A system is only as good as its cables, interconnects, snakes, and networks -- period
April 04, 2014, by Craig Leerman
A few weeks ago one of the neighbors in the industrial complex where I keep my shop came over to say hello while I was in the middle of doing some PM (preventative maintenance) on cables.
As I sat at a bench surrounded by piles of microphone and loudspeaker lines, he asked why I was spending so much time on “stupid cords.” I replied, simply, that without the stupid cords, the rest of my equipment is worthless. A system is only as good as its cables, interconnects, snakes, and networks—period. Fortunately, we have a wide variety of analog and digital options to choose from, and it’s getting better on a constant basis.
Digital audio transport technology (a.k.a., digital snakes and networks) have taken pro audio by storm in the last few years, pushed at least in part by the proliferation of digital consoles, with virtually every manufacturer offering some way to move audio over Cat-5/6, coax, and fiber optic cabling. While digital networking certainly offers a lot of advantages and flexibility, it hasn’t pushed analog completely out of the picture—and in my opinion, at least, I don’t think it will, at least in the foreseeable future.
One reason is personal preference, another is the sheer amount of cabling that will have to be replaced, and yet another big one is that digital systems need A-D and D-A conversion at each end of the cable (or fiber), which increases cost, and this is particularly dramatic for smaller systems that only run a few channels of audio. As technologies improve and prices come down, I’m sure we’ll see even more digital, even on the smallest of shows, but there’s still the issue of preference.
That said, let’s take a look at the various cables, connectors, and audio transport used in production audio systems.
Here To There
The first cable in the signal chain is usually the humble XLR cable sporting 3-conductor connectors at each end. These cables connect low-impedance microphones and direct boxes to consoles, as well as send line level signals around to various gear.
Left to right: XLR (female end), TRS 1/4-inch, signal 1/4-inch, loudspeaker 1/4-inch, loudspeaker 1/4-inch with larger barrel, 4-pin Speakon, 19-in Soco male, and 19-pin Soco female. Note the use of colored heat shrink to quickly ID signal (blue and red) and loudspeaker cables.
They operate on the balanced principle and contain two insulated conductors that are twisted together inside a shield under the outer jacket. The audio signal is applied to the pair of conductors differentially, that is to say that one wire has the polarity of the signal reversed but the levels are the same. Any noise or outside interference that gets into the signal lines will mostly be defeated because one conductor transmits the noise with a positive polarity and the other is at a negative polarity.
When signals with opposite polarity (in this case, the noise) are combined, they will cancel each other out. The reason the inner conductors are twisted is that it allows external noise to be introduced to both signal conductors equally (or as equally as possible) and improves the common-mode rejection ratio. Some cables use four inner conductors (two pairs of two) that offer better rejection from outside electromagnetic interference like transformers and fluorescent lighting ballasts.
The conducting shield that wraps around the inner wires is used for the signal common and can be a spiral winding or a braided winding. Braided shields provide more surface area coverage and better rejection of radio frequency interference (RFI) than spiral wound shields.
Similar in construction to the XLR is a cable that instead has 3-conductor 1/4-inch phone plugs at each end, usually called a TRS cable. The TRS refers to Tip, Ring, and Sleeve, the three conductor positions on the connector.
These are commonly used as interconnection cables between rack gear and are a popular option for manufacturers who want to use balanced connections but have limited real estate on the product in which to squeeze in XLR connections.
Many consoles have insert jacks that allow patching of external processing into a channel or group. They normally use a TRS 1/4-inch jack and a special Y cable called an “insert cable” that is outfitted with a TRS plug on one end and a 2-conductor 1/4-inch plug at each end of the Y that is used to route to the inputs and outputs of the external processor. The TRS end is usually wired so the tip is the send to the external unit, the ring is the return and the sleeve is the shield or common.
While similar in looks to a TRS cable, a regular 1/4-inch signal cable is quite different. It has only one inner conductor surrounded by a spiral or braided shield. They are used with high-impedance signals from a guitar or keyboard to connect them to a stage amplifier or DI. The outer braid acts as both a conductor and a barrier to help keep RFI and other noises from reaching the center “hot” conductor.
When used with a guitar or other high-impedance input, the cable’s capacitance couples with the high impedance to create a low-pass filter that varies depending on cable length. The longer the cable, the more highs it rolls off, so 1/4-inch cable runs are usually kept under 25 feet in length unless they’re serving electronic keyboards, which output a hot line-level signal that can drive longer runs.
Another cable that may look identical to these first two is the 1/4-inch loudspeaker cable. While these may have a 1/4-inch plug on each end, the loudspeaker cable is a different animal altogether, designed to move large amounts of output current from an amplifier to a loudspeaker, not the mere milliamps that signal cables handle. Constructed of two heavy-gauge inner-insulated conductors housed in an outer jacket, these cables are commonly used to connect a stage amplifier head to its loudspeaker cabinet, or a small PA loudspeaker to a powered mixer.
Just a reminder—signal cables should never be used for loudspeaker lines, and vice versa. Signal cable isn’t designed to handle high current, and loudspeaker cable is not shielded from outside interference.
The most popular loudspeaker connector in pro audio is the Speakon (stylized as speakON) from Neutrik. They come in 2-, 4- and 8-pin varieties, allowing a multitude of connections options. The wire size (gauge) of loudspeaker cable depends on a few factors, chiefly the load impedance and the length of the cable. Simply put, the longer the cable, the larger the conductors should be. Common sizes for audio production include 12- and 14-gauge, with a few manufacturers also offering multi-conductor cable in 13-gauge.
Two 50-foot, 6-channel boxes to fan stage snakes.
Some sound companies deploy an 18-conductor cable with a 19-pin connector called a Soco, borrowed from the lighting world. The term Soco comes from the trade name of the most common 19-pin connector manufactured by Socapex, but companies like Veam and Kupo also make compatible connectors. Lighting folks use the cable for six circuits of power, while audio folks wire up their systems differently and can get up to nine speaker circuits in one cable. A “Soco to fan out” distributes signal to the various loudspeaker cabinets.
Speaking of multi-circuit cables, snakes are the answer for running multiple channels of audio from one place to another. These cables could have a breakout fan on one or both ends to individual channel lines, or could use a box at one end (usually at the stage end) that individual XLR cables can be plugged into. Snakes can also integrate multi-pin connectors that make it faster and easier to hook up a system. To save weight and size in the cable many snakes use a foil shield around each pair of channel conductors instead of a braided or spiral wrapped shield.
Aside from the obvious stage to FOH mixer application, smaller “stage” snakes are a popular way to help manage cable runs on stage and keep things neat and organized. Another use of snakes is for “crosslink cables” running the signal to the PA system from one side of the stage to the other.
Many snakes have the capability to run signals from and to the stage. The “sends” are for the mic inputs to the mixer and the “returns” get the output of the mixer to the amp rack or powered loudspeakers. Larger systems may use a separate return snake for the line-level outputs to keep any crosstalk (interference from adjacent snake channels) to a minimum.
Splitter snakes provide more than one output off the send side of a snake, so the same inputs can be sent to multiple consoles (i.e., when using a separate monitor or broadcast console along with the house console). Some splitters are passive and simply hardwire a “Y” off each channel.
A better practice is to use isolation transformers to isolate each console from potential noises and hums and buzzes caused when plugging them into different power sources. In a split snake system, usually one split is hard-wired to the inputs so that the console can pass phantom power to mics and DIs.
Another version, called a “power snake,” combines a few loudspeaker lines along with the signal channels. These can work well for a small system on short runs but their use is usually limited to about 100 feet. Yet another multi-circuit version that has become popular recently is cable systems that include signal and power in one jacket. These are perfect for getting audio and AC power to a powered loudspeaker or floor wedge.
While analog cables still fill the road trunks, digital systems are starting to take over many of the audio transport duties. They offer a host of signal routing benefits that analog simply can’t match, including using a small thin cable to route multiple channels of audio. Smaller cable equals less stagehands required to lay out a digital network as opposed to large, heavy multi-core snakes.
Digital cables are also less prone to RFI and crosstalk. Networks, as we now call our digital transport systems, can offer audio almost anywhere along the line, and can easily interface with multi-track recording systems, personal monitoring rigs and broadcast trucks.
Transport networks use one of three types of cable: fiber optic, coax or Ethernet Cat-5/6. Coax cables offer up a rugged solution and are used by a few manufacturers to transport signals between stage boxes, consoles and recorders. Fiber optics offer the ability to send signals over very long distances, and because the signals travel as light, are immune to all outside electromagnetic disturbances and RFI. Ethernet Cat-5/6 cables are the most popular, found in many different systems to transport audio at distances of up to about 330 feet (100 meters). Some of these have accessories that can extend this distance.
A typical molded RJ45 connector (left) with an Ethercon connector.
Ethernet cables have RJ45 8-pin connectors that are stout enough for home computer use but not rugged enough for most gig uses, so they’re best replaced with rugged Ethercon connectors that surround the plastic crimp-on with a metal barrel that provides added protection in addition to better locking.
Ethernet cables come in a variety of styles. Some have solid wire conductors that offer the best performance, while others have stranded conductors that provide greater flexibility. They can be unshielded but it’s better to go with shielded in noisy environments.
Ethernet cables can also be wired in different ways. The “standard” wiring scheme runs pin 1 to pin 1, pin 2 to pin 2, etc. “Crossed over” cables wire pin 1 to pin 3, pin 2 to pin 6, pin 3 to pin 1 and pin 6 to pin 2. Before choosing an Ethernet cable, check manufacturer recommendations on which cable is recommended for interconnection of specific gear.
As noted earlier, a downside to digital transport is that there is the need for analog to digital conversion, and further, manufacturers utilize a variety of variety of different protocols that are not compatible. However, that’s been changing rapidly, as more and more devices support multiple protocols, and the Audinate Dante protocol in particular has really caught on the past few years. And, AVB (Audio/Video/Bridging) is a standard that manufacturers are also starting to embrace. The bottom line is that our job of interconnecting various gear from various manufacturers is getting easier.
Senior contributing editor Craig Leerman is the owner of Tech Works, a production company based in Las Vegas.