Decoding Balanced XLR Cables: How They Deliver Noiseless Audio
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Understanding balanced XLR cables
XLR cables are a staple in live sound. We use them to connect different audio peripherals together to pass audio to one another. It is one of the most common methods of analogue transport due to its long standing track record of delivering clean audio over long distances. But how does it achieve that without allowing noise to disrupt your gig, and what does the term “balanced” mean? Let’s unpack the very simple yet effective technology behind it.
Pin Configuration
XLR cables have a 3-pin configuration. Unless there is a specific reason, you would almost never find a 2-pin configuration. The standard pin configuration is shown in the image below.
XLR Pin Out
Pin 1: Signal Ground
Pin 2: Hot/Positive
Pin 3: Cold/Negative
Now that we know how an XLR is wired, let’s understand audio phase relationship before moving on.
Phase Relationship
Audio signals can be altered by the phase relationship with other audio signals. Two identical signals with the same amplitude and phase(at 0°) will yield summation, making the overall summed signal louder. However, if one of them is in total opposite phase to the other i.e. 180° out of phase, the signals will cancel each other out. Inverting the polarity on one signal puts it at 180° phase relationship. When that happens, you get silence because the signals cancel each other. You can listen to a demonstration with a 1kHz sine tone below to understand this phenomenon.
As you heard, when two identical signals are in phase, the signals are summed. When the polarity is inverted on the second sine tone, it causes cancellation and, therefore, silence.
Differential Signalling
In the case of a balanced XLR cable the cold wire will carry the inverted audio signal, while the hot wire will carry an un-inverted copy of the same audio signal. This method is called Differential Signalling. This is done by the device that is sending (transmitting) the audio signals, where it sends two copies of the audio signal but inverts the signal on the cold pin. XLR cables are susceptible to interferences from electromagnetic fields, after all, they are made to conduct electric signals. The longer the cable, the more prone it will be to such interferences. As the audio signal travels through the cable to the end point, for example, from a mixer to a speaker, any noise that is picked up by the cable will be copied onto both the hot and cold signals. That means that the cable is transporting:
Pin 2 (Hot/Positive): Un-Inverted Audio Signal + Un-Inverted Noise
Pin 3 (Cold/Negative): Inverted Audio Signal + Un-inverted Noise
At this point, theoretically, the audio signals will cancel each other out and the noise should sum.
Common Mode Rejection
To achieve the desired result, the device receiving the audio has to be able to invert the signal on the cold pin again. At the receiving device, the cold pin is inverted, and the inverted audio signal is now un-inverted, causing it to sum with the signal from the hot pin. The noise on the cold pin gets inverted, thereby cancelling against the un-inverted noise on the hot pin. This is called Common Mode Rejection. And there you have it, a good clean signal without the noise.
Wrap Up
Balanced XLR cables are essential for delivering clean audio signals in professional audio setups. By utilizing differential signalling and common mode rejection, these cables effectively eliminate noise and interference, ensuring high-quality sound transmission. Understanding the principles behind these technologies not only enhances your knowledge but also empowers you to make informed decisions when setting up your audio equipment. With balanced XLR cables, you can achieve pristine audio performance, making them a reliable choice for any audio professional.
