PART III Preamplifiers, Mixers, and Interconnects Fillers , nonconductive elements placed around the conductors, help keep the conductor spacing con- stant while semiconductive materials, such as carbon-impregnated cloth or carbon-plastic layers, help dissipate charge buildup. Triboelectric noise is measured through low noise test equipment using three low noise standards: NBS, ISA-S, and MIL-C-17. Mechanically -induced noise is a critical and frequent concern in the use of high-impedance cables such as guitar cords and unbalanced microphone cables that are constantly moving. The properties of spe- cial conductive tapes and insulations are often employed to help prevent mechanically-induced noise. Cable without fillers can often produce triboelectric noise. This is why premise/data category cables are not suitable for flexing, moving audio applications. There are emerging flexible tactical data cables, especially those using bonded pairs, that might be considered for these applications. CONDUIT FILL To find the conduit size required for any cable, or group of cables, do the following: 1. 2. 3. 4. 5. Square the OD (outside diameter) of each cable and total the results. To install only one cable: multiply that number by 0.5927. To install two cables: multiply by 1.0134. To install three or more cables: multiply the total by 0.7854. From step #2 or #3 or #4, select the conduit size with an area equal to or greater than the total area. Use the ID (inside diameter) of the conduit for this determination. This is based on the NEC ratings of single cable two cables three or more cables 53% fill 31% fill 40% fill 190 If the conduit run is 50 ft to 100 ft, reduce the number of cables by 15%. For each 90° bend, reduce the conduit length by 30 ft. Any run over 100 ft requires a pull box at some midpoint. LONG LINE AUDIO TWISTED PAIRS As can be seen in Table 6.29 , low-frequency signals, such as audio, rarely go a quarter-wavelength and, therefore, the attributes of a transmission line, such as the determination of the impedance and the loading/matching of that line, are not considered. However , long twisted pairs are common for telephone and similar applications, and now apply for moderate data rate, such as DSL. A twisted-pair transmission line is loaded at stated intervals by con- necting an inductance in series with the line. Two types of loading are in general usage--lumped and continuous. Loading a line increases the impedance of the line, thereby decreasing the series loss because of the conductor resistance. Although loading decreases the attenuation and distortion and permits a more uniform frequency characteristic, it increases the shunt losses caused by leakage. Loading also causes the line to have a cut-off frequency above which the loss becomes excessive. In a continuously loaded line, loading is obtained by wrapping the complete cable with a high-permeability magnetic tape or wire. The induct- ance is distributed evenly along the line, causing it to behave as a line with distributed constants. In the lumped loading method, toroidal wound coils are placed at equally spaced intervals along the line, as shown in Figure 6.18 . Each coil has an inductance on the order of 88 mH. The insulation between the line conductors and ground must be extremely good if the coils are to function properly. Loading coils will increase the talking distance by 35 to 90 miles for the average telephone line. If a high-frequency cable is not properly terminated, some of the transmitted signal will be reflected back toward the transmitter, reducing the output.