Jacketing -

The jacket of a cable serves as a bundle and physically protects the inner conductors. This makes the criteria for materials used unique. In certain cases, the jacket material may be required to meet standards set by a governing body such as underwriter’s laboratory (UL). In this case, ratings are assigned to the jacket material which relate to its flammability. Cables with a UL rating of CMP, CL3P, and CL2P are considered to be plenum rated. This means that the cable is suitable for use in a plenum, which is an environmental air space or duct. Cables that have a UL rating of CMR, CL3R, and CL2R are considered suitable for riser use. This means that the cable is can be used in vertical riser shafts from floor to floor in a building. A cable with a rating of CM is considered suitable for general use. This means a cable is suitable for general purpose use, with the exception of risers and plenums. Each rating utilizes different mixtures of materials to craft the jacket. Plenum cables tend to be the least flexible, riser rated cable are flexible, but tend to have a memory, and general cables tend to be the more flexible. One of the most commonly used types of jacket material is PVC or polyvinylchloride. PVC is available in numerous blends customized to many applications. It resists most solvents, oil, flame, and sunlight. Although, standards like these have been set you must consult your local authority to determine the appropriate cable rating for the job.

Cable Configuration -

Cable configuration is also a determining factor of function. The major types of cable configurations to be analyzed are a pair of parallel conductors, a twisted pair of conductors, a shielded single conductor, a shielded twisted pair of conductors, and a shielded quad conductor.

A parallel pair of conductors is generally used for a balanced signal with no ground (i.e. speaker cable). However, it can be used for an unbalanced signal with a ground and is generally used when the benefits of twisting are irrelevant such as AC power wiring. In the case of loud speaker cabling the most critical factor is the conductor size. It is important to use the appropriate size to minimize losses over length of run so the amplifier may still apply its control over a loud speaker’s driver. The primary factor is the total impedance presented to the amplifier by the speaker. This value is derived from the number of speakers their impedance and the cables direct current resistance described in Ohms/Mft. However, in most applications only one speaker is connected to one channel of an amplifier. This simplifies the equation and we can now use a general rule of thumb: the longer the cable the higher the resistance; in addition, the thicker the wire the lower the resistance.

Load Z	Length of Run
	<100’	>100’
16 Ohms	16AWG	14AWG
8 Ohms	14AWG	12AWG
4 Ohms	12AWG	10AWG

As a simple reference the table to the left can be used to determine the wire gage necessary. If multiple speakers are connected to a single amp channel the effective load must be calculated first. Formulas for calculating the resistance of serial and parallel loads can be found in most basic electronic texts. However, in the case of high impedance loudspeaker systems or 70V systems wire gage is the key design consideration. For a 70V system, wire gages as small as 18AWG can be used to connect multiple speakers. This is due to transformers wired in parallel across the amplifiers output and an amplifier that provides a constant output voltage is used. A twisted pair of conductors offers the same functionality of a parallel pair with the additional benefit of common mode rejection. This makes the cable less susceptible to electro magnetic noise by reducing the loop area of the cable. When a signal is applied differentially between the two conductors it creates what is known as a common mode signal. Due to the signals equal yet opposite position relative to ground, the common mode signal will be attenuated.

Shielded single conductors (known as coaxial cables) can be used for a wide variety of signal types ranging from analog audio to digital video. The physical characteristics are crucial when choosing a single shielded conductor for video or digital audio. However, less demanding when used for instrument level or analog unbalanced audio. In the case of digital audio the characteristic impedance is the determining factor. If a cable is terminated at its characteristic impedance electronically it will appear infinitely long, consequently reducing signal reflections. The characteristic impedance of a cable is determined by the relationship of the center conductor to the shield and the dielectric that separates them. For example, the characteristic impedance of an unbalanced digital audio cable (S/PDIF format) should be 75Ohms. In the case of unbalanced analog audio the cables shielding is important. These cables are generally used for instrument level signals as well as line level audio signals. Given that a single conductor offers no common mode rejection to electromagnetic coupling, the shield plays an important role of protecting the cable from EMI. However, this is not to say that shielding is unimportant for digital audio. In the case of instrument cabling, a semi conductive PVC tape is utilized as well as a shield to minimize handling noise.

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