Frightened by the world of audio? Plagued by unwanted electronic noises in your video soundtracks? Think the difference between balanced and unbalanced audio signals is their weight? If you answered yes to any of the above questions, you’ve come to the right place.
Follow me into the dimension of audio that’s often overlooked, the balancing act. In this study, we’ll explore the basics of balanced and unbalanced audio signals. We’ll also focus out attention on microphones, covering the nuances between professional balanced microphones and unbalanced models. Understanding how audio devices create and receive electric signals is the first step toward making better audio for your video.
The world is full of electrical signals that can cause unwanted noises in your soundtrack. Radio broadcasts, TV stations, computers and cellular phones all emit electromagnetic energy. These stray electromagnetic signals can infect audio signals that travel unprotected. The farther the audio signal travels, the more susceptible it is to interference. And the longer the cable carrying the audio signal, the more it acts as an antenna for picking up stray signals.
Standard unbalanced audio cable contains a protective shield which guards against some electromagnetic interference. The shield is usually a solid foil layer or a braided metal blanket which encases the signal-carrying conductor for the length of the cable. Most unwanted signals hit this shield and dissipate. Any remaining signal radiation penetrates the shield and infects the signal traveling underneath. Noise interference often manifests itself in the form of an audible buzz, crackle or hum.
By design, balanced signals are much more impervious to noise. To compensate for noise interference, a balanced line contains two conductors, each one carrying a separate version of the desired signal. The two signals carried are actually electrical opposites of each other–when one rises higher in voltage, the other lowers accordingly (see figure 1). We call these signals out-of-phase.
Taken at face value, this doesn’t appear to make any sense. If you combine two out-of-phase signals, won’t they cancel themselves out? Well, yes and no. This self-cancelling property of out-of-phase signals is what makes balanced lines so effective against unwanted noise. The trick is, we invert the signal from one of the balanced conductors before combining the two signals at the receiving end. This avoids cancellation of the desired signal, but spells the end of any noise picked up on the trip.
Here’s why. When noise penetrates the shield and reaches the conductors of the balanced line, it affects both signals in exactly the same way. If the noise rises two millivolts on one of the balanced lines, it rises two millivolts on the other.
Then, when the audio arrives at our receiving device, we invert one of the balanced lines. The once out-of-phase signals are now in-phase, while the noise from both conductors is now out-of-phase. The noise still rises two millivolts on one line, but it drops two millivolts on the other due to the inversion process. Merging the two signals now cancels anything that’s out-of-phase–the noise–resulting in clean signal.
Mikes and Cables
The strength of the signal and the distance it travels determine how great an effect interference will have. The smaller the signal, and the farther it has to travel, the more threatening outside noise becomes.
Microphones generate some of the smallest signals in the world of consumer electronics, a signal that sometimes has to travel hundreds of feet before being amplified. For this reason, you’ll often find balanced wiring on most pro mikes.
Balanced microphones have a three-pin connector at the base of the microphone. This connector is most often referred to as an XLR or Cannon connector. This type of connection locks the cable to the microphone, resulting in a solid link between components.
Most unbalanced microphones have their cables permanently attached. In addition, if the cable ends in a mono 1/4-inch phone jack, it’s definitely not balanced. Remember that a balanced signal requires three connections–two conductors and a shield. Mono jacks have only two conductors–one signal-carrying wire and a ground or shield.
Impedance is another characteristic of microphones that affects the way they send signals along lengthy stretches of cable. Impedance, indicated by the letter Z, measures opposition to a changing voltage. The electrical design of most microphones is either high impedance (hi-Z) or low impedance (low-Z). Most professional balanced microphones are low impedance.
Every piece of cable has a certain amount of built-in capacitance. Capacitance refers to the wire’s ability to collect a charge of electricity. In audio cables, capacitance draws away and reduces high frequencies. Longer cable runs have more built-in capacitance, hence more potential for high-frequency loss.
The combination of these two characteristics, impedance and capacitance, ultimately affect the microphone signal. The impedance of a hi-Z microphone reacts with the capacitance in a given cable, reducing the high frequencies of a signal. The result is a noticeable dulling of the sound. For example, when you run a hi-Z mike signal over a hundred feet of cable, you risk a muddy, indistinct audio signal.
Low-Z microphones experience less high frequency loss, because there’s less mike impedance to interact with the cable capacitance. You can run low-Z microphone signals through hundreds of feet of cable without suffering serious high frequency loss. Even if you’re only patching cables together for a 50-foot run, a low-Z mike will better preserve the high frequencies that lend clarity to your audio.
Whether or not a mike needs external power to operate is one of the major differences between types of professional balanced mikes. Dynamic microphones need no power supply whatsoever, while condenser mikes require an external or internal power supply.
Condenser microphones pick up their power from one of two places: built-in batteries or mixers with something called “phantom power.” Professional audio mixers send DC voltage–phantom power–down the audio cable to power the internal electronics within the microphone. The mixer then filters the DC voltage out of the resulting audio signal.
Most camcorders don’t have phantom power output, and many professional condenser mikes don’t have a built-in battery.
So how can we use one of these mikes in the field? One solution is to use a field mixer that offers phantom power output. These types of mixers power a handful of microphones, in addition to combining their signals and routing them to your camcorder. The DM12, a field mixer from Electro-Voice ($936), offers flexibility and enough phantom power to handle twelve microphones. In addition, Shure Brother’s offers the FP32A stereo field mixer for $1795.
Dynamic mikes and condenser models with built-in batteries offer a less expensive alternative to phantom power. A shotgun microphone is almost always a self-powered condenser. Again, for convenience, you may want to shop around to find one with its own built-in battery.
A consumer camcorder’s mike input jack won’t accept a professional microphone cable’s XLR connector. There are incompatibilities with both the physical connections and the electrical specifications of the signal. So before you can use a professional balanced microphone with your camcorder, you’ll need some additional adapters and converters.
The physical difference between your camcorder and microphone connectors require the help of adapters. Your local electronics stores carry a wide range of audio adapters, connectors and converters. But before you buy anything, let’s go over your audio signal needs. Do you want to preserve a balanced signal?
If you want to enjoy the benefits of a balanced mike with your camcorder, you’ll need a line transformer. This device converts the balanced line to an unbalanced signal right at the camcorder, eliminating any noise previously picked up in the cable.
However, most line transformers raise the voltage in preparation for a line-level input. This is necessary for plugging the mike into a line mixer, but it will overdrive your camcorder’s mike input.
To avert this, place an attenuator in line after the transformer. This will drop the voltage down to a suitable level that your camcorder can handle. Now you can pull out your wallet–Radio Shack sells both line transformers (part #274-016 or 274-017, $13) and line attenuators (part #274-300, $3). Also, Shure Brother’s (Evanston, IL) offers a low-to-medium impedance line transformer that makes using a balanced mike with a camcorder even easier. Shure’s A97F ($48) is a small cylindrical device that terminates in a minijack for direct connection to your camcorder. With this system, you won’t need an attenuator or adapter.
There is a simpler method, but it defeats the noise-canceling properties of balanced wiring. It involves unbalancing the microphone and running it straight into your camcorder. You’ll achieve the improved frequency response and clarity of a professional mike without the balanced line, and without the transformer. The only drawback is the system’s limited resistance to noise. If there is a large amount of radio frequency (RF) noise on your location you’ll find that unbalanced cables won’t work for cable runs over 20 feet.
An easy way to unbalance a professional microphone to work with your camcorder is to buy or make a connector that uses signal ground plus just one leg of the balanced connection. Using converters, take the two conductors down to a minijack. The result is an unbalanced, mike-level signal compatible with the camcorder’s mike input. No attenuator is need with this type of connection.
You’ll find a balanced line in almost every professional production studio. High-end tape decks, VCRs and audio mixers use balanced lines almost exclusively. Most studios maintain signal levels that are thousands of times stronger than mike signals. And very seldom do you find a studio cable stretched beyond 10 or 20 feet. At the professional level, audible noise or hum is unacceptable.
There are times when you’ll need to connect your camcorder to a professional deck or mixer with balanced outputs. Although the outgoing signal may have the same noise-canceling properties of a balanced mike, it’s line-level output will greatly overdrive your camcorder’s mike input if not reduced in voltage.
A normal line transformer is not the answer. The transformer will only increase the voltage to your camcorder’s mike input. The solution is to use just one half of the balanced line as previously discussed. Use the same connector for mike or line level outputs, and add an attenuator just before the camcorder’s mike input. Depending on the strength of the signal, you may have to use an additional attenuator in-line between the source and the camcorder.
Okay, we’ve covered how to “unbalance” a balanced signal. What if we needed to balance an unbalanced signal in order to protect a weak signal over a really long cable run?
Here’s the scenario. You’re in charge of a two-camera shoot, documenting your local city council meeting from the back of its conference room. Your city is experiencing a financial crisis, so the council can only supply you with two unbalanced microphones. One sits on a table in front of the panel, the other is mounted on the plebeian podium where the public addresses the council. Your cable runs extend well over 100 feet from the back of the conference room to where the council members sit. The signals from the high-impedance unbalanced mikes won’t make the trip, no matter how much hot air reaches the microphone.
A transformer is your only hope. You see, transformers also work in the other direction. Turn a transformer around and it makes a balanced signal from an unbalanced one.
Simply place the transformers at the table and podium where you can balance the mike outputs and lower their impedance. The signals from the microphones then travel the lengths of each cable without significant high frequency loss. You’ll still need to place a second transformer at each camcorder to unbalance the signal and cancel any noise picked up along the way.
The parts and pieces for making good audio are out there, you simply need to expand your audio horizons and learn how to make them work together. Such is the audio balancing act.