Overview
The era of mainstream digital audio kicked off with the advent of the compact disc in the early 1980s, which means it’s been around for about twenty years now. Today, we have two primary serial interfaces in use: AES3 (also known as AES/EBU) for professional applications and S/PDIF for consumer electronics. While it’s relatively simple—and even straightforward—to convert between these interfaces using low-cost, passive methods, there are still plenty of pitfalls to watch out for.
Back in the day, the rule of thumb was to avoid directly connecting AES/EBU and S/PDIF devices, but things have changed. Modern receiver chips are now more versatile and can handle either interface. Even so, navigating the intricacies of these interfaces can still feel like untangling a knotted nest. Let’s dive in and explore this complex web.
Initials, Acronyms & Standards
The professional digital audio interface commonly referred to as AES/EBU stands for Audio Engineering Society/European Broadcasting Union. These are the two organizations that jointly developed the first two-channel digital audio serial interface standard back in 1985. Initially released as AES3-1985, the standard has undergone revisions and reissues over the years. The most recent version, as of 2001, is AES3-1992 (r1997), titled “AES Recommended Practice for Digital Audio Engineering — Serial transmission format for two-channel linearly represented digital audio data.” This standard has also been adopted as an American National Standard (ANSI S4.40-1992) and an international standard (IEC 60958-4). The key point is that all these documents essentially cover the same serial interface.
Ray A. Rayburn notes an important distinction: “AES3 allows the use of either transformer or transformerless interfaces, while the corresponding EBU standard mandates the use of transformers. As a result, it has become common practice to refer to the interface as AES/EBU when it is transformer-coupled, and simply AES3 when it is not, or if the type of interface is unclear.“
In the consumer realm, we encounter the acronym S/PDIF (sometimes written without the slash as SPDIF), which stands for Sony/Philips digital interface format. This too has been standardized internationally and is published as IEC 60958-3 (note the same number but different dash compared to the professional version). It also aligns with the EIAJ (Electronic Industry Association Japan) standard CP-1201 (later renumbered as CP-340).
Another notable standard is AES-3id-1995, which is an AES information document for digital audio engineering. It outlines the transmission of AES3 formatted data via unbalanced coaxial cable. Essentially, it uses the same format as AES3 but employs a 75-ohm unbalanced line with BNC connectors, similar to the coaxial interface used in consumer S/PDIF systems. AES3id (the hyphen is often omitted for simplicity) is a specialized subset of AES3, primarily used in broadcasting and for long-distance runs. For high-frequency transmission over long distances, unbalanced coaxial cable is preferred over balanced lines due to the high capacitance of shielded twisted-pair cables. Converting between AES3 and AES3id is quite similar to converting between AES3 and S/PDIF.
[It’s important to note that AES3 is a professional-only audio standard, while S/PDIF is a consumer-only audio standard. In contrast, IEC 60958 and EIAJ CP-1201 cover both consumer and professional applications.]
Connector
Type I connections use balanced, three-conductor, 110-ohm twisted pair cabling with XLR connectors. Type I connections are most often used in professional installations and are considered the standard connector for AES3.
AES/EBU signals can also be run using unbalanced BNC connectors a with a 75-ohm coaxial cable.
The signal is transmitted over either a coaxial cable using RCA or BNC connectors, or a fibre-optic cable using TOSLINK connectors. S/PDIF interconnects components in home theaters and other digital high-fidelity systems.
Comparation
