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WPI Technical Theatre Handbook: Line-Level
 
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Line-Level

Many different connectors and cables are used for line-level signals. All good line-level cabling is what is known as shielded cable. A braid or wrap of wire around a center conductor in a cable serves to help stop electrostatic and electromagnetic noise from entering the cable. Electromagnetic noise is emitted from several devices commonly found in a theatre situation, such as fluorescent lighting, lighting dimmers, and electric motors.

There are two types of line-level cables used in audio systems -- balanced and unbalanced. Most consumer hi-fi equipment is unbalanced, meaning the signal is carried on only one wire of a cable. In an unbalanced cable, the shield is used as a return path for the signal, while the center conductor carries the signal. Most professional sound equipment offers balanced connections, which are carried on two wires of a cable. Each wire carries a copy of the audio signal, but one is at the opposite polarity than the other. Balanced cables are far less prone to picking up interference, thus their appearance on professional gear, where cable runs may exceed hundreds of feet.

A few types of connectors are used on cables to connect equipment together. The most common types are the XLR cable and the 1/4 inch phone plug. Each can be used to carry balanced or unbalanced signals. Another type of connector sometimes used is the RCA phono connector. This connector is the standard used to interconnect consumer hi-fi gear, but is rarely found on professional equipment. RCA connectors can only carry unbalanced signals. Figure 6.11 shows the different types of audio connectors.

Figure 6.11: Various line-level audio connectors.
\begin{figure}\psfig{file=audio/lineconns.eps,width=3in}\end{figure}

Cable that has a male XLR connector at one end and a female XLR at the other is most commonly referred to as mic cable, simply because that's what it's most often used for. Other variations exist, such as male XLR to male phone plug, female phone plug to male phono plug, etc. These types of cables are referred to as adapters, converters, or at WPI, funnies. Finding all of the appropriate cables to hook the line-level components of a system together can sometimes be a challenge, and one should feel some sense of accomplishment after doing so. Often finding the required cables is more of a challenge than actually hooking up the system!

Often times it is necessary to carry a large number of separate audio signals a substantial distance. While it is possible to run several lengths of individual cables, this is less than convenient. For this reason, multicore snakes have been developed that carry anywhere from a few to forty or more signals. These cables are essentially several individually shielded cables bundled together. Most snakes have one end that is a box with panel-mounted connectors, such as the one shown in figure 6.12. The other end is generally a mass of wires and connectors, and is called the tail end of the snake. The tails of a typical snake are shown in figure 6.13. Snakes are extremely useful for carrying signals from several microphones and other equipment located on a stage or remote location to a sound console.

Figure 6.12: The box end of an audio snake. Thirty two XLR channels (typically used as inputs to a sound board) are available, as are six XLR/phone plug channels (often used as sends from the sound board). Typically this is the end placed on or near the stage.
\begin{figure}\psfig{file=audio/snake2.eps,width=3in}\end{figure}

Figure 6.13: The tails of an audio snake. These connectors are typically connected to the inputs and outputs of a sound board.
\begin{figure}\psfig{file=audio/snake3.eps,width=3in}\end{figure}


next up previous contents index
Next: Speaker-Level Up: Cabling and Connectors Previous: Cabling and Connectors   Contents   Index
Steve Richardson 2000-07-06

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