Why Networking Cable Works as Audio Cable

Tetrapatch products use the AES-72 standard, which is intended to take advantage of commonly available, standardized network cable. This kind of cable was designed for high-bandwidth digital networks, not audio, but its electrical properties happen to make it excellent for audio as well as networking. this article, we will discuss why this cable works so well for professional-level analog audio.

How AES-72 Uses Quad Twisted Pair Cable

There are many kinds of network cable, but AES-72 was developed with a particular type of cable in mind, known as quad twisted-pair (QTP). Such cables contain four pairs of wire, making eight wires altogether, and each pair of wires is twisted (we will soon discuss the significance of this). Each wire is connected to one of the eight pins on an 8p8c modular jack. AES-72 cables therefore can carry four independent channels of audio.

How AES-72 Puts Standard Network Cable to Work

Although AES-72 does not actually require its use, by far the most common type of network cable is governed by a standard called ANSI/TIA-568 (there is an ISO equivalent as well). Developed beginning in the 1980s to standardize building wiring and connectors for then newly emerging data networks, it specifies not only the construction of the cable, but also electrical specifications which must be met by these cables, including bandwidth and crosstalk.

TIA-568: The Standard Behind Your CAT Cable

TIA-568 specifies many different types of network cable, including coaxial, twisted-pair, and fiber optic, but twisted pair is what interests us, and this is described in part C of the standard. This in turn describes several categories of twisted-pair cable, but the most commonly used today is Category 5 ("Cat-5"), with categories 6, 7, and 8 rapidly growing in popularity. (Categories 1 through 4 are largely obsolete today and seldom used in new installations.)

There are a number of differences between these categories, but the three most important are bandwidth, crosstalk, and immunity.

Bandwidth: Audio Doesn't Come Close to Maxing It Out

Bandwidth refers to the range of frequencies which the cable can carry without a certain specified amount of loss (usually about -3dB). This is important for computer networks, because bandwidth directly limits the data rate of the network. For TIA-568, bandwidths range from 100MHz for Category 5, all the way up to 2GHz for Category 8.

For audio, this is good news, since even professional audio occupies a far smaller bandwidth than data networking. Most audio engineers consider the working range of audio to be between 20Hz and 20kHz. A few say it actually goes higher, to 40kHz or beyond. But even those few hardy souls who teach that audio really goes up to 100kHz will find little to be dissatisfied with in the bandwidth of networking cable: even lowly Category 5 cable exceeds their requirement by over 1,000 times.

Crosstalk: Keeping Channels From Bleeding Into Each Other

Crosstalk is the degree to which the different signals in the cable tend to blend with each other. Ideally, there would be no blending at all: the signals would remain completely isolated. In practice, there is always some blending, so the standard specifies a minimum acceptable amount. Lowering crosstalk is of critical importance in high-speed networking cable, as more crosstalk will cause signal disruptions and degraded throughput. Crosstalk is also of great significance in audio: unwanted mixing of signals can cause all sorts of problems.

Ideally, crosstalk would be below the level of the dynamic range of audio, which can be as high as 120dB (or even 140dB, in free air, although your ears probably can't handle that). However, networked data has much less stringent requirements for crosstalk and noise, and the specifications reflect this. Minimum crosstalk levels are much higher than 120dB, going as low as about 75dB at lower frequencies for CAT-6 cable. This seems well short of the requirements of pro audio, so let's take a closer look at the numbers.

(math math math)

Therefore, crosstalk in the audio band can be estimated to be minimum 115dB at 10kHz, 121dB at 5kHz, and so forth. This is reflected in baseband measurements taken up to this time, and is well within the requirements of pro audio applications.

Immunity: Twisting, Shielding, and Balanced Transmission

Immunity, for electronics, refers not to freedom from disease, but to the insensitivity of circuitry to the roar of radiation constantly flying all around it. The world is full of electromagnetic waves, generated by both natural sources (e.g., the sun) and man-made sources (radios, motors, other electronics), and radio waves induce signals in electronics. A long cable can easily pick up these waves, which usually manifest as various noises, and occasionally even sound from AM radio stations (less common now, with digital broadcasting). Systems designed for high dynamic range, like pro audio systems, must incorporate safeguards against this.

Twisting the Pairs

The first line of defence is to twist the wires in each signal pair. Every audio signal needs at least two wires: signal, and ground. Simply putting a twist in the cable greatly enhances immunity to external sources of interference. This is the purpose of the "twisted" in "quad twisted pair", so all QTP network cable has this feature, including CAT-5.

Shielding

Another defence is (literally) shielding. A shield is a metallic envelope around a cable. Typically the shield is connected to ground. It acts as a (usually incomplete) Faraday cage, and also as a drain for received interference -- the energy is shunted to ground through the shield, rather than continuing on to the cable. Most CAT-6E cable, and all CAT-7 and CAT-8 cable, have some degree of shielding.

Balanced Transmission

An even more powerful trick is to send signals in a balanced format. Balanced transmission works by sending each signal twice: once as itself, and once as a mirror image. By combining the two versions at the receiving end, any interference picked up along the way can be thrown out. Microphones are a common example of this: to preserve the very small microphone signal, they are almost always balanced. Most balanced connections use three wires: positive, negative, and ground. This requires a shield connection, so CAT-6E and up mostly supports balanced transmission of audio signals, while CAT-5 does not.

The Bottom Line for Your Studio

None of this is a coincidence. AES-72 didn't just borrow ethernet cable because it was convenient. It works because the same properties that make CAT cable reliable for data networking, namely bandwidth, controlled crosstalk, and immunity through twisting, shielding, and balanced transmission, happen to line up with what balanced analog audio needs. That is the engineering case for building Tetrapatch around AES-72 in the first place.