Handbook for Sound Engineers

Transmission Techniques: Wire and Cable 431

14.20 Conduit Fill

To find the conduit size required for any cable, or group

of cables, do the following:

1. Square the OD (outside diameter) of each cable and
   total the results.


2. To install only one cable: multiply that number by
   0.5927.


3. To install two cables: multiply by 1.0134.


4. To install three or more cables: multiply the total
   by 0.7854.


5. 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 53% fill


• two cables 31% fill


• three or more cables 40% fill

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.

14.21 Long Line Audio Twisted Pairs

As can be seen in Table 14-29, low frequency signals,

such as audio, rarely go a quarter-wavelength and, there-

fore, the attributes of a transmission line, such as the

determination of the impedance and the loading/match-

ing of that line, are not considered.

However, long twisted pairs are common for tele-

phone and similar applications, and now apply for

moderate data rate, such as DSL. A twisted-pair trans-

mission line is loaded at stated intervals by connecting

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 char-

acteristic, it increases the shunt losses caused by

leakage. Loading also causes the line to have a cutoff

frequency above which the loss becomes excessive. In a

continuously loaded line, loading is obtained by wrap-

ping the complete cable with a high-permeability

magnetic tape or wire. The inductance 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 Fig. 14-18. 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.

14.22 Delay and Delay Skew

The fact that every cable has a velocity of propagation,

obviously means that it takes time for a signal to go

down a cable. That time is called delay, normally mea-

sured in nanoseconds (Dn). Vp can easily be converted

into delay. Since Vp is directly related to dielectric con-

stant (DC ), they are all directly related as shown in Eq.

14-8 and determine the delay in nanoseconds- per-foot

(ns/ft).

(14-8)

While these equations will give you a reasonable

approximate value, the actual equations should be

(14-9)

Delay becomes a factor in broadcasting when

multiple cables carry a single signal. This commonly

occurs in RGB or other component video delivery

systems. Delay also appears in high-data rate UTP, such

Figure 14-18. Loading coil connected in a balanced trans-

mission line.

L 2

L 1 L 1

L 2

Dn^100

Vp

---------=

= DC

Delay 101.67164

Vp

=-------------------------

=1.0167164 DC.