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Balancing Act

Understanding balanced and unbalanced audio connections can help you record better audio. As a youngster, I loved to go to the circus. The sights and sounds of the big-top fascinated me especially the tightrope walkers. These were people who risked life and limb, balanced high above the crowd with only a thin wire between them and certain peril. That fascination stayed with me over the years and, when I started dealing with audio equipment, I realized there was a lesson to apply from my childhood.

In this article, we'll look at the difference between balanced and unbalanced audio connections, where they are used and how they benefit you as a video producer.

Consider the intrepid tightrope walker. He climbs up to the wire and starts to walk across arms extended for balance. Then, a gust of wind comes along and blows directly in his face, but because of his outstretched arms, the wind affects him evenly. His journey is a bit more difficult, but the wind does not throw him off balance. Now imagine that we tie one arm behind his back for the next trip across the wire. Once again the wind gusts, except now he has only one arm for balance. You guessed it, he falls this time because the wind pushed on his one arm, throwing him off balance (don't worry, he's OK - it was a practice wire and just a few feet off the ground).

But let's apply this analogy to your audio connections. A balanced audio signal, like our tightrope walker, travels down the wire with an equal amount of signal on either side of an imaginary line. There are three wires in a balanced audio connection (Figure 1B): ground, hot (or positive) and cold (or negative). The ground wire functions as the tightrope, with the positive and negative portions of the audio signal serving as the outstretched arms. In this configuration, an audio signal can travel for hundreds of feet, virtually unaffected by interference or signal loss. Why? Because any interference that might harm the signal is equally applied. At the receiving end of the cable (your mixer or camcorder), the audio circuit understands the correlation of the positive and negative portions of the signal to the ground wire. Since each part of the signal is distinct, any extraneous noise or interference is considered an outsider and is cancelled.

Interference takes many forms, but falls into two main categories: RFI (radio frequency interference) and EMI (electro-magnetic interference). RFI comes from anything that generates radio waves radio and TV stations, cell phones, intercoms, etc. Of course, all these items have legitimate uses, but when the radio energy intrudes on your audio signal, it becomes interference. EMI comes from things that generate a magnetic field by electrical means; camcorder/laptop power supplies and computer monitors are notorious for this. You can minimize the effects of interference by keeping your audio cables and connections away from these noise generators.

Unbalanced cables use only a ground wire and one other to transfer their signals (Figure 1A). As a result, all the audio activity takes place on one side of the tightrope, so to speak (Figure 2A). This allows interference to corrupt the signal easily due to the inability to cancel the noise at the receiving end of the cable. Of course, unbalanced connections still have their applications. A microphone signal can usually survive up to 25 feet of cable length before the risk of interference is too great. Unbalanced line-level signals work at lengths of 50 feet or more in many applications. Your results will vary based on how much potential interference exists at your recording location. A remote shoot at the base of a radio station tower is a bad idea.

You'll encounter a wide variety of audio connectors in the video world (see photo). Professional microphones use balanced three-pin connectors, generally referred to as "XLR." Some microphones and mixers use a 1/4-inch connector for audio connections (also called a "phone plug" from their origin in telephone switchboards). These connectors come in two and three conductor flavors for unbalanced and balanced applications. The little brother of the 1/4-inch connector is the 1/8-inch or mini-plug connector. This type of plug is often the only way to get external audio into your camcorder. Modern digital cameras use a three-conductor version for stereo audio, but don't confuse three conductors with a balanced connection. This application provides two unbalanced inputs in one connector. Finally, there is the RCA connector or phono plug - a classic unbalanced connector used for simple audio and video hookups.