Introduction to AES-72
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What is AES-72?
AES-72 is a framework for making professional-grade multichannel analog audio connections using ordinary Ethernet cable.
Connecting multiple analog audio signals is as necessary in the modern audio production facility as it has ever been. Over the years, various schemes have been used for this, usually involving existing widely-used connector types like EDAC or subminiature D-shell (DB25). This is because the professional audio industry is generally too small to make its own standards, so it borrows from larger industries which can.
AES-72 continues in this tradition by borrowing what has come to be known as "Ethernet cable": TIA/EIA-568, categories 5 and up, with RJ-45 connectors. Thanks to the vast growth of Ethernet 100BASE-T, this cable and connector combination has become ubiquitous, which means two important things: cheap, and widely available. Very few music specialty shops keep DB-25 snake in stock, but you can buy high-quality shielded Ethernet cable at many home centers for a very reasonable price.
The modular jacks used with AES-72 sometimes cause confusion due to their familiar use with Ethernet, so it's worth emphasizing that AES-72 is entirely analog -- there is nothing about Ethernet involved. AES-72 is not "audio over Ethernet", but audio over Ethernet cable. Ethernet is digital, but AES-72 is completely analog.
Some Advantages of Using AES-72
The Connector
There are many advantages to the AES-72 scheme. The familiar modular connector is a major one: cheap, simple, and small, it packs more signals into a smaller space than its DB-25 rival. And just as the cables are easier to get than DB-25, so the connectors are cheaper too.
The Cable
The cable itself is another advantage. The most commonly used forms of Ethernet, 10/100/1000BASE-T, use TIA/EIA-568 cable. These cables have eight conductors, each conductor brought out to one of the eight pins on an RJ-45 connector (technically an 8p8c modular connector). Eight wires are enough (almost) to carry four audio signals independently. And unlike many other cabling standards (looking at you, HDMI!), if it says CAT-5, you are guaranteed that all eight wires will be there.
Twisted Pairs and Crosstalk
If those eight wires were simply bundled up willy-nilly, there could be crosstalk; wires which run very close to each other for a long distance tend to "talk", with a little signal bleeding from one to the other. This is a serious problem for networking, so to fight it, TIA/EIA-568 uses a clever solution (apparently invented by Alexander Graham Bell!): each pair of wires is twisted, which not only helps the pairs keep their signals from mixing up, but also reduces the amount of interference they pick up while running from place to place. This is why this kind of cable is often referred to as "quad twisted pair", or "QTP".
AES-72 also ensures that each audio signal gets its own twisted pair -- you won't have two signals sharing a pair, which could be a disaster for crosstalk. With AES-72, what you put in the cable at one end, you will get out on the other, with each channel preserved from unwanted crosstalk and interference.
Balanced signals over CAT cable
Unbalanced audio cable has two wires: signal, and ground. Balanced audio cable has three: positive, negative, and shield (ground). CAT cable has eight wires, so how can it carry four balanced signals?
The answer is shielded cable, like CAT-8. This adds a shield, connected to both ends of the cable. The RJ-45 connector is given a metal shell, which is terminated to a drain wire, and also makes a connection to a ninth shield pin in the RJ-45 connector.
The compromise, versus balanced cable, is that all four balanced connections shares a shield. In practice this is no problem: there is little reason to maintain independent shield connections, especially for connections going to the same or similar earth domains.
AES-72 pinouts
The attentive reader may have noticed that we have hesitated to use the word "standard" to refer to AES-72. That's because AES-72 doesn't specify just one pin arrangement, but nine:
| Type | Pin # | Polarity | |||||||
|---|---|---|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | ||
| Type 1E | 1− | 1+ | 2− | 3+ | 3− | 2+ | 4− | 4+ | Even = + |
| Type 1O | 1+ | 1− | 2+ | 3− | 3+ | 2− | 4+ | 4− | Odd = + |
| Type 1M | 1+ | 1− | 2+ | 3+ | 3− | 2− | 4+ | 4− | Mixed |
| Type 2O | 2+ | 2− | 3+ | 1− | 1+ | 3− | 4+ | 4− | Odd = + |
| Type 3E | 3− | 3+ | 2− | 1+ | 1− | 2+ | 4− | 4+ | Even = + |
| Type 3M | 3+ | 3− | 2+ | 1+ | 1− | 2− | 4+ | 4− | Mixed |
| Type 4E | 1− | 1+ | 3− | 2+ | 2− | 3+ | 4− | 4+ | Even = + |
| Type 5E | 3− | 3+ | 4− | 1+ | 1− | 4+ | 2− | 2+ | Even = + |
| Type 6E | 4− | 4+ | 3− | 2+ | 2− | 3+ | 1− | 1+ | Even = + |
Pinout data per the AES-72 standard (Audio Engineering Society). Ashton Research Tetrapatch products use the Type-1M pinout, highlighted above.
The standard does not recommend any one of these pinouts above the others. Ashton Research Tetrapatch products use the Type-1M pinout.
One thing that all of these pinouts have in common is their conformance to the T568A/B arrangement of wire pairs. This ensures that any AES-72 pinout will keep audio signals on their own twisted pairs, and not mixed up across pairs. The worst thing that can happen when mixing AES-72 pinout types is polarity inversion and channel order swap -- not insignificant problems, but extremely unlikely to cause damage.
Conclusion
None of this is exotic. AES-72 takes the most ubiquitous cable ever made and puts it to work carrying clean, balanced, fully analog audio — just wires doing what wires do. The trade is a good one: standard cable in place of specialized snake, a few tidy CAT runs in place of one fat multicore. The audio industry has always borrowed its cabling from larger industries. This time it borrowed well.