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.