Handbook for Sound Engineers

Transmission Techniques: Wire and Cable 425

Vp of 78%, and nitrogen gas injected foam has a Vp up to
86%, with current manufacturing techniques. Some
hardline, which is mostly dry air or nitrogen dielectric,
can exceed 95% velocity.
Velocity of propagation is the velocity of the signal
as it travels from one end of the line to the other end. It
is caused because a transmission line, like all electrical
circuits, possesses three inherent properties: resistance,
inductance, and capacitance. All three of these proper-
ties will exist regardless of how the line is constructed.
Lines cannot be constructed to eliminate these
characteristics.
Under the foregoing conditions, the velocity of the
electrical pulses applied to the line is slowed down in its
transmission. The elements of the line are distributed
evenly and are not localized or present in a lumped
quantity.
The velocity of propagation (Vp ) in flexible cables
will vary from 50% to a Vp of 86%, depending on the
insulating composition used and the frequency. Vp is
directly related to the dielectric constant (DC) of the
insulation chosen. The equation for determining the
velocity of propagation is

(14-5)

where,
Vp is the velocity of propagation,
DC is the dielectric constant.

Velocity can apply to any cable, coax or twisted
pairs, although it is much more common to be expressed
for cables intended for high-frequency applications. The
velocity of propagation of coaxial cables is the ratio of
the dielectric constant of a vacuum to the square root of
the dielectric constant of the insulator, and is expressed
in percent.

(14-6)

or

(14-7)

where,
VL is the velocity of propagation in the transmission
line,
VS is the velocity of propagation in free space,
H is the dielectric constant of the transmission line insu-
lation.

Various dielectric constants (Hare as follows:

14.14 Shielding

From outdoor news gathering to studios and control

rooms to sound reinforcement systems, the audio indus-

try faces critical challenges from EM/RF interference

(EMI and RFI). Shielding cable and twisting pairs

insures signal integrity and provides confidence in audio

and video transmissions, preventing downtime and

maintaining sound and picture clarity.

Cables can be shielded or unshielded, except for

coaxial cable which is, by definition, a precise construc-

tions of a shielded single conductor. There are a number

of shield constructions available. Here are the most

common.

14.14.1 Serve or Spiral Shields

Serve or spiral shield are the simplest of all wire-based

shields. The wire is simply wound around the inner por-

tions of the cable. Spiral shields can be either single or

double spirals. They are more flexible than braided

shields and are easier to terminate. Since spiral shields

are, in essence, coils of wire, they can exhibit inductive

effects which make them ineffective at higher frequen-

cies. Therefore, spiral/serve shields are relegated to low

frequencies and are rarely used for frequencies above

analog audio. Serve or spiral shields tend to open up

when the cable is bent or flexed. So shield effectiveness

is less than ideal, especially at high frequencies.

14.14.2 Double Serve Shields

Serve or spiral shields can be improved by adding a sec-

ond layer. Most often, this is run at a 90° angle to the

original spiral. This does improve coverage although the

tendency to open up is not significantly improved and so

this is still relegated to low-frequency or analog audio

applications. This double serve or spiral construction is

also called a Reussen shield (pronounced roy-sen).

Vp^100

DC

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

VL

VS

----- -^1

H

=------

VL

VS

H

= ------

Material Dielectric Constant

Vacuum 1.00

Air 1.0167

Teflon 2.1

Polyethylene 2.25

Polypropylene 2.3

PVC 3.0 to 6.5