Mixing in any source that produces a coherent image in/across the center of the playback stage will allow you to space an omni pair wider than your otherwise could without problems.
How much farther you can space the omnis, and how far you might actually want to space them, will depend on a few things.
Simplifying by ignoring other things for now, here are two primary aspects the spacing between omnis effects on playback: 1) image distribution, and 2) ambient pickup feel and sense of space. We want a lot of spacing for the second - more spacing produces a more open, airy, diffuse, enveloping, "it feels like I'm there" kind of ambient impression. Audience reaction becomes is less specific, less in your face, specific audience voices tend to be pulled to one side or the other and tend to compete less for attention with the music.
How much spacing we want for "1)" depends on distance to the source, the width of the ensemble or PA, and the desired playback width of that part of the recording, but is in general less than what we want for "2)". The correct spacing for this is somewhat more exacting in terms of getting the playback image distribution right. Too little spacing is going to sound narrow and produce a small ensemble image. Too much produces the disjointed "hole in the middle" imaging problem. In some ways the term of "hole in the middle" as a description of the imaging problem produced by a pair of over-widely spaced omnis is a bit of a misnomer. A more apt description is that sources which were located to the left or right of center get pulled toward the closer playback speaker, while any source(s) that was truly centered remains centered, yet becomes sort of stranded like an island in the middle of the "hole".
If we mix something in with a pair of spaced omnis, we gain some freedom. In most cases that addition allows us to space the omni pair farther apart, making "2)" better without "1)" suffering for it. Of course there is a limit to how much wider we can go, and one of those things is the nature of whatever it is you will be mixing in with the omnis. A monophonic soundboard element like a isolated vox feed, mono SBD mix, or even a single directional center microphone placed between the spaced omni pair will tend to solidly anchor the center and allow for a somewhat greater spacing between omnis than would otherwise be acceptable. However, because that kind of mono source rather tightly occupies the center position without much apparent width to it, the omnis still need to be close enough together to achieve sufficient blend across the entire playback stage. Mixing in a tight-monophonic source with wide-spaced omnis might not sufficiently fill the gap to either side of center. You sort of end up with three clumps of sound again- only with a stronger center now, probably with enhanced clarity and better overall balance, but not really blending smoothly to achieve a seamless image all the way across.
Mixing in something that provides more width across the center will help that contribution to feather into and blend smoothly with what the wider omnis are providing. That addition will allow you to space the omnis farther apart than you would want to when mixing in something monophonic in the center, and significantly more than you might risk when using an omni pair on its own.
A stereo SBD or stereo microphone pair in the center instead of a single microphone helps achieve a good image distribution that fills the entire sound stage, while the wider omni spacing the incorporation of that stereo center allows for provides a more natural open sense of space, ambience, audience reaction, and bass quality.
The addition of a coincident microphone pair in the middle, much like the addition of stereo SBD, produces mostly phase-coherent level-difference stereo with a strong monophonic center component, yet one with some degree of center image width. This is exactly what a spaced omni pair tends to need, and is in a way the opposite of what the spaced omni pair provides. It makes for a highly coherent center image portion that blends smoothly into the more open, ambient, wide omni bed.
A near-spaced center pair can do that too. But a near-spaced pair is designed to achieve a reasonable balance of all this in isolation. If you add spaced omnis, the near-spaced pair ideally needs to be narrower to compensate. If you add a center microphone or coincident pair the near-spaced pair needs to be wider. A-B and coincident are sort of opposite ends of the spectrum already, and combine in a complementary way, naturally compensating for the shortfalls of the other. Ideally, the combination of the two will include some modification of the setup of each (the omnis get wider, the coincident pair narrower) than either would use by itself in isolation. Yet somehow those altercations seem less of an excursion from normalcy than altering standard, well-known near-spaced configurations by doubling the spacing for use in combination with a coincident center pair, or halving it for use in combination with a pair wide omnis.
Topic: Recording bass in a stage lip setup (Read 2394 times)