Audio multicore cable
An audio multicore cable (often colloquially referred to in the US and Canada as a snake cable or just a snake) is a thick cable which contains from four to 64 individual audio cables inside a common, sturdy outer jacket. Audio multicore cables are widely used whenever multiple audio signals, for example from a number of microphones, need to be conveyed between common locations. Typical professional audio applications include audio recording, sound reinforcement, PA systems and broadcasting. The "snake" is typically used to make it easier to route many signals from the microphones or other input transducers, to the audio console or sound equipment.
Without a snake, a rock band performing onstage, for example, would have to have 20 or more individual mic cables running from the stage to the mixing console, which is typically located at the rear of a venue. It would be easy for the cables to become tangled, and it would be very hard for the audio engineers connecting the mic cables to the mixing board to determine which cable is for which mic (or other input).
The term can be used to refer to the basic cable as may be obtained from a supplier, or a cable with the ends terminated in either a multipin connector, a number of separate connectors on the individual pairs, or a box or panel with a number of chassis-mounted connectors. Different termination methods can be used on each end to suit the application. When individual connectors are used, three pin XLR connectors are most common, although 1/4" jack connectors are also occasionally used. Assembled audio multicore cables are found in both portable applications, where they are stored loose or on a cable drum, so that they can be taken from one location to another, or are permanently installed in a building. The availability of portable snakes enables sound engineers to set up sound systems at temporary outdoor locations, such as music festivals. Snakes are permanently installed in the walls of recording studios and performance venues such as arts centres and large nightclubs.
The vast majority of audio multicore cables consist of a number of twisted-pair copper wires, suitable for balanced audio. However, the broadcast industry tends to use audio multicores containing star quad cables, due to their increased rejection of radio-frequency interference. Some multicore cables designed for unbalanced audio are made, and they contain a number of single-core screened cables.
The individual cables inside the multicore are usually individually screened. This reduces crosstalk between cables and also enables each cable to have a screen or ground that is isolated from other cables, significantly reducing the possibility of earth loops (also known as "ground loops"), which can cause unwanted hum.
A foil-screen is typically used, comprising thin aluminium wrapped around the conductors. An uninsulated drain wire is contained inside the screen to facilitate ground connection. A less common construction is a lapped or braided screen comprising thin wires wrapped around the conductors. This gives improved flexibility, but less effective screening.
This describes the method used to isolate the screening between individual cores; two approaches are common. The first method has an extruded plastic sheath around each individual core, making the multicore appear like a number of individual audio cables bundled together inside an outer sheath. The second method dispenses with the individual sheath and typically wraps the screening with very thin plastic; in some cases this plastic is bonded to the foil sheath.
The individually sheathed construction has the advantage that individually cores can easily be terminated to individual connectors at the ends of the cable. Its disadvantage is that the size and weight of the cable are increased. When a cable without individual sheaths has to be terminated to multiple connectors, it is common practice to create individual sheaths using heat-shrinkable tubing.
A small number of multicore cables compromise twisted-pairs with an overall screen. These are virtually obsolete and typically only found in older broadcast installations, the use of individual screens is now virtually standard. Some multicore cables do contain an overall braid screen in addition to the individual screens on each core. This also increases the mechanical durability of the cable.
Different cable manufacturers use different methods of identification for the shielded pairs of cable. Belden have a sequenced color code and a number for each of the conductors in their products of up to 52 pair cables, for their plenum cables each pair is covered with their patended Belfoil shield, that is only conductive on the inner surface. For their portable cables they use a French braid. Canare, Mogami, Clark Wire & Cable and GEPCO mark numbers on the PVC insulation of the individual pairs.
Composite multicores combine different types of signals in the one cable. They may contain coaxial cores for video, twisted pair for data or low voltage cores for mains power. Composite multicores are usually used to connect professional video cameras, but they are now gaining usage in live event support with the introduction of the Yamaha PM1D which uses a composite cable to connect it to the stage box.
The multicore cable runs from the stage box or microphone splitter and then to the front-of-house sound desk or mixing console. Permanent installations have stage boxes mounted in the floor or side of stage and the plenum cable runs through the ceiling or false floor to the console, located either in the auditorium or the bio-box.
For temporary shows the stage box is placed at the side or rear of the stage. The stage box contains around 75% sends, and 25% returns. Both sends and returns tend to use XLRs; however, some systems use TRS A Gauge 1/4" phone connector for the returns. Returns tend to be used for foldback (on-stage monitoring) or FOH returns at line level. Larger systems can be fitted with split points integrated into the stage box, providing both an input and an isolated output on each line. In this system, the split point could be connected to the foldback desk, via another multicore. Larger shows tend to use a system with subsnakes which plug into the main stage box so one can use shorter microphone leads on stage and have less clutter.
The output of most microphones is balanced to reduce the pickup of electrical noise, and therefore requires two signal wires and an earth per channel. This necessitates the use of three-pin connectors. To reduce noise, the shields of each channel are often isolated from the other channels. Phone jacks may be used instead of XLRs.
The end of the multicore where the channels fan out is called the tail and it generally goes at the mixing desk end. Tails used for patching may have a mixture of male and female XLRs, or TRS phone connectors on each end.
An audio multicore may also function as a breakout cable, if it has a compound connector on one end and component connectors on the other. This is more common in short multicores meant for in-studio connections, such as audio engines, analog to digital converters or digital mixing consoles. Multicore cables may also connect to either the front or back of patch panels, when the patch panel is used as an access point or breakout box for connecting external inputs and outputs.
The ideal system uses multipins to connect the subsnakes to the main stage box (which may be rack mounted) and multipins to the main cores. This allows the system to be expanded by channel count or length. The multipin connectors are based on the MIL C5015 standard and are made by companies like Link in Italy or Amphenol in the United States. Alternative connectors like EDAC and Burndy are also used but lack the reliability of the alloy mil-spec style connectors. Examples include;
|Multicore||MIL C5015 Multipin||Sends/Returns|
|8 core||25 pole||8/0|
|12 core||37 pole||8/4|
|16 core||54 pole||12/4|
|24 core||85 pole||20/4|
While the sound reinforcement industry generally uses proprietary multicores depending on the application, the television broadcasting industry has standardized on a 12-channel snake with a common 37-pin connector on each end known as DT-12. Only 36 pins are used and sometimes the unused pin is omitted. DT-12 snakes are commonly built into sports venues and run from where the TV truck is parked to areas such as the press box or playing field where audio is required by the TV production. The truck can then simply run a DT-12 patch snake between the truck and the house cabling, with actual XLR connectors then only being required at the very end of the snake in order to plug in microphones and other equipment.
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Audio systems have the option to use digital systems instead of multicore cables. In these systems, the audio signal is encoded to digital using analog-to-digital converters, the channels are bundled together to be transported on a single wire to a destination then re-converted back to analog using D-to-As. Sometimes all that is run to the mix positions is control and all audio processing occurs in the stage box.
The main disadvantage with these digital systems is latency, the time taken to encode into digital and then back into analog at the mixing console. Most standalone digital consoles take their inputs in analog format and the cumulative latency is too great to go unnoticed. Cost can also be a disadvantage. It can more expensive to use a digital system especially for small systems. Most digital systems use Audio over Ethernet though some use MADI or proprietary transmission technologies.
- http://www.prosoundweb.com/article/unbalanced_vs_balanced_lines/ Balanced vs. Unbalanced Lines
- http://www.yamahaproaudio.com/products/interfaces/ygdai_cards/index.html Yamaha Mini-YGDAI interface cards