AES 72 Standard for Quad Twisted Pair Audio Transmission
When people are setting up devices to send analog audio over CAT5 or CAT6 using RJ45 or Ethercon connectors, they’re often using the AES 72 Standard and Quad Twisted Pair Audio Transmission.
To get a good grasp of the AES 72 Standard for Quad Twisted Pair Audio Transmission, it’s important to know that there isn’t a single standard. In fact, there are nine different standards, and some companies are creating their own versions.
AES 72 Standard
The AES 72 standard is all about balanced analog and AES3 digital audio transmission over quad twisted pair (QTP) cabling, using RJ45 (8P8C) connectors—not to be confused with Ethernet.
QTP cabling, if it’s CAT5, CAT5e, or CAT6, is a great, affordable way to handle multiple audio channels in one cable. This simplifies things and saves money compared to traditional audio cabling. It’s a good idea to use a grounded or shielded cable. The twisted pair and balanced signals help minimize EMI or radio signals getting on your cable, but having a shield and ground really protects the signal, especially when it’s a microphone signal.
Twisted pair cabling is a tried-and-true technology that was invented in the telephone industry to reduce electromagnetic interference (EMI) by twisting pairs of wires together. Broadcasting and pro audio have adopted telephone wiring practices, but there are some common mistakes, like using too many adapters and unclear wiring maps. The AES0505 group looked at how people are using cables and noticed that there was not a standard way to assign pairs, determine polarity, use phantom power, or shield cables for analog audio.
The RJ45 connector we adopted from Ethernet carries 4 twisted pairs, which means it can carry 8 channels of AES3 Digital Audio or 4 Analog Audio channels. Just because you see two manufacturers making a box with RJ45 and 4 XLR does not mean they’ll work together. They’ll pass audio, but the channels might not match. Channel one might not show up in channel one of the other box, or pin 2 might show up in pin 3 on the other box. So, when you’re shopping from different manufacturers, it’s a good idea to double-check that they’re using an AES72 standard and that it works with what you already have.
The audio industry has a bunch of different standards, and AES has been a big help in bringing them together. Take the XLR connector, for example. It was created by Cannon Electric (now part of ITT) back in the early 1950s. It took about 15 years for it to become the go-to standard for microphones and balanced cables, and by the early 1960s, it was widely used. So, the industry found a connector they could agree on, but it took over 30 years for the industry to agree on the wiring configuration. Pin 2 was officially set as the “hot” (positive) standard for XLR cables in 1992 with the AES14-1992 Standard (later adopted as ANSI S4.48-1992) by the Audio Engineering Society. Before that, you had to check if your equipment was connected with Pin 2 or Pin 3 as the hot leg.
Now, we’re facing similar challenges with the RJ45 connector in the industry. Cat cables usually have about 100Ω impedance, which means they fit into both AES3 digital audio and Analog audio standards. However, there are still many different cable types and electrical characteristics to keep in mind. Is the cable solid or stranded, and how is it shielded? These details can affect how well it handles EMI and noise.
Since this is an older connector, we’re dealing with legacy and manufacturer variations. Two main RJ45 pinout types, TIA-568A and TIA-568B, can cause polarity and pair mapping issues. This is super important when you’re building or repairing cables, that you know which TIA-568 variant you’re using. There’s not much difference in performance between the two wiring variations. The T568A was chosen because it was backwards compatible with analog telephone lines. As the RJ45 became popular in commercial settings, the T568B was adopted and is still the standard for commercial work today.
Ethernet networking uses transformers on every port to manage polarity and impedance. Most Analog and AES3 digital audio on RJ45 do not use transformers for isolation, which brings challenges: so having a good shield on our cable is important. Shielded cables provide better electromagnetic interference rejection, especially for analog microphone-level signals. It also allows you to provide phantom Power for condenser mics and active direct boxes. Phantom power application over twisted pairs requires dedicated return paths over the cables shield
Even if you don’t need a shielded cable to send analog or AES3 digital audio over twisted pair, microphone applications really benefit from shielded cables with good bonding and phantom power return paths. Line-level AES3 and analog audio signals are less susceptible to such interference and can often use non-shielded cable for cost savings and ease of installation but its not recommended.
Audio Engineering Society (AES) has been a large help organizing what people and manufacturers are currently doing to implement the RJ45 connection to our industry’s work flow but there is a lot of work to do before we are able to get the standards down to a single standard. If you look at the XLR connector it took over 30 years before there was agreement on the standard and you look at other connectors like the BP25, they were able to get it down to 2 primary standards. So you have 50/50 chance of getting the correct adapter. With RJ45 to analog audio you have 9 standards and people still picking other ways to use the 8 conductors. So be careful buying generic adapters and check with the manufacturer to confirm their pinout.
From my research it looks like the most popular AES73 is Type 1M some of the most well known manufacturers of this style of adapter have chosen other configurations. Please do your due diligence to confirm my findings but the chart below may help steer you in the correct direction.
If you need help wiring your next project please contact OCD-Labs at INFO@OCD-Labs.com
Primary resources
AES Standards Webinar – AES72: Audio over Quad Twisted Pair
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