Audio connections in a studio can be broken down into two categories:
Unbalanced
An unbalanced connection uses cabling and a connector with only two points of contact.
On a jack connector, this would be the tip and the sleeve.
Another type of unbalanced connector is the RCA/Phono connector.
These are the simplest types of connections as they use one signal cable and a screen.
Balanced
Balanced connections use connectors with three points of connection.
These are referred to as the hot (+), cold (-) and screen (or ground/earth).
The cable therefore needs to have two cores, plus the screen.
Some connectors may look similar to their unbalanced equivalents, but they will have an additional signal connection. In the case of a balanced jack plug it will have a tip, ring and sleeve.
Tip = Hot
Ring = Cold
Sleeve = Ground
XLR connections can be balanced and use the three pins for the three required connections. In the UK:
Pin 1 = Ground
Pin 2 = Hot
Pin 3 = Cold
Why Use Balanced Audio?
The purpose of balanced audio is to reduce the risk of external interference.
When running cables around a studio, or on location, there are many electrical devices nearby which can have an effect on the audio if the cable runs past them. These might be mains cables causing hum or radio devices causing high frequency crackles.
An unbalanced cable uses its screen to give it some protection but this isn’t always enough.
A balanced audio system uses a clever method for eliminating any external interference when it is received at its destination.
This means that the cables can be longer and as a side effect the audio signal will be louder.
How Balanced Systems Work
In a balanced audio system the exact same audio signal is sent along two wires (hot and cold), but the polarity of one of the signals is inverted. This happens in the output device, not in the cable itself.
Polarity Inversion
When interference or noise is introduced into the cable, it is introduced equally to both the original and the inverted signal.
When the signal arrives at its destination, the inverted signal is inverted back to its original orientation and both signals are combined. The wanted signal will now be in phase with itself and will increase in level but the noise will now be out of phase with itself and will therefore cancel out to nothing.
This is called common mode rejection and ensures that any noise in the signal is eradicated.
Canceled Noise
If a fault occurs in a balanced cable and either the hot or cold connection is broken, then the connection will no longer be balanced and the level will drop by around 6dB.
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