Some cables are capable of balance connections,
others are not. In this video we'll take a look inside some of the most common audio cables to
see how they work. At the end of the video, I'll demonstrate how big of a difference a balanced
connection makes over a long distance. All that's coming up but if this is our first time meeting
my name is Kyle. Welcome to Audio University. There are many ways that noise can enter an audio
signal chain. One of the most common sources of noise is other electronic devices. When I place
this transformer next to other electronic devices, some of the current passing through those
devices is induced onto the transformer and enters the audio signal. The electric
currents flowing through any device create magnetic fields that could induce currents on
nearby devices and we're honestly very lucky to have the cable technology we have today to
prevent that noise from ruining our recordings. When connecting audio devices together, you'll
most likely be using one of these cables: XLR, 1/4-inch TRS, 1/4-inch TS, or RCA. Let's expose
the wires on these cables to get a closer look at what's going on inside. The XLR and 1/4-inch
TRS cables both contain a shield, a positive, and a negative. The 1/4-inch TS and RCA cables
each have a signal wire and a shield (or ground). The construction of these cables is
important to understand why the first two cables can support balance connections
and the second two cables cannot. In Handbook For Sound Engineers, Bill Whitlock
shares this definition of a balanced circuit: "A balanced circuit is a two conductor
circuit in which both conductors and all circuits connected to them have the same impedance
with respect to ground and all other conductors. The purpose of balancing is to make the noise
pick up equal in both conductors, in which case it will be a common-mode signal that will be made
to cancel out in the load." If I'm being honest, when I first read this definition it just left
me feeling more confused and that's probably just because I don't have a very strong background
in electrical engineering. You might feel the same way, so let's simplify this a bit. In any circuit
you'll find a driver, a line, and a receiver. A balanced receiver uses a differential device which
will only respond to the difference in voltage between the two wires on the interconnecting
cable. So if there's a voltage on one wire and not the other, the voltage will go through.
This is an example of a differential-mode signal. If there's an identical voltage on each wire,
they will cancel out in the differential device. This is an example of a common-mode signal,
which results in common-mode rejection. Now this is where it gets really interesting
because we can use this to our advantage. Imagine an audio signal sent across one of the wires. When
it reaches the differential device it's allowed to pass through, because it's a different voltage
on each wire. However, along the way there's some noise from nearby electronics that induces
voltage onto the wires. The voltage that's induced is equal in both wires so the differential
device will completely cancel out that noise! There are two important details that make this
cancellation possible. First the two wires have the same impedance, which means that the strength
of the voltage will be equal in each wire. And second the two wires occupy the same point in
space and they're the same distance away from the source of the noise. Again, so that the strength
of the voltage will be the same in each of the two wires. Looking back at these audio cables, we can
see that each of them consists of two conductors. The XLR and TRS cables do both have a shield but
the shield is irrelevant to its ability to form a balanced connection. So let's ignore
that for right now. If we look closely, we can see that the wires in the XLR and
TRS cables are the same size given that the impedance of a wire varies based on its size. This
suggests that the wires in the TRS and XLR cables have the same impedance. We can also see that the
wires are twisted in the XLR and the TRS cables. This helps to ensure that the wires occupy the
same average position over the length of the cable so that any noise from nearby
electronics will be equal in both wires. Before demonstrating the practical difference
between a balanced and unbalanced connection, I want to make sure to mention a very
common misconception about balanced audio. Sometimes an equal but opposite audio signal
will be sent across each wire. While this has some additional benefits, it's not at all
necessary to establish a balanced connection. The benefit of this is that the opposite signals
will be summed together in the differential device. This is often said to be a requirement for
a balanced connection, but in reality the noise cancellation will still occur even if only one
copy of the signal is sent across the audio cable. Now for a demonstration of the difference
between a balanced and unbalanced connection. Right here, I've got a thousand feet of
4-conductor cable, which gives me two pairs of conductors to work with. With a bit of
soldering, I can wire one pair to be balanced and the other pair to be unbalanced. The balanced
pair will be wired between pins two and three of the XLR connectors and the unbalanced
pair will be wired between pins one and two. Right now you're hearing my voice through
the unbalanced connection. As you can hear, there is a lot of noise. Now you're hearing my voice
through the balance connection. The noise is still being picked up by the cable, but it's being canceled out by the differential
device within the input of my audio interface. If you're like me and you enjoy appreciating
the science and engineering that goes into creating audio devices, make sure
to check out some other posts on the Audio University website
- audiouniversityonline.com. If you enjoyed this video, hit the "Like"
button and I'll see you in the next video.
