One of the most important equipment selection decisions made when
building a facility is also one that is typically given the least
thought. The wire and cable used to connect everything within the
facility is an important element. It is a part of the overall circuit
path. The wire choice is often made without really considering the
vital role it plays in the facility's operation.
Simply put, not all wire is the same. Some manufacturers
(particularly on the consumer side) insist that the overpriced,
supposedly special-formula copper blend is better for a particular
application. There are variations in manufacturing quality, but more
importantly, there are electrical differences to consider. A cable's
impedance, capacitance and immunity to unwanted signals are important
in any application.
The physical design is the first consideration. The conductors can
be either solid or stranded. Both offer certain advantages. Stranded
wire should be used any time the cable will be flexed in any way.
Solid-conductor wire should only be used when the wire will not move.
This said, it is very common to use stranded wire in fixed
installations, such as a studio interconnect.
The 66-style telephone punch blocks are still regularly used in
facilities. These blocks were designed for use with solid wire. They
will work with stranded wire if the terminations are carefully made.
Uninsulated shield or drain wires should be covered with tubing to
prevent the individual strands from coming loose. Because of the
tapered opening, stranded wire tends to push out of the punch
Newer block styles, such as those from Krone and Bix, work well with
either type of wire because the forked terminal is parallel and does
Microphone cables are prone to excessive flexing, so use stranded
wire. In addition, a rugged, flexible jacket is important. Some
microphone cables also have additional filler to add strength and
reduce stress from cable tension.
A foil shield offers complete coverage and is easy to prepare for a
Also keep in mind that microphones are low-level devices. The cable
used between the mic and the preamp should provide the highest immunity
to noise and interference. Avoid using fixed-installation wire for
microphones because some types of this wire typically are microphonic.
As the wire is moved or touched, it will mechanically pick up the
vibration and result in unwanted pops in the audio feed.
When two or more conductors share a common jacket, particularly two
wires in a balanced signal application, an outer shield may be used,
especially for most audio applications. A braided shield is very
flexible and adds additional strength to the cable, but a braided
shield can be difficult to work with when attaching a connector. Also,
a braided shield does not completely shield the inner conductors. A
second braided shield will improve overall shielding, but this likewise
adds more tedium to connector assembly.
Connector preparation of a
braided-shield cable does not have to be difficult. See an explanation
of an easier method in the
Fixed-installation cables often make use of a foil shield. A foil
shield can provide 100 percent shield coverage, which makes it
electrically favorable. An additional drain wire is added to the cable
to carry the shield connection to the connector. If the wire is subject
to any movement or flexing, the foil shield can become deformed, which
lessens its effectiveness. While a shield can offer some rejection to
unwanted noise, there is a better defense.
By twisting the two conductors, a cable increases its ability to
reject external interference. To be most effective, the twist must be
consistent through its path. The tightness of the twist is also
important in improving the noise rejection ability. A tighter twist has
a better specification.
A braided shield stands up to cable flexing and adds strength to a
There is a variation on a twisted-pair cable that offers even better
noise rejection properties. Star-quad mic cable (a term coined
by Canare but not trademarked and now a generic term) is a
four-conductor cable exhibiting very low noise and hum pickup. This is
also called double-balanced cable.
Analog vs. digital
All wire has a characteristic impedance. Coaxial cables are
specified by impedance. Analog audio and control wire has typically
ignored the characteristic impedance. In most situations, the
electrical characteristics are not important. For long analog wire
runs, a lower capacitance value cable should used to prevent
attenuation of the higher frequencies.
Digital signals ideally should be passed through a cable with
constant, specific impedance. The AES-3 digital audio standard
(balanced) calls for a characteristic impedance of 110Ω. The
unbalanced version, AES-3ID, calls for 75Ω cable. For reference,
common analog audio, shielded twisted pair cable has characteristic
impedance of about 30Ω. Because digital audio cable is designed
for a specific impedance, it tends to be more consistent throughout the
entire section. You can pass digital audio over analog cable, and for
short distances you may not have any difficulties. However, if only one
type of cable can be used for an audio installation, digital audio
cable would be a preferred choice. Digital audio cable is the best
analog cable you can find.
Turn up the heat
There are several different ways to classify wire and cable, but one
application-specific classification is plenum-rated cable. When cables
are run in an air return space for ventilation, such as within a drop
ceiling, electrical codes and fire codes may call for a plenum-rated
cable. Materials commonly used for cable jackets, such as PVC, melt at
high temperatures. Some also produce toxic gases when burned. During a
fire, these toxic gases could be distributed through a building's HVAC
system. In these installations, a plenum-rated cable must be used. The
most common material used for a plenum cable jacket is Teflon, but
other materials can be employed.
Cables may contain additional filler material to maintain the spacing
of the conductors or to provide additional strength.
Non plenum cables in a plenum space are an obvious target for
building inspectors during construction. Once an inspector approves an
installation, it is rare that you will see him again, and the
temptation to use less expensive, non-plenum cable may need to be
overcome. But building codes, electrical and good engineering practice
dictate that this should never be done.
The Telecommunications Industry Association and Electronics Industry
Association have established a set of cable categories to describe
certain characteristics of data cable performance. The TIA/EIA
standards define categories 1 through 5 (CAT1 through CAT5) for
unshielded twisted-pair cables (UTP).
CAT1 is rated for 19.2kb/s;
CAT2 is rated for 786kb/s;
CAT3 is rated for 16Mb/s and is unshielded;
CAT4 is rated for 16Mb/s and is shielded; and,
CAT5 is rated for 100Mb/s, with additional levels, such as CAT5e,
rated up to 400Mb/s
CAT6 and CAT7 are not yet officially TIA/EIA standards and not
rated. Cables that are called CAT6 by manufacturers are typically rated
at 350Mb/s; CAT7 is typically rated at 600Mb/s. A CAT7 standard is
likely to be adopted, and it will probably be a shielded cable.
Twisted-pair data cables can also have a shield. These cables are
referred to as shielded twisted-pair cables (STP).
Because of the proliferation of CAT5 cable for computer networks,
other industries are finding uses for this cable. Digital audio is one
of those uses. I previously noted that the AES-3 standard defines use
with 110ohm; cable. This standard has a tolerance of +/-20 percent,
resulting in a usable range of 88ohms to 132ohms. CAT5 cables and
higher are specified at 100ohms +/-15 percent, resulting in a usable
range of 85ohms to 115ohms. There is a significant overlap in the
ranges of these two standards. Except for the minimum impedance rating
range, it is obvious that the CAT5 cable standard is within the limits
of the AES-3 audio standard. It has been shown in practice that CAT5
indeed works very well for AES-3 audio applications.
Plenum cable is rated to withstand high temperatures for use within an
Coaxial cables are used for both data and RF signals. As mentioned
earlier, the AES-3ID digital audio standard calls for 75ohm cable. The
same handling, routing and connector considerations apply to coaxial
cables as to twisted-pair cables.
A lighter load
Fiber optic cable is gaining popularity for intrafacility
interconnection because of its small size and massive signal capacity.
Fiber is already used for long distance connections, something for
which it is well suited. Fiber optics are also impervious to
electromagnetic interference. However, the cable is more delicate than
its copper counterpart. Physical stress of any kind should be avoided.
Likewise, any connectors or splices must be installed properly to
prevent signal losses.
Most wire and cable manufacturers offer extensive resources to
assist you in selecting the best cable for a particular need. Some post
these resources online and in their catalogs. It's easy to simply
specifiy the same cable you have always used, but with improvements in
design and electrical specifications constantly occurring, there likely
is something better out there.
Photos courtesy of Belden Electronics and Gepco.
Working with UTP
When installing or moving data-rated cables, several basic practices
should be followed to ensure maximum signal integrity.
|Use the correct
|Any impedance mismatch will cause a signal error.
Digital signals can handle small errors with few undesired effects. In
time, enough signal problems will cause noticeable data errors. This
rule also applies to coaxial cables.
|Do not physically stress
the cable with tension, hard bends, kinks or tight cable
|Physical stress to the cable will change its
impedance or capacitance ratings. Any change in the transmission path
will result in an error in the signal.
|Space cable ties at
|When cable ties are used, they will naturally
deform the cable slightly. By spacing the ties at irregular intervals
you avoid creating a problem at the wavelength relative to the
|Maintain the twist as far
as possible to the connector or termination
|A cable's twist and conductor spacing are an
integral part of it signal rating capability. By maintaining the
cable's electrical parameters, you will ensure a higher level of signal
Manufacturers of wire and cable and wiring systems