU is the professional balanced connection using XLR plugs. The standard is being set
by the
Audio Engineering Society
based on the AES3-1992. For the 'home user', SONY and
Philips have omitted the balanced connection and use either phono plugs or optical cables
(TOSLINK). The format called S/P-DIF (SONY/Philips Digital Interface) is described by IEC
60958.
Type AES3-1992
IEC
60958
Connection
XLR
RCA / Optical
Mode Balanced
Un-balanced
Impedance
110 Ohm
75 Ohm
Level
0.2 V up to 5 Vss
0.2 V up to 0.5 Vss
Clock accuracy
not specified
I:
±
50ppm
II: 0,1%
III: Variable Pitch
Jitter
< 0.025 UI (4.4 ns @ 44.1 kHz)
not specified
Besides the electrical differences, both formats also have a slightly different setup. The two
formats are principally compatible, because the audio information is stored in the same place in
the data stream. However, there are blocks of additional information, which are different for both
standards. In the table, the meaning of the first byte (0) is shown for both formats. Already in the
first bit there is the decision, whether the following bits should be read as Professional or Con-
sumer information.
Byte Mode Bit
0
1
2
3
4
5
6
7
0 Pro P/C
Audio?
Emphasis
Locked
Sample
Freq.
0 Con P/C
Audio?
Copy
Emphasis
Mode
As can be seen, the meaning of the following bits differs quite substantially in both formats. If a
device like a common DAT recorder only has an SPDIF input, it usually understands only this
format. In most cases, it will switch off when being fed Professional-coded data. The table
shows that a Professional-coded signal would lead to malfunctions for copy prohibition and
emphasis, if being read as Consumer-coded data. This actually happened in former times, but if
found today then it was implemented to force the costumer to buy a more expensive device.
Nowadays many devices with SPDIF input can handle Professional subcode. Devices with
AES3 input almost always accept Consumer SPDIF (passive cable adapter necessary).
