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Video Production Glossary
Unbalanced and Balanced Audio
Balanced audio is a method of interconnecting
audio equipment using impedance-balanced lines. This type of connection is
very important in sound recording and video production because it allows for
the use of long cables while reducing susceptibility to external noise.
Balanced connections use three-conductor connectors, usually the XLR or TRS
jack plug. XLR connectors, for instance, are usually used with microphones
because of their durable construction, while TRS jack plugs are usually used
for mixer inputs and outputs because of their smaller profile.
Many microphones operate at low voltage levels and some with high output
impedance (hi-Z), which makes long microphone cables especially susceptible
to electromagnetic interference. Microphone interconnections are therefore a
perfect application for a balanced interconnection, which cancels out most
of this induced outside noise.
If the power amplifiers of a public address system are located at any
distance from the mixing console, it is also normal to use balanced lines
for the signal paths from the mixer to these amplifiers. Many other
components, such as graphic equalizers and effects units, have balanced
inputs and outputs to allow this. In recording and for short cable runs in
general, a compromise is necessary between the noise reduction given by
balanced lines and the cost introduced by the extra circuitry they require.
Balanced audio connections use a number of techniques to reduce noise.
A typical balanced cable contains two identical wires, which are twisted
together and then wrapped with a third conductor (foil or braid) that acts
as a shield.
The term "balanced" comes from the method of connecting each wire to
identical impedances at source and load. This means that much of the
electromagnetic interference will induce an equal noise voltage in each
wire. Since the amplifier at the far end measures the difference in voltage
between the two signal lines, noise that is identical on both wires is
rejected. The noise received in the second, inverted line is applied against
the first, upright signal, and cancels it out when the two signals are
subtracted.
This differential signal recombination can be implemented with a
differential amplifier. A balun may also be used instead of an active
differential amplifier device.
The wires are also twisted together, to reduce interference from
electromagnetic induction. Twisting makes the loop area between the
conductors as small as possible, and ensures that a magnetic field that
passes equally through adjacent loops will induce equal but opposite
currents, which cancel out.
The separate shield of a balanced audio connection also yields a noise
rejection advantage over an unbalanced two-conductor arrangement (such as
used in typical home stereos) where the shield must also act as the signal
return wire. Any noise currents induced into a balanced audio shield will
not therefore be directly modulated onto the signal, whereas in a
two-conductor system they will be. This also prevents ground loop problems,
by separating the shield/chassis from signal ground.
While 3-pin XLR connectors are the most common balanced connector,
quarter-inch (¼" or 6.35 mm) TRS connectors (tip-ring-sleeve) are also
commonly used. Many hybrid jacks are now designed to take either XLR or TRS.
2.5, 3.5 and 6.35 mm TRS plugs
XLR3 cable connectors, female on left and male on right
XLR and 1/4" TRS combo jack.
On TRS plugs, the tip is "hot" (positive), the ring is "cold" (negative),
and the sleeve is ground (earthed or chassis). If a stereophonic or other
binaural signal is plugged into such a jack, one channel (usually the right)
will be subtracted from the other (usually the left), leaving an
un-listenable L − R (left minus right) signal instead of normal monophonic L
+ R. Reversing the polarity at any other point in a balanced audio system
will also result in this effect at some point when it is later mixed-down
with its other channel.
Telephone lines also carry balanced audio, though this is generally now
limited to the local loop. It is called this because the two wires form a
balanced loop through which both sides of the telephone call travel.
Data lines, including digital audio, are also frequently balanced, normally
using AES/EBU (AES3) with XLR connectors for pro audio.[citation needed]
Eight-channel analog balanced audio connectors like ADAT use DB25
connectors, which can also carry up to 16 digital channels.
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