Audio cables can be the weakest link in the chain of sound recording. When deploying external microphones, videographers need to know the difference between unbalanced and balanced cables.
The best way to understand the difference between balanced and unbalanced cables to think in terms of signal separation. You can remember how it works by thinking of a balanced cable with three wires, one for the positive, one for the negative and one for the ground: X=ground, L=left or positive and R=right or negative. XLR, get it? Unbalanced cables have two wires, one for the positive and one that acts as both the negative and the ground. Such a configuration means that the negative wire is doing more "work" than the other: therefore, we could call it "unbalanced."
Which is Better and Why?
Balanced cables are better because they have a separate wire for the ground, which means they are less susceptible to extraneous noise. Their XLR connectors often have a locking mechanism that makes the connections more reliable and more difficult to accidentally unplug. Unbalanced phone, mini and RCA connectors are not hearty and can be easily damaged and dislodged from their inputs. Balanced cables often also include a tight metal mesh wrapper that acts as an electronic shield to protect them from interference. Electricity can generate hum through a broken cable or a damaged connector. It can even arise simply by laying a power cable parallel to an audio cable. The upshot to all this is that hum in a cable can ruin an audio recording.
Adapters and How to Use Them
Balanced and unbalanced audio adapters potentially need to do two things. First, an adapter might physically change the connections to match each other, say from an XLR plug to a 1/4-inch phone jack. Second, an adapter might electronically change the impedance of a signal so that it matches the source to the destination. Impedance is a characteristic that refers to the resistance a sound signal encounters as it goes through a system. Some adapters only do one of these things and some do both. Or you might need to combine two different adapters in a chain to connect your equipment fully.
Professional microphones are balanced, use XLR connection and are of low impedance. Consumer equipment is unbalanced, does not use XLR connections and often has high impedance. Be careful when using adapters because some lack the electronic component necessary to properly match the signal levels used in XLR cables and those used in mini-inputs. This can definitely create buzz in your audio and could potentially damage your equipment. Fortunately, you can purchase adapters and boxes that can compensate for the impedance differences. Beachtek, Canon, Sign Video and Studio 1 (among others) manufacture the boxes that take care of both the physical connection and any impedance difference.
Many professional camcorders have XLR connections ready for microphones. Sometimes there are inputs for two channels, left and right for stereo recording, but a single XLR input is always mono. If a consumer camcorder has a microphone jack, it is almost certainly a stereo mini plug (1/4-inch).
Microphone Level and Line Level
Audio and video recorders (and playback devices) usually have two types of inputs (outputs): microphone level and line level. Microphone inputs are for microphones and line inputs are for other equipment, such as CD players and tape decks. The difference between microphone and line inputs is the amount of power of the signal. CD players and similar equipment usually have some means of amplifying the audio signal. Microphones do not, so they need the extra amplification provided by the recorder. Therefore, you should always make sure the microphone is plugged into a microphone input and not into the line input. Actually, this is a rather easy problem to diagnose: if you plug a microphone into a line input, the signal will be way too soft.
Connecting a line source to a microphone input can be even easier to diagnose: you'll know it immediately by the loud and painful sounds screaming through your headphones. Be more careful about plugging in line sources. A microphone into a line jack will yield poor audio, but a line source into a microphone jack has the potential to damage your equipment.
Who Should Use Balanced and Why?
Unbalanced cables and high impedance accessories are fine when you can get away with it, for example with very short cable runs. Also, an inexpensive microphone and unbalanced cable is almost always better than just using the on-camera microphone. In a professional situation (i.e. one where you're being paid), you want to isolate the sound source, and eliminate any potential for hum. Balanced cables eliminate hum. Using an external microphone of low impedance is best in an interview situation or when it is not possible to get close to your subject.
Actually, much of this discussion is not up for discussion: we can't think of any decent microphones that come with anything other than XLR connections and, by extension, work with balanced cables. So you'll almost certainly be using balanced connections, whether you care to or not. You'll just have to be flexible about the whole situation and adapt.
Garret Maynard is a film-video maker and guest lecturer, and lives in Connecticut & Vermont.
[Sidebar: Testing Cables]
It makes sense to test your audio connections and recording quality before you videotape an important event. The best way to do that is to connect everything and do a test recording. Make sure that your test conditions are similar to the event conditions. In playback, if you get low audio or a buzz through the headphones, it's time to troubleshoot. If you are using an external microphone, check to see if the impedance matches, check for parallel cables and ill fitted adapters. It also could be a broken cable. If you are connecting to a mixer, check it for signal continuity. It's a good idea to get to the location early to allow enough time for problem solving. Finally, if you have a buzz you just can't get rid of, try using only battery power with your camcorder: sometimes the source of your hum is the AC power to your camera.
[Sidebar: Longer Runs]
If you need to move farther away than fifty feet, consider using a wireless microphone. With a wireless microphone, you eliminate the cables but you often end up using an XLR connection to attach the receiver to your camera anyway. Wireless audio recording has its own unique challenges. Wireless microphones rely on radio frequencies (RF) to send the audio signal from the transmitter attached to the microphone to the receiver attached to the camera. Wireless units transmit either VHF (Very High Frequency) or UHF (Ultra High Frequency) transmitters. VHF is a very crowded bandwidth in our modern world and is used for everything from pagers to remote control toys to broadcast television. Wireless microphones used in these frequencies can be very susceptible to RF interference from these other sources. This is especially true as the distance between the transmitter and receiver grows or obstructions like walls block the signal. UHF is less susceptible to interference, but of course it's more expensive and even that bandwidth is becoming crowded. Try to use a wireless system that identifies itself as a True Diversity System. Such a system uses more than one signal path and records the strongest signal at any given moment.
[Sidebar: Safe Cabling Tips]
Treat your connectors and cables with care. If you have no choice where you lay the cables, cover them with a padded mat and gaffer tape them so they won't move. Wind your cables in a manner that follows the natural bend in the cable. Also, it's a good idea to wrap your cables once around your camera tripod just in case someone trips and pulls them. This simple preventive act will protect the inputs on the camera.