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Sound Reasoning: Recording Stereo Sound

Recording Stereo Sound

Remember the last time you went to a movie and you heard the actors on the right side of the screen--and their voices came from the same side? Or you saw a monster stomp from one side of the screen to the other, and the sound of the footsteps moved through the theater?

That's stereo sound. And though it's not even available at most movie theaters, you can do it with a camcorder, provided you own the right equipment.

Webster's defines stereophonic as an adjective meaning "to render the illusion of true auditory perspective through the use of two or more mikes for recording." In more basic terms, it means breaking the audio signal into two parts--right and left--which, on playback, gives a more realistic sound.

Needless to say, any decent mike, whether it's built into your camcorder, attached as an external mike or connected to your VCR or audio recorder, should create a reasonable reproduction of the sound you're recording. But on playback, a mono (single-channel) recording loses the rich, spread-out sounds of a live classical orchestra, or the auditory positioning of the actors on a stage. When you think about it, a single source of audio is pretty limiting when things are happening all over your video screen.

The addition of stereo sound to your videos will provide the same illusion of perspective to the audio you hear that the video image does to what you see. But there's more to it than just sticking a stereo mike in the scene and hoping for the best.

In this article, we'll look at the various types of equipment involved in recording stereo audio. We'll consider the proper use and arrangement of different types of mikes to get the best stereo sound possible. We'll also note some of the problems in recording stereo that you might encounter along the way.

Equipment Needs

If your camcorder or VCR is limited to a single audio track, you cannot record true stereo. You could use any number of mikes connected to an audio mixer and this might give you a better representation of all the elements in the sound you record. But the final mix would have to be a mono signal to be compatible with your camcorder or VCR's single audio track. Even if you found a stereo audio tape recorder to record your stereo sound, it would be of little use if the video recorder you wanted to transfer it to had only a single audio track. The stereo effect would be lost at your camcorder or VCR as you shot or edited your tape.

For the purposes of this article, we'll assume that your camcorder or VCR has VHS hi-fi if you're using equipment in the VHS family, or AFM stereo if you're using 8mm gear. Both VHS hi-fi and 8mm AFM stereo provide both tracks necessary for good stereo recording.

Some industrial-level S-VHS decks offer dual linear audio tracks along with the VHS hi-fi. This gives you two ways to record stereo on one deck. Usually, the sound quality of these linear audio tracks is inferior to the hi-fi tracks, though considerably better than the single linear track on consumer VHS VCRs.

A few high-level Hi8 decks also offer PCM stereo audio along with the AFM stereo. PCM stereo offers near-CD quality sound--much better than the quality of AFM stereo. For a full explanation of these audio tracks, see "Camcorder Sounds" in the January 1995 issue of Videomaker.

Camcorders that have stereo audio generally come with a built-in stereo mike. This may seem a great advantage until it's realized that while shooting, the best placement for the camcorder and the best placement for the mike are seldom the same. Say, for example, that you're shooting a stage play. If you're shooting with a single camera, you'll want to be back in the audience where your lens can capture the whole stage. Here, your built-in stereo mike is as far from the talent as you are. Most theater halls deliberately have hard, bouncy acoustics to assist the talent in projecting their voices. That, combined with the distance to your mike, can muddle the sound by the time it reaches you and severely limit the stereo effect.

Fortunately for our purposes, most stereo camcorders have external stereo mike inputs. You can send signals from an external stereo mike, or several mike signals from a portable audio mixer, to your camcorder's external stereo mike input. These mikes can be placed anywhere you wish, regardless of the placement of your camcorder or VCR.

If your camcorder does not have an external stereo mike input, or has only a single audio channel, you can still get by if you have a stereo audio mixer and a VCR with stereo audio. You can run audio signals from the audio mixer to the VCR inputs while you run the video output of your camcorder to the video input on the VCR. The audio mixer is required because the audio inputs on your VCR require a much stronger signal (called a line-level signal) than a mike alone can provide.

If you use this method, you'll have to do the actual recording on your VCR instead of your camcorder. This requires carrying around a VCR to your shoots, and making sure A/C power is available. However, it offers you a chance to record stereo audio if your camcorder is not capable of recording or accepting external stereo mike signals.

Connections and Mixers

The stereo mike input on most camcorders accepts a 1/8-inch mini stereo phone plug. A standard mono phone plug is made up of a tip and a sleeve (insulated from each other) to conduct the signal from a single mike into your camcorder, while a stereo phone plug has a tip, a ring and then the sleeve. On the stereo phone plug, the tip and ring conduct the separate stereo signals, while the sleeve acts as a common connection for both signals (see figure 1a).

The cable used with stereo mikes has two center conductors and a shield. At the camcorder end, the center conductors go to the tip and ring while the shield goes to the sleeve. At the mike end, the center conductors go to two separate mike elements while the shield goes to both mike elements and acts as a common ground. It's possible to divide this cable from your stereo mini jack into two mono cables should you wish to use two separate mikes rather than one stereo mike (see figure 1b).

This will also allow you to connect your camcorder to the outputs of a stereo or multi-channel mixer. Many good battery-powered portable mixers are available. It's always advisable to use a mixer if you're using two or more individual mikes to record stereo. When using a mixer, you have individual control of the level of each mike, allowing you to fine tune the balance of your stereo signal.

One major advantage here is that, unlike camcorders, most mixers provide a level meter for setting your separate audio levels. If you plan to connect a mixer directly to your camcorder's external stereo mike input, make sure the mixer offers mike-level outputs. Line-level outputs (with much stronger signals) will badly distort your audio if connected to your camcorder's external mike input. You use line-level signals only when you wish to connect a mixer's outputs directly to your VCR. Several good quality portable mixers offer both line- and mike-level outputs.

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.

M/S Miking Technique

M/S, or middle/side miking, uses one cardioid mike facing directly forward at the middle of the sound source, and one bi-directional mike just above or below it to pick up side and indirect sound (see figure 2c). A bi-directional mike picks up sound most strongly from two sides, but unlike a stereo mike, a bi-directional mike outputs only a single signal. The outputs of both the cardioid and the bi-directional mikes are sent to a special matrixing electronic circuit (basically a transformer and supporting circuit) which derives the left and right stereo signals.

Several manufacturers sell M/S miking systems which include the matrixing box. It's usually not necessary to use an audio mixer with M/S miking systems, since the electronics act as their own mixer, unless you wish to include other sound sources in your audio tracks.

Among all these systems we've discussed, M/S miking can produce the best possible stereo sound when properly used. M/S miking systems generally incorporate adjustments between the center and side signals, allowing flexibility in the amount of separation you desire. Most stereo zoom mikes (which zoom in on sound much the same way your lens zooms in on pictures) use this adjustment for their effect. In fact, most built-in stereo camcorder mikes use the M/S miking system in either a fixed or zoom configuration.

To Sum Up

By now, you should have a good basic idea of how to record quality stereo audio. You should know if your equipment can record stereo, or what you'll need if you plan to buy new gear. You should understand the connections you'll need to make, and see the advantages of using a mixer when recording. You should understand the advantages and disadvantages of recording stereo using the external stereo mike, the different X-Y miking placements and M/S miking.

All of these techniques will provide you with the basic tools you need to record much better sound tracks than simple mono setups will allow, and to excite your audiences with the full-bodied quality of stereo sound. So let's dust off those mikes and go make some stereophonic sound.

Tags:  June 1995
Thu, 06/01/1995 - 12:00am