Sound Reasoning: Recording Stereo Sound (page 2)

External Stereo Mikes

Several popular mike manufacturers make high-quality stereo mikes which you connect to your camcorder's external stereo mike jack. These will almost always give you better performance than your built-in mike.

Stereo mikes come in as many different designs as other mikes, but most are condenser types. They will provide very high quality sound and give you the great advantage of positioning your camcorder and mike at separate locations.

A stereo mike is nothing more than two separate cardioid mike elements built into a single case. Cardioid means that the elements are most sensitive to sound coming from a specific direction, rather than equally from all directions. The pickup pattern is in the shape of a heart, with the bottom of the heart shape pointed out--hence the term cardioid.

Cheaper stereo mikes have their elements fixed into position and are not adjustable, but better versions have one element capable of up to 270 degrees of rotation mounted over a second fixed element. This allows you to choose from several different coincident miking patterns.

Coincident miking means that one element of the mike sits directly above the other and the two pickup patterns are aimed to roughly cover the left and right sides of a sound source. Note that coincident miking refers not only to one-piece stereo mikes, but to separate mikes as well, when their elements are placed as described above.

Quality stereo mikes, when used properly, are capable of reproducing excellent stereo sound, but their major drawback is their high cost.

Spaced Pair Miking

Spaced pair miking is probably the simplest form of stereo miking using more than one mike. As the name implies, two mikes, usually omni types (which pick up sounds from all directions equally) are set to the left and right sides of the sound source. Spacing can range from only a few feet apart to the whole width of the room.

This is generally the first way a beginner tries to record stereo because it makes sense--a mike for the left side and a mike for the right. Unfortunately, this can cause problems should the need arise to sum the signal to mono.

Summing to mono means mixing your stereo signals into a single mono signal. You do this to retain compatibility with mono VCRs, to transmit mono audio over the airwaves or to dub to a mono film track, among other possibilities.

With this setup, if the two mikes are placed too far apart, they can record sounds that are out of phase on the two audio tracks. This means that if one mike is closer to a sound source than the other, the sound waves that enter one mike may be out of phase with the other. Although they might sound fine when played back in stereo, when mixed together as a mono track, these out-of-phase sounds will cancel each other out, making the audio thin, empty and lacking in harmonics. The best way to avoid this is to keep your mikes close together, as in crossed X-Y miking.

Crossed and Spaced X-Y Miking

You can use two mikes in a coincident pattern to get results as good as, if not better than, a stereo mike. This is called crossed X-Y miking.

In crossed X-Y miking, two cardioid mikes are placed one across the other (and one slightly above the other) with their elements as close together as possible (see figure 2a). To achieve this, they are often mounted on separate stands with short booms so that the stands do not interfere with each other. The mikes should be of the same brand and model for equal sound quality and pickup pattern. The advantage of crossed X-Y miking is a more pronounced stereo separation than available from a stereo mike.

The angles you set between the two mikes depends a lot on their placement in relation to the entire sound source. Sometimes, you can't control where you're able to place your mikes. While certain degrees of mike element rotation are often suggested by manufacturers, it's generally best to experiment when possible to find out what sounds best. This is true of spaced X-Y miking as well.

Spaced X-Y miking (often simply called spaced stereo miking) separates the mike elements by about 7 to 12 inches and aims them at the right and left areas of the sound source (see figure 2b). Using this method, the stereo separation is even more pronounced by adding greater space--and therefore time delay--between the mike elements and the incoming sound.

There are some disadvantages to spaced X-Y miking that you should be aware of. First, the farther away the mikes are spaced, the more reinforced the recordings are to the left and right sides. As a result, there can be a loss of sound from the middle area. Separation will be too good and the overall sound will have a rather hollow quality on playback.

Second is the problem of acoustic cancellation. Described above under spaced pair miking, this occurs when the distance from a sound source is so much further from one mike than from the other that the mikes receive the sound waves out of phase, which can reduce or even cancel out the sound during recording. Fortunately, when spaced X-Y mikes are properly placed, both of these problems are rare.