Here’s a simple truism about sound: the more it seems to wrap around the listener, the more dramatic and realistic it is.
When sound comes from just one spot, the brain is far from intrigued. We call this monaural or mono sound, and it’s very common in consumer-level video. Up a notch in dramatic impact is stereo sound, where sounds appear to be coming from any number of points between two speakers. Some camcorders will record stereo sound, and the end result is far better than the sterile, stagnant effect of mono sound. Most dramatic of all is surround sound, which envelops the viewer in a realistic wash of sound. Surround-sound videos are great fun to listen to, but we home videographers probably won’t be making them for several years.
Stereo sound, however, is a possibility for many of you–even if you don’t have a stereo camcorder. For those who do, these techniques will allow you to record sound superior to that of the built-in stereo mike. With a firm grasp on a few simple guidelines, you’ll soon be capturing spacious, engaging stereo sound to accompany your videos.
In the same way that stereo sound requires two speakers for playback, recording stereo sound requires at least two microphones. Even one-piece stereo mikes have two discrete capsules inside, wired and aimed to capture stereo sound. When using two separate mikes for recording stereo, the mikes can take any number of different shapes and styles. Likewise, there are numerous ways to position microphones when recording stereo sound.
The main goal of stereo miking is to capture a spread of sounds–a stereo image–that accurately reproduces the position of various sources. If all your recorded sound seems to come from a small area between the speakers, the stereo image is too narrow. Conversely, if sounds just slightly off-center of the mikes come entirely from one speaker or another, then the stereo image is unnaturally wide. Good stereo recordings make it easy to localize individual sounds in the stereo image. See figure 1 for details.
A secondary goal of stereo miking is to maintain as much mono compatibility as possible. In other words, when a mixer, VCR or television combines the left and right signals into one (a frequent occurrence), a mono-compatible signal will still sound relatively accurate. It’s pretty much inevitable, especially with consumer-level video production, that stereo sound will at some point be played back or recorded in mono. If combining your left and right signals makes the sound turn to mud, you’ve got a stereo signal that’s not very mono-compatible. Thankfully, it’s not that tough to capture stereo sound that holds up well when played back in mono.
Volume and Time
Before we discuss mike type and positioning, it’s important to understand how our brains discern where a given sound is coming from. One of the main cues our brains rely on is volume–if a sound is louder in one ear than the other, we assume the sound is coming from the louder side. Another significant cue is time. If a sound arrives at one ear sooner than the other, we deduce that the sound source is closer to the ear that hears it first. If we set up a pair of mikes to correctly capture one (or both) of these cues, our listeners will hear stereo sound on playback.
The directional mike is the key to capturing volume differences between sounds coming from left and right. If you point two directional mikes so their patterns extend in different directions, sounds originating from anywhere but dead center will appear stronger on one side than the other (see figure 2a). Even though mikes in this arrangement pick up no time difference between left and right signals, a decent stereo image results. We call this type of stereo miking coincident pair because sounds arrive at both mikes at the same time. Some folks refer to this miking technique as X-Y miking.
Most built-in camcorder mikes use two elements in a coincident-pair arrangement. Camcorder “zoom” mikes use a variation on the coincident pair called mid-side or M/S. With mid-side miking, it’s possible to adjust the width of the stereo image electrically, without physically moving mikes or capsules. At the tightest “zoomed-in” setting, most stereo camcorder mikes are recording just a mono signal from the front-pointing directional mike. Coincident pair and mid-side miking have good mono compatibility, primarily because there’s no time difference between the two mike signals. On the down side, these techniques don’t generate a very dramatic stereo image.
Putting two mikes even a short distance from one another generates a time difference for sounds coming from left or right of center. This occurs as off-center sounds hit one mike before the other. Even a pair of non-directional mikes will generate a stereo recording if spaced apart from one another. Cues in this case are almost exclusively time difference, with little level difference occurring between the two mikes. See figure 2b for a picture of spaced-pair stereo miking.
This technique can create relatively dramatic stereo imaging, depending on the distance between the mikes and whether directional or non-directional mikes are used. The main drawback to spaced-pair miking is that sounds coming from midway between the mikes (in the center) tend to be “blurry” and hard to localize. Spaced-pair recordings sometimes have poor mono compatibility, making for a hollow sound when the left and right signals are combined.
The most dramatic stereo imaging usually comes from a combination of these two techniques. One very effective stereo mike setup places two directional mikes a few inches from one another, pointing in opposite directions. This method picks up volume difference cues from the directional mikes, as well as a time difference from the space between them. Called near-coincident-pair miking, the width of the stereo spread from this setup depends on the angle and distance between the mikes (see figure 2c).
Because this method creates such a wide stereo image, you have to be careful not to let the spacing or angle between the mikes become too exaggerated. If you do, most of the recorded sounds will appear to come from one speaker or the other, with little coming from between them. Mono compatibility with near-coincident mikes is not great, due to the time delay between the two mikes.
More Mikes, Better Mikes
Combining more than two mikes through an audio mixer is another way to capture a spread of sound. Most stereo mike/line mixers have a “pan” knob for each input; this knob controls where that sound appears in the stereo field. By placing mikes in front of various sound sources and panning them to different degrees, you can create a panorama of sound.
Imagine you’ve got a medium shot with three people on-screen. Each is wearing a lavalier (tie-clip) mike, and the three mikes are running into a mixer. If your record deck is stereo, you can pan the voices slightly to match their position on the screen. Keep the middle person centered between the speakers, and pan the left and right talent part-way toward their respective speakers. The result is an interesting spread of sound, with the stereo placement adding further distinction to the voices. You can create the same kind of stereo effect with a stage production, talk show, drama, interview, music program or any other production. Provided you don’t overdo the panning, some stereo separation can really enhance the message of your video.
With an ample sampling of theory out of the way, let’s explore some specifics of using stereo mikes for videography.
The first thing to consider is what type of mikes to use. In general, you want a pair of mikes of the same make and model. The more similar the two stereo mikes sound, the more realistic the recording will be. For best results, use a pair of condenser mikes. Most condensers offer the clean, crisp sound and flat frequency response required for good stereo miking. Handheld dynamic mikes, in contrast, often have a built-in treble boost to add clarity to speech. At more than just a few inches from the sound source, this presence boost causes dynamic mikes to sound bright and thin.
Some condenser mikes use an on-board battery for power. If such mikes have a minijack output, they’re ready to plug straight into your camcorder. If the mikes have three-pin XLR outputs, you’ll need XLR-to-minijack converters. If the condenser mikes have no batteries, they’ll need phantom power supplied through their cables. Since no consumer camcorders supply power in this fashion, you’ll have to put an external phantom supply box in line with the mikes.
If your camcorder doesn’t offer stereo recording, you’re not out of luck. Provided you’re shooting in a fixed location, you can use a stereo VCR to record your audio and video. Simply run your camcorder’s video output into the VCR. If the mikes are plugged into the camcorder, take the stereo signal from the camcorder’s RCA outputs (if it has them). If you’re using an audio mixer, plug its output into the VCR. As a final option, some VCRs will accept the mikes’ outputs directly.
There are several ways to hold stereo mikes. The most common method mounts the mikes on one or two traditional microphone stands. You can get special holders to place two mikes on one stand for coincident pair miking, or you can position two stands next to each other. If you’re recording stereo on-the-move, you can mount the two mikes atop your camcorder. Better yet, recruit a friend or family member to hold the mikes on a boom, positioning the mikes just outside the frame.
When recording in stereo, it’s more crucial than ever to monitor your recordings with headphones. The phones will allow you to hear the stereo image you’re picking up, and make any necessary adjustments to microphone placement. When the sound you hear in your headphones is uncannily real, you’re on the right track.