Just about all of us working with audio systems in our churches have used both “instrument” audio cables that utilize 1/4-inch tip sleeve (TS) connectors, and “microphone” audio cables that utilize those barrel-shaped connectors with three pins (or holes at the other end) called XLR connectors. However, we’re not all familiar with why the two types exist, where exactly is it appropriate to use each type—and what’s the difference anyway?
The difference between the two types of cables goes beyond the different connectors on the ends. The difference is actually in the way the signals are generated and processed at the sending and receiving ends, and in the distance that the audio signals need to go.
Unbalanced Audio Signals
Instrument cables are designed for carrying “unbalanced” audio signals. The reason why the name “unbalanced” is used will be come clear when we discuss the “balanced” signals later in this article.
Unbalanced audio cables consist of a single wire for transferring the audio signal, and the ground shield that surrounds the signal wire. This type of audio signal is primarily used for musical instruments and consumergrade audio components, and is meant for transferring audio signals over a very short distance. It’s an inexpensive type of signal to generate and transmit; however, it has the drawback of being highly susceptible to electro-magnetic interference.
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Figure 1: AC currents generate magnetic fields, which can introduce noise in unbalanced audio cables. |
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Figure 2: Balanced audio cables have the original audio signal placed on one wire, and an opposite polarity version of the signal placed on the second wire. |
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Figure 3: Unlike the audio signals, noise introduced in a balanced audio cable is equal in polarity. |
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Figure 4: When the differential input section of a mixer combines the positive and negative signals, the audio signal remains the same, but the noise signal cancels out. |
Electro-magnetic what, you say? Here’s the scoop: When electricity passes through a cable, it generates a magnetic field around the cable that varies as the voltage changes in the cable. The larger the current, the stronger the magnetic field. When that magnetic field has the opportunity to pass through another wire, it will induce an electrical voltage and current in the other wire. And as audio signals moving through wire are simply an AC electrical signal, if this magnetic field passes through an unbalanced audio cable, the electrical current generated in the unbalanced cable creates noise in your audio system (Figure 1). The most common occurrence of this type of interference is that which is introduced by your AC power cables, creating a 60-hertz hum in one or more of your audio input channels. The use of unbalanced audio signals is a common culprit for such noise. And the longer the run of unbalanced audio, the more noise that is likely to be introduced.
So, if unbalanced audio signals have these issues, what’s a better thing to use? That would be your balanced audio signals.
Balanced Audio Signals
To greatly reduce the noise problems that unbalanced signals are susceptible to, a rather ingenious mechanism was developed. Instead of running a single signal wire inside of the shield, two insulated wires are run, and the two wires are twisted together throughout the length of the cable.
One of the audio cables carries the audio signal (the positive signal); the other carries the same signal, but inverted (the negative signal (Figure 2). When the balanced audio signal is connected to your professional audio mixer, the input stage of the mixer channel is sensitive to the difference between these two signals, which results in the same signal waveform as was generated by the instrument.
Big deal, you say—what’s so special about that? Well, let’s take a look at what happens with you introduce electromagnetic interference into the cable. Because the two audio signal wires are wrapped tightly around each other, when an electromagnetic field passes through the cable, an identical voltage is introduced into both cables. If at any given moment a noise voltage of +10 millivolts is introduced in the positive signal wire, a +10 millivolt noise voltage is also introduced in the negative signal wire (Figure 3). Note that this noise voltage is the same polarity in both wires—unlike the audio signals that were placed on the cable by the microphone, where they are opposite polarities.
So, when the audio signal reaches the mixer input channel, because the input stage of the mixer is sensitive to the difference between the two signals, the intended audio signal is left intact, but the noise is ignored (Figure 4).
Balancing Unbalanced Signals
So, you have your electric guitar, or your electronic keyboard, and you need to get that signal 100 feet back to your mixer. You now know that unbalanced signals are not the way to go, because they are too susceptible to noise. So, how do you make it happen? When you need to transmit an unbalanced signal over a longer distance (say, more that 10–15 feet), you need to balance them. This is accomplished through the use of a direct-injection box, or DI box. The sole purpose of the DI box is to take an unbalanced signal in, and turn it into a balanced signal going out. They have a 1/4-inch TS jack for its input, and an XLR jack for its output. This will enable you to connect your unbalanced signal into your mixer or snake, and obtain all the great noise-isolation that balanced signals are known for. Note: Many DI boxes also drop the signal level in addition to balancing it. This is necessary if you want to plug your guitar into a microphone input.
Connectors
There’s one last thing that is worth mentioning: connectors. Each type of audio signal cable can have a variety of connectors on the ends of the cables. It’s important to understand what the connectors are, and what they are typically used for.
All images courtesy Neutrik USA. |
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Figure 5: A 1/4-inch TS plug, used for unbalanced audio signals. |
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Figure 6: An RCA, or phono, audio plug, used
for unbalanced audio signals on consumer-grade |
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Figure 7: Male and female XLR plugs for balanced audio signals. |
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Figure 8: A 1/4-inch TRS plug, used for balanced audio signals. |
For unbalanced signals, the typical connector is the 1/4-inch TS plug for professional cabling (Figure 5). In this connector type, the tip of the connector carries the audio signal, and the longer sleeve portion of the connector is connected to the shield of the cable. This connector is also known as a tip-sleeve (TS) plug. One word of caution—the TS plug is also commonly used for speaker connections. The only way to tell if a cable with TS plugs is intended for a speaker connection or an unbalanced audio signal connection is to check the cable itself. If the wiring is a thick diameter, with two insulated wires running side-by-side, it’s a speaker cable. If it is a thin insulated wire run inside of a wire shield, it’s an unbalanced audio signal cable.
Less common in the professional audio environment, but probably more familiar to the average person, is the RCA (or phono) plug (Figure 6), also used for unbalanced audio signals. This connecter is used predominantly in consumer-grade audio equipment. The signal is carried on the center pole, and the outer ring is connected to the shield.
For balanced audio signals, there are two styles of plugs used. The more common is the XLR plug, which consists of three pins (for male connectors), or three holes (for female connectors) inside of a metal housing (Figure 7). With these connectors, one end of the cable will have a male connector, the other will normally have a female connector. Why are they called male and female? Well, we’ll leave that as an exercise for the reader to figure out. With XLR connectors, the three pins are numbered. Pin 1 is connected to the shield; pin 2 carries the normal audio signal, and pin 3 carries the inverted audio signal.
The other type of plug frequently used for balanced audio signals is the 1/4-inch three-conductor plug (Figure 8). As you see in the illustration, this plug is similar to the TS plug, but adds a third element—the ring, located between the tip and the sleeve. This type of connecter is often referred to as a tip-ring-sleeve, or TRS, connector. The most common wiring configuration has the tip carrying one audio signal; the ring carries the second, inverted audio signal; and the sleeve is connected to the shield. Some manufacturers wire the tip as negative and the ring as positive.
With this knowledge, you can now build custom cables for any interconnection. Need an unbalanced, RCA-to-TR cable to connect an unbalanced output to an unbalanced input? Connect the center pin on the RCA to the tip of the TS, and the sleeve of the RCA to the sleeve of the TS. Need a TRS-to-XLR-male balanced cable? Connect pin 1 of the XLR to the sleeve of the TRS via the shield of the cable; Pin 2 to the tip; pin 3 to the ring.
One last word—just because two pieces of equipment have similar connectors does not mean they can be connected together. Your amplifier might have a 1/4-inch TS jack as its output; this does not mean you can connect it to your 1/4-inch phone jack on your mixer. Likewise, some lighting equipment uses 3-pin XLR connectors to transfer lighting data. This does not mean it’s sending a balanced audio signal that you can connect to your mixer. It’s not enough to know your connector type—you need to know the signal type as well.
