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RaneNote

Interfacing AES3 and S/PDIF

Dennis Bohn, Rane Corporation

RaneNote 149 written 2001; last revised 2014

Initials, Acronyms & Standards

AES3 (AES/EBU) & S/PDIF Differences

AES-3id Variation

Conversion Cautions

Converting AES3, AES-3id & S/PDIF

Overview

Mainstream digital audio dates from the introduction of the compact disc in
the early '80s, making it about twenty years old. Today two serial interfaces
coexist: AES3 (aka
AES/EBU) for professional use and S/PDIF
for consumer products. Simple low-cost passive conversion between them is
possible -- even easy -- but it is also filled with cautions. The old rule that
direct connection between AES/EBU and S/PDIF equipment is bad practice is
relaxed today with new receiver chips tolerant to either interface. With that
said, let's explore this tangled nest.

Initials, Acronyms & Standards

The professional digital audio interface known as AES/EBU is initialism for
Audio Engineering Society/European Broadcasting Union, the two
organizations that created the first two-channel digital audio serial interface
standard in 1985. Issued as AES3-1985, it was subsequently revised &
reissued, with the latest version (as of 2001) being AES3-1992
(r1997) AES Recommended Practice for Digital Audio Engineering _
Serial transmission format for two-channel linearly represented digital audio
data. It was also made an American National Standard, issued as ANSI
S4.40-1992, and an international standard, issued as IEC 60958-4. The
importance is that all of these documents cover the same serial
interface. Ray A. Rayburn adds "AES3 allows the use of transformer or
transformerless interfaces, while the corresponding EBU standard requires the
use of transformers. Therefore it has become a common shorthand to say AES/EBU
when the interface is transformer coupled, and AES3 when it is not or if the
interface type is unknown."

In the consumer universe we find the acronym S/PDIF (also seen without
the slash as SPDIF) created from Sony/Philips digital interface
format. This was also made an international standard and issued as IEC
60958-3 (same number, different dash as the professional version), and it
conforms with the EIAJ (Electronic Industry Association Japan) standard
CP-1201 (renumbered CP-340).

AES-3id-1995 AES information document for digital audio
engineering -- Transmission of AES3 formatted data by unbalanced coaxial cable
is the same format as AES3 but instead of 110-ohm balanced line, it is a
75-ohm unbalanced line using BNC connectors and carried over the same coaxial
interface as consumer S/PDIF. AES3id (the hyphen is dropped for
simplicity) is a special AES3 subset for broadcast applications and long
distance runs. (For long distance high-frequency transmission, unbalanced
coax is superior to balanced lines due to the high capacitance of shielded
twisted-pair cable.) Conversion between AES3 and AES3id is very similar to
S/PDIF.

[Note AES3 is a professional (only) audio standard and S/PDIF is a
consumer (only) audio standard, while IEC 60958 and EIAJ CO-1201 cover both
consumer and professional definitions.]

[Caution Do not confuse S/PDIF with SDIF (no P); they are
very different. SDIF, developed and used exclusively by Sony on early
professional machines, is mono and not self-clocking, consequently requiring
three cables for interconnection: two for the stereo channels and one for the
synchronization clock.]

Differences between AES3 and S/PDIF

The following table summarizes the differences in the electrical
characteristics of AES3 and S/PDIF interfaces:

AES31

AES36

AES3id2

S/PDIF3

Interface

Balanced

Balanced

Unbalanced

Unbalanced

Connector

XLR-3

CAT 5

BNC

RCA

Impedance

110 ohms

110 ohms

75 ohms

75 ohms

Output Level

2-7 Vp-p

2-7 Vp-p

1.0 Vp-p

0.5 Vp-p

Max Output

7 Vp-p

7 Vp-p

1.2 Vp-p

0.6 Vp-p

Max Current

64 mA

64 mA

1.6 mA

8 mA

Min Input5

0.2 V

0.2 V

0.32 V

0.2 V

Cable

STP4

CAT 5

Coax

Coax

Max Distance

100 m

400 m

1000 m

10 m

Table 1 indicates a large difference
between the minimum output level and the minimum input level (2 V vs. 0.2 V for
AES3 for instance). The difference is accounted for by two factors:

1) Half the signal is loss due to the impedance matching required for high-speed
transmission lines (output impedance equals input impedance creating a 6 dB
pad); and
2) Signal loss driving long cables.

Conversion Caution

You can convert one electrical interface to another with just a few parts,
but the protocol used in AES3 and S/PDIF is not exactly the same and that can
cause problems. The basic data formats are identical, but there is a bit in the
channel status frame that tells which is which, and assigns certain bits
different meanings.

This sets the stage for incompatibilities. Many older units are sticklers
about what's what in each bit, and even though a given signal faithfully
complies with the standard, some equipment will still reject it. Fortunately,
many units are flexible and tolerant so simple resistor or transformer
converters work. But be warned that a converter that works fine with one unit is
no guarantee that it will work fine with all units.

Remember that even though the audio data is the same between
AES3 and S/PDIF, they have different subcode formats. AES3 converted to
75-ohm coax is not S/PDIF, and S/PDIF converted to XLR balanced is not
AES3. Nor is AES3id 75-ohm BNC the same as 75-ohm RCA S/PDIF -- it may work,
but it is not the same. They are still in their native format; just the
transmission medium has changed. Going from S/PDIF to AES3 has a higher degree
of success than the other way around. AES3 signals often are not recognized as
valid by S/PDIF inputs. Whether they will work in your application depends on
the equipment chosen.

Therefore the following passive circuits convert only the signal level and
impedance, and not other protocol details (e.g., sample rate,
consumer/professional status, nor correct any block errors in the data
stream).

Examining the table reveals the difference in levels as the most troubling.
The minor impedance mismatch is easily taken care of, but the levels and drive
currents are another matter entirely for a passive converter. (If you add
power supplies to the converter then it becomes straightforward, but that is the
subject of another Note and another day.)

AES3 to AES3id or S/PDIF

IF (BIG if) you know that either the transmitter or the
receiver is transformer coupled and the interconnect distance is short then a
simple resistor divider will match the impedances and change the level as shown
in Fig. 1. This is the AES3id recommended network for creating a 12-dB pad (4:1
voltage divider) and converting the AES3 110 ohm balanced output impedance into
75 ohms for driving the AES3id input. Therefore an average output level of 4
volts will be reduced to 1 volt. Since this exceeds the max allowed for S/PDIF,
use the values shown in parenthesis to create an 18-dB pad (8:1 voltage divider)
producing 0.5 volt output for the same 4 volts input. (Other average AES3
output voltages require different resistors -- consult AES3id for value
graph).

Transformers make the best passive impedance matchers, plus provide the
benefits of ground isolation, high-frequency rejection, DC blocking and
short-circuit protection. Impedance matching is easily handled by selection of
the appropriate turns ratio (1.21:1 for 110-ohm to 75-ohm -- it's the square
of the turns ratio for impedance) and careful attention to winding details
allow wideband high-frequency transformers (you need ~12.5 MHz). The best and
safest converter includes the transformer.

If you are not sure about the transformer isolation of the equipment
interfacing, use a store-bought impedance matching transformer that comes
complete with connectors and a separate resistor voltage divider network as
shown in Fig. 2. This T-network is a 75-ohm:75-ohm bi-directional attenuator.
The attenuation is a little less than that of Fig. 1 for the same
(assumed) 4-volt AES3 input because the transformer reduces the voltage
level by a factor of 1.21:1 (down to 3.3 volts) as well as matching the
impedances.

AES3id or S/PDIF to AES3

This conversion only requires impedance matching since the levels are smaller
than AES3, but compatible. No satisfactory resistor-only network exists to
convert between AES3id or S/PDIF and AES3 even though one appears in the AES3id
document. While the network given matches the impedance, it does so with an
attenuation of the input signal, running the risk of making it too small for the
AES3 receiver, and it does not create the balanced 110-ohm lines that an AES3
receiver needs. Therefore it is recommended to use a transformer for this
conversion as shown in Fig. 4. Note that this adapter is identical to that shown
in Fig. 3, only driven backwards. Using the store-bought converter with a
male-to-female adapter allows it to be fully bi-directional _ or build your own
using the recommended transformers. BUT DO NOT FORGET that the levels coming
out of an AES3 transmitter are too large for the typical AES3id or S/PDIF input
so you must add the attenuator. Going the other way you do not need the
attenuator since all AES3 receivers can handle the smallest in-spec AES3id or
S/PDIF output signal as long as a minimum of 200 mV arrives.

AES3id to S/PDIF & Vice-versa

Since both formats use 75-ohm coaxial cable, connecting an AES3id
output to an S/PDIF input is simple. All that is needed is to provide
compatible hardware hook-up and a 6-dB pad (2:1 attenuator). As long as
connecting distances are short, the simple T-network shown in Fig. 5 does the
job.

S/PDIF outputs may be connected directly to AES3id inputs -- no
adapter is required (but don't forget the formatting caveats mentioned
above).

NOTE that even though the network is symmetrical and functions
normally in either direction, it is to be used only to interface between an
AES3id output and an S/PDIF input. If this adapter is used the
other way around, to connect an S/PDIF output to an AES3id input, the
attenuation may make the S/PDIF signal too small for the AES3id receiver to
acquire.

Off-the-Shelf Sources

Retail Sources:

Neutrik AES/EBU
Impedance Transformers

C4 Audio Systems SA
XM BF series

RDL HR-UDC1
Universal Digital Audio Converter

Henry Engineering Digimatch

Hosa CDL-313
Digital Audio Interface

Canare BCJ
Adaptor

Alternatively, build the impedance matching transformer into the resistor
network, as shown in Figure 3.

Component Sources:

Scientific Conversion SC961-04 (SMT) (evaluation PCB available [good tech
notes])

Or Schott P/N 22523
(thru-hole)

1AES3-1992 (r1997); 2AES-3id-1995;
3IEC 60958-3; 4Shielded twisted-pair; 5
Vmin for eye pattern Tmin of 50% of Tnom
(eye pattern is a measurement method for determining transmission
quality.) 6AES3-2003 (Annex D).

"Interfacing
AES3 & S/PDIF" This note in PDF.

© 2014 Rane Corporation

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