One of the most common types of cabling in professional audio systems is a shielded twisted pair. Generally used for mic and line level analog audio it can also be used for AES EBU digital audio. When using a shielded twisted pair for analog mic/line level signals, there are a couple factors to take into consideration, both of which relate to the cables flexibility and durability. We can break this down by the cables intended use. Generally for permanent installations the cable should utilize a foil shield with a UL listed jacket. These cables tend to be less flexible because of the materials utilized to make the jackets. Also, the foil shield offers a more cost effective cable. For portable or live use, the cable should utilize a braided shield, which offers a longer flex life. However, the jacket choice depends on the environment the cable will be used in. If the cable will be used outside it must have a jacket resistant to sunlight and water. This will make the cable less flexible. If the cable will be used for AES EBU signals the most important factor, yet again, is characteristic impedance. These cables should have 110 Ohm characteristic impedance.

A shielded quad conductor is generally used for microphone level signals where external interference is likely to be a problem. When terminated the conductors that are diagonally opposite are connected together. This is done to reduce EMI coupling by making the effective center of each conductor pair the center of the cable. This gives the cable a higher capacitance. Although the number of conductors and their physical relationship improves the cables EMI rejection, it does not make it useful for long runs. The most important aspect of this cable should be its shield. Since these cables are generally used for portable or patching purposes they must be flexible and utilize a braided shield for a long flex life.

Conclusion -

Using the guidlines presented in this paper to select the proper type of audio cable for a project will optimize system performance and in most cases reduce the overall cabling expense. The chart below lists some common audio cable applications and expamples of recommended product types. For further recommendations please email to info@avcable.com or call 877-229-3601 to discuss your application with one of our signal distribution experts.

Example Application Chart

Application	Permanent Install	Portable	Patching
Speaker	SSC-122SSA105  12 AWG stranded BC twisted pair and PVC jacket	Gepco GSC132 13AWG stranded BC twisted pair and TPE all weather jacket	Gepco GSC122OFC 12AWG stranded OFC BC w/ transparent PVC jacket
Instrument	N/A	Gepco GLC20 20AWG stranded TC w/ 95% BC braid and semi-conductive PVC tape and flexible matte PVC jacket	Gepco GLC20 20AWG stranded TC w/ 95% BC braid and semi-conductive PVC tape and flexible matte PVC jacket
Mic/Line	Gepco 61801EZ 22AWG stranded TC twisted pair w/ 100% foil shield and PVC jacket	Gepco M1042 20AWG stranded TC w/ 95% TC braid and TPE all weather jacket	Gepco XB401 24AWG stranded OFC BC w/ 95% TC braid and flexible matte PVC jacket
Digital Audio S/PDIF	Gepco VPM2000 20AWG  Solid BC w/ 95% TC braid and 100% foil Shield and PVC jacket	Gepco VE61859M 22AWG stranded BC w/ 95% BC braid and flexible matte PVC jacket	Gepco VE61859M 22AWG stranded BC w/ 95% BC braid and flexible matte PVC jacket
Digital Audio AES/EBU	Gepco 5596EZ 24AWG Stranded OFC BC w/ 100% foil shield and PVC jacket	Gepco 5596M 24AWG Stranded OFC BC w/ 95% OFC BC braid and flexible matte PVC jacket	Gepco 5596M 24AWG Stranded OFC BC w/ 95% OFC BC braid and flexible matte PVC jacket

Bibliography

G. H. Phillip Giddings, 1990. “Audio Systems Design and Installation,” p. 140-144, 301-344

Gary Davis, Ralph Jones, 1990. “Yamaha Sound Reinforcement Handbook,” p. 281-355

National Fire Protection Association 2005. “National Electrical Code,” p. 70-602 – 70-605

Definitions

Wire: metal in the form of a usually very flexible thread or slender rod

Cable: an assembly of electrical conductors insulated from each other but laid up together usually by being twisted around a central core.

Conductor: a material or object that permits an electric current to flow easily

Insulator: a material that is a poor conductor (as of electricity or heat)

Current: a flow of electric charge; also: the rate of such flow

Inductance: a: a property of an electric circuit by which an electromotive force is induced in it by a variation of current either in the circuit itself or in a neighboring circuit b: the measure of this property that is equal to the ratio of the induced electromotive force to the rate of change of the inducing current

Capacitance: a: the property of an electric nonconductor that permits the storage of energy as a result of the separation of charge that occurs when opposite surfaces of the nonconductor are maintained at a difference of potential b: the measure of this property that is equal to the ratio of the charge on either surface to the potential difference between the surfaces.

Balanced Line: a: an audio transmission line where the signal is applied differentially between two conductors, each of which has equal impedances to ground or common.

Impedance: a : the apparent opposition in an electrical circuit to the flow of an alternating current that is analogous to the actual electrical resistance to a direct current and that is the ratio of effective electromotive force to the effective current

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