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Surround Sound Mixing Techniques
Written by Mike Sokol - © 2005, PRIMEDIA Business Magazines & Media Inc. Reprinted with Permission.
Wouldn't you know it? Just when recording engineers have mixing in stereo down cold, 5.1-channel surround sound comes along. Of course, movie soundtracks have been using surround sound for years, but the basic formula is pretty straightforward: dialog in the center, music in the front left and right, suround effects in the rear, low-frequency effects (explosions, earthquakes, and so forth) in the subwoofer, which is the type of setup you'll find in a 5.1 surround sound home theater system.

Now audio-only music recordings are being mixed in 5.1 surround, and the old rules have been thrown out the window. Where do you place the listener with respect to the performer — in the “audience” or in the midst of the ensemble? With five main channels surrounding the listening position, the number of mixing decisions has increased substantially compared with stereo, and engineers are only starting to comprehend the enormity of the task.

I like to think of 5.1 surround mixing as similar to using different camera angles for shooting a movie. Sometimes up close and personal is the right approach, but other situations call for a wide panoramic shot. In any event, you need to understand surround-mixing technology before you jump in head first; if you are unfamiliar with this technology, see Introduction to Surround Sound and Setting Up Your Studio for Surround Sound.

Sound Field Of Dreams
Consider a typical stereo sound system with two speakers placed in front of a listener centered between them. The space between the speakers is called the stereo sound field, and individual sounds in a mix can be placed at any location within this space. Two basic principles of psychoacoustics let engineers do this: relative level and interear time delay.

Relative level is the way in which a sound's volume at each ear helps determine its source's location. In a stereo mix, each input channel's pan pot determines the relative level of the corresponding signal in the right and left speakers, and the main fader controls the signal's overall level. If the pan pot is centered, the signal's level is equal in both speakers, and the listener's brain is fooled into believing that the sound is coming from the point halfway between them. It's as though there were another speaker at that location; in fact, this virtual speaker is often called a phantom center. If you move the pan pot to the left, the signal's level is greater in the left speaker, and the apparent sound source moves to the left of center. Move the pan pot to the right, and the apparent sound source shifts to the right because that sound's level is greater in the right speaker.

The other psychoacoustic principle, interear time delay, helps you localize a sound source according to the difference between the instants at which a sound arrives at each ear. For example, if a sound source is to your left, the sound arrives at the left ear before it arrives at the right.

This principle is best simulated in a stereo recording by using a pair of microphones to pick up an entire ensemble, rather than combining multiple tracks through a mixer. If the microphones are placed too far apart, you lose the interear effect. An Office de Radiodiffusion-Television Française (ORTF) configuration works quite well: simply place two cardioid mics at an angle of about 110 degrees with the capsules roughly seven inches apart (the average ear-to-ear distance on a human head).

With this technique, what you hear is what you get. You can adjust each instrument's volume by moving the musicians nearer to or farther from the mics, and you can change each instrument's stereo placement by moving the musicians to the left or right in front of the mics.

Unfortunately, listening to such a recording's playback on speakers can result in speaker crosstalk, which occurs when the left speaker's sound reaches the right ear and vice versa. This can obscure the interear effect in the recording, but separating the mics by 10 to 12 inches reduces the problem. Listening on headphones eliminates the problem altogether.

This procedure has been employed on some of the finest orchestral and acoustic recordings. It's more difficult to do it well because you must think about the mix from the very beginning of the recording, and many engineers and musicians don't want to give up the luxury of fixing it in the mix with punch-ins and pitch correctors. That's a pity; this technique can make a beautiful stereo sound field that simply can't be duplicated with separate tracks and pan pots. All you need to record in this manner is a nice pair of microphones, a great room, and a stereo recorder.

Going Surround
The same two principles can be applied to 5.1-channel surround recordings, which are played back with a surround-speaker system that includes front left, center, and right speakers; left and right surround speakers; and one or more subwoofers, all arrayed around the listening position. You can start with a multitrack master and send all the tracks through a surround mixer. But instead of a simple right-left pan pot, each input channel includes a surround-panning control, which functions like a joystick. Such a mixer might be a hardware device, or it might be implemented in software that runs with a multitrack digital-audio program.

If you want an instrument or voice to sound as though it's coming from a particular speaker, simply grab the panning control and pan the sound to that speaker. What's more, you can adjust each track's apparent location anywhere between the speakers by moving the panning control to any available position. For instance, you can spread the drum kit across the three front speakers and place the guitar anywhere you like in the front or back. It's just like stereo mixing, but now you can decide whether you want to place the listener in front of the band, in the middle of the stage, or in some other strange place.

You can also record ensembles with a surround-microphone array, which is just an extension of a stereo array. However, you have to choose where to place the instruments in the surround sound field, which determines the listener's perspective. For example, you can position the ensemble in front of the array, using the rear microphones to pick up the room's ambience, which puts the listener at the conductor's position. Alternatively, you can place the array in the center of the ensemble, thereby putting the listener within the group. In either case, the sound field in the surround speakers is particularly effective because these speakers are frequently positioned to the sides of and just behind the listener, sort of like oversize headphones. This is an excerpt from the following article: Mixing in the Round.

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