40
4.2.1 The AES/EBU and S/PDIF standards
Essentially, two standards exist for the transfer of digital audio data. Their most important electrical specifica-
tions are summarized in Table 4.1. AES/EBU is a professional, balanced connection using XLR connectors.
For semiprofessional users, Sony and Philips have opted against balanced connections and use either cinch
connectors or optical waveguides. The process which was standardized as IEC 958 and is generally known
as S/PDIF (Sony/Philips Digital Interface) became best known for the associated efforts to introduce copy
protection, however.
Typ
AES / EBU
IEC 958 (S/PDIF)
Connection
XLR
Cinch / Optisch
Mode
Balanced
Unbalanced
Impedance
110 Ohm
75 Ohm
Level
0,2 V bis 5 Vpp
0,2 V bis 0,5 V pp
Clock accuracy
Not specified
I: ± 50 ppm II: 0,1 % III: Variable Pitch
Jitter
± 20 ns
Not specified
Tab. 4.1: Important data for the AES and IEC specifications
Unfortunately, the professional standard was soon watered down, as the design of many units didn't provide
enough room for XLR connectors. Instead, stereo jacks, miniature jacks and special adapters for sub-D
connectors were used.
In addition to the electrical differences, the formats also have a slightly different structure. The audio informa-
tion occupies the same position in the data stream, making the two formats compatible in principle. The
differences between the two standards can be found in the information blocks. Table 4.2 shows a section of
the professional format data structure generally used in AES/EBU connections.
Byte
Bit
0
1
2
3
4
5
6
7
0
P/C
Audio?
Emphasis
Locked
Sampl. Freq.
1
Channel Mode
Use of User bits
2
Use of AUX bits
Length of audio sample
Reserved
3
Reserved for description of multichannel recording
4
Audio reference
Reserved
5
Reserved
Tab. 4.2: Professional format (AES/EBU) ID data
Table 4.3 shows the corresponding data of the consumer format commonly used for S/PDIF connections. The
first bit serves to identify the following data as professional or consumer format.
Byte
Bit
0
1
2
3
4
5
6
7
0
P/C
Audio?
Copy
Emphasis
Mode
1
Category code
Gen.St.
2
Source number
Channel number
3
Sampling frequency
Clock accuracy
Reserved
Tab. 4.3: Consumer format (S/PDIF) ID data
As can be seen, there are major differences in the significance of the following bits between the two formats. If
a device such as a commercially available DAT recorder only has a S/PDIF input, the device will normally only
understand that format. Generally, it will switch off when receiving professional-format data. The reason is
simple: As the illustrations show, a signal coded in professional format would lead to copy-protection and
emphasis errors in a device only capable of processing the consumer format!
As was already the case with the connectors, this point is often not immediately obvious. Many units do not
switch off, others can process both formats despite having only a single connector type, and still others are
4. TECHNICAL BACKGROUND
