Sound Advice: Stereo Recording Techniques

Today’s camcorders are marvelous devices. They allow us to relive events of the past with vivid color and "you-are-there" picture quality. Unfortunately, the audio we record with our camcorders often pales in comparison. Usually noisy, distant and unfocused, typical camcorder audio leaves quite a bit to be desired. But it doesn’t have to be that way. With some planning (and a little extra equipment), the audio on your video recordings can rival that of mega-buck productions — with a broad stereo soundscape you’ll be proud to play on any home theater system.

Is stereo audio worth all the extra effort? In many cases, yes. In this article, we will explore several of the most popular techniques for recording stereo audio and how you can apply them to your next video shoot.

First Things First

Before we start talking about microphone patterns, distance measurements and the like, lets take a quick look at what you’ll need to record stereo audio. First, forget the "stereo" microphone built into your camcorder. Yes, you might put it into service in an emergency, but your recording will include noise from the tape drive, zoom motor and focus mechanism. It will also pick up any noises you make, including your breathing and hands. Simply put, you need external microphones.

There are several hardware items to acquire as well: a tall (10-15 feet) microphone stand, mounting hardware and long microphone cables are necessities. Depending on your microphone choice, you may also need an audio mixer to combine and power the microphones. Of course, a good pair of headphones and a 50-foot power cord will come in handy too.

Finally, you have to decide where you will record the audio: direct to the camcorder, to a master video recorder, or to another audio recording device (MiniDisc, CD recorder, etc.). This decision affects both your hookup for the recording and your options during post production. Recording to the camcorder is the most direct method, but will require some special adapters. Using a master video deck means that you won’t actually record any audio in your video camera, but is simpler to set up. A separate audio recorder is the most complicated method, since you will have to recombine the audio and video elements later, most likely in a computer. Just make sure you are completely prepared for whatever technique you choose.

A Method to the Madness

There are several methods for recording stereo audio. Some are very simple while others require hours of setup time and a crew to keep everything straight. Let’s focus on a handful of the most popular techniques — those requiring a minimum of equipment and producing maximum results.

Spaced Omni

Also called A-B Stereo, this technique uses a matched pair of omnidirectional microphones positioned 40 to 60cm apart on a very tall mike stand or suspended above the recording subject . This stereo technique is well suited to large concert events — choirs, bands and orchestras — where you set the mike stand several feet behind the conductor and position the microphones 10 to 15 feet in the air. Of course, several real-world factors will determine your actual microphone position: size and liveliness of the room, depth of the stage and loudness of the recording subject. As a general rule, the further back you position the mike stand, the more ambient the recording becomes – picking up more of the room and less of the subject.

There are strong benefits to this technique. Spaced omnis usually produce a large, spacious soundfield with excellent detail. Low frequencies are faithfully reproduced and the recording will have a very natural sound. If there is an audience at the performance, its applause will be just as clear and detailed as the music. On the other hand, the spaced omni technique can sound very different when played back over a typical mono Television speaker. That leads us to the next technique:


X-Y Pair

This method, also called X-Y Stereo or "coincident," uses a matched pair of cardioid (unidirectional) microphones. The microphones are positioned 90 degrees opposite of each other with one microphone above the other and the diaphragms vertically aligned . This assembly mounts on a tall mike stand and raised above the recording subject, similar to the A-B stereo method above. Again, position is critical for the best recording, but the same real-world factors apply. The further back your mike stand is positioned, the more distant the sound will become. This can actually be a benefit in a smaller or acoustically "dead" room. Put on the headphones and let your ears guide you.

Although the stereo image from this arrangement is not as strong or detailed as the previous method, it is still very good, and mono compatibility is near perfect. The X-Y stereo method is a favorite in the broadcast industry for its clear playback on any audio system, whether it is mono or stereo.

ORTF

The video world is full of acronyms. This one stands for Office de Radio et Television Franaise, the French equivalent of the FCC. Also called "near-coincident" this recording technique is very similar to the X-Y method above, with one small difference. Instead of placing the microphone diaphragms directly over each other, the ORTF technique locates the diaphragms 17cm (about 7 inches) apart, at a 110-degree angle . This spacing more accurately represent the spacing and directional characteristics of our ears.

The ORTF technique is a great compromise. It retains a great deal of stereo information while maintaining good mono compatibility. This is important when you want maximum sound detail while producing a video that will sound great on any home system.

MS (Mid-Side)

This unique method uses two different microphones – one cardioid (unidirectional) and one figure-8 (bi-directional). The cardioid mike is mounted to face the recording subject while the bi-directional mike is mounted with its directional faces pointing side-to-side . The microphone elements may be separate or enclosed in the same body.

Due to its "single point" setup, M-S microphones yield perfect mono compatibility yet retain the ability to provide spacious stereo content as well. To record M-S audio to your tape, you’ll need a matrixing transformer. This mixes the two signals and allows you to control the width of the stereo picture. You may have a hard time monitoring the audio in the field, as you’ll have to "un-matrix" the audio on playback. Several manufacturers provide switches to blend the microphones in the field and record standard left-right audio to your tape.


Recording in Style

The results of all this will pay big dividends in personal satisfaction and will elevate your services above those with less sophisticated approaches. A good stereo recording is a wonderful thing to behold, so a little research is in order. Prior to your shoot, find a well-produced CD of the type of material you will record. Listen closely (with headphones if possible) to the sense of space in the recording. Pay careful attention to the placement of instruments, voices or other sounds in the recording. You should be able to estimate their distance from the microphone and their left-right position. Keep this information in mind as you set up for your shoot. Stereo recording adds a unique dimension to your videos.

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