11 Unbalanced Connections
Balanced Connections 12
Understanding unbalanced audio.
An unbalanced connection or cable is made up of two conductors (wires). One
wire carries the audio signal, while the other serves as the shield. The most
common example of this is the RCA or phono cable, where the center conductor
carries the signal and the outer ring is the shield and signal
return.
Unbalanced audio is most often encountered in consumer
electronics equipment. It is suitable for very short
interconnections, especially when the source and destination
are plugged into the same power source. Unfortunately, when
used to connect more distant equipment, its limitations can
become quite bothersome.
If the source equipment and the destination equipment are plugged into different
power outlets, any ground differential between the outlets will appear as a
common-mode voltage, present on both the signal and the shield. This is often
referred to as a ground loop and may result in hum, buzz, hiss, or RF interference.
Additionally, the level of unbalanced audio is typically fairly low, with the majority
of consumer (non-pro) gear having an output of -10dBV, which means even small
induced noise is more apparent.
Understanding balanced audio.
A balanced connection uses three wires, one for shield and two for signal. Most
professional gear uses XLR connectors to convey balanced
signals. The two signal wires are twisted together and
surrounded by the shield in a balanced cable.
Balanced audio, by definition, means the two signal
wires have the same impedance. A balanced audio
receiving device will employ a transformer or differential
amplifier input which will cancel any noise that is
common to both signals and pass only the signal which
is different.
Often, one signal line will carry the audio and the other will carry the same audio
with opposite polarity. This is not strictly necessary, however. The same principle
works if the audio return is grounded at the source. The only absolute requirement
in balanced audio is that the two signal lines are impedance matched. The shield
is never used as an audio return.
The audio levels on a balanced signal are typ4dBu which further improves
the noise suppression performance of the interconnect. These features are why
balanced audio can be used for very long distances.