Sound Advice: A Little Balance

Hal Robertson
August 2005

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We could all use a little balance in our lives, right? In fact, we're encouraged to exercise balance in our eating habits, careers and personal relationships. Video production is another area to practice balance. I'm not talking about production methods or lighting, but audio. That's right, video sound can benefit from some balance too -- specifically balanced connections and cabling. If you're new to video, you've probably heard this term batted about, but with little explanation. Let's dig in and see if we can add some balance to our audio and video world.

First, lets look at an unbalanced audio connection. An unbalanced connection employs two connectors, one for the signal and one for the ground, and are much more susceptible to interference than balanced cables. This arrangement works fine for line-level signals (line-level is usually somewhere in the range of 40-60dB higher than mic-level) like CD and DVD players and other consumer video equipment. Your camcorder probably came with a short cable with these connectors, allowing you to attach it to a standard home entertainment system or television. Unbalanced connections work well for line-level signals over moderate distances. Unfortunately, an unbalanced microphone signal starts to degrade after about 25 feet. Beyond that distance, it's easy for stray electrical, magnetic and radio frequency noises to overcome the small signal traveling on the cable.

Years ago, telephone engineers needed a way to send phone conversations over long distances, and decided to use a balanced approach. It worked so well that the broadcast industry decided to adopt the same principles. The recording and professional sound industry picked it up from there and now, it's available for use in your video productions. Here's how it works: the signal is split in two and one version is inverted 180 degrees out of phase with the other. Both signals share a common ground connection -- essentially two unbalanced versions of the same signal. The two phases are combined at the receiving end. That's where the magic happens. Over any significant distance, some stray noise is bound to sneak into the cable, but that noise will be equal in each phase of the signal. When the opposing phases are combined, the noise is effectively cancelled.

If you're still a little fuzzy on the physics, don't worry. Just know that balanced connections can run noise-free for hundreds of feet -- further than you'll probably ever need. From a practical standpoint, all you need is a balanced source such as a microphone, balanced cabling and a way to attach it to your camera.

Check the audio input of any random camera and you'll likely see a small, 1/8" stereo jack as the sole audio entrance. That's bad enough, but it's also an unbalanced connection. Don't let the three conductors on the plug fool you -- it's two unbalanced connections. One for the left channel, one for the right. In fact, very few cameras today accept professional balanced connectors for audio input. Those that do typically sell in the low to mid four-figure range -- well out of reach for many video shooters. So how do you use balanced audio in a situation like this? There are several ways to go and the July 2004 issue of this column outlines them in detail. The short version is this: you need some adapters. But what if you need to combine various audio sources, balanced or otherwise? In that case, you'll want to consider an audio mixer. If you don't already own one, an audio mixer is a great investment. It will accept audio from microphones, instruments and various other audio and video equipment, both balanced and unbalanced. Some mixers even run on battery power if you need their assistance in the field. A mixer is an excellent, easy way to mix the outputs of any type of audio device.

Let's say you have a balanced microphone and need to attach it to your unbalanced stereo camcorder input. Of course, there is the mic and its cable, and then the fun starts. First, you need a balanced-to-unbalanced adapter. This little jewel uses a transformer to convert the connection. Then, through a series of adapters, you end up with an 1/8" plug that matches the input on your camera. If you'd rather not deal with all these items, there are alternatives. A handful of companies make adapter boxes that mount under your camera or clip on a belt. It's more expensive than the DIY approach, but everything you need is built into the box and you can even connect two separate microphones. Recently, some video equipment suppliers have begun offering "cheat cables" that do the adapting for you. The signal isn't actually balanced, but works fine in many situations. Is it really worth all the trouble and expense? The answer is 'yes' because you can run the mic cable 10 or 100 feet without worrying about noise or interference. An added advantage to the XLR connector is the locking mechanism that stops your cable from accidentally being pulled out.

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