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The microphone is one of the basic tools of the sound recordist, and to attempt to cover every possible micing situation and technique would be an overwhelming task, so I'll restrict this article to stereo microphones useful to the field recordist.
Compact, lightweight, portable stereo recording - that's what this article is about.
To try and cover the whole gamut of microphone technology and techniques could prove to be a lifelong task filling several weighty tomes. Fortunately, several tomes have already been written. Here are a few that I've found quite informative:
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Stereo Microphone Techniques by Bruce Bartlett |
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The New Stereo Soundbook by F. Alton Everest and Ron Streicher
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For micing of musical performance in a non-studio setting, | take a peak at On Location Recording Techniques by Bruce Bartlett. The title is slightly misleading, it is not about location recording for film or video.
To learn about a pioneer in stereo recording, read | Inventor of Stereo: The Life and Works of Alan Dower Blumlein by Robert Charles Alexander.
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Why Stereo?
Well, you've got two ears, don't 'cha? Having two ears is as necessary to locating the source of a sound as having two eyes is to perceiving visual depth of field. Here, in a nutshell (and painting with broad strokes), is how this works:
Suppose a tree falls in the forest, and that you are there to hear it.
The sound waves travel away from the sound source until they reach your ears. High frequencies have short wavelengths (relative to the size of your head) and are reflected off of your skull. This reflection alters the sound spectrum slightly as it reaches the ear furthest from the sound source. Lower frequencies have longer wavelengths that pass by your head unaffected.
The sound waves also reach each ear at slightly different times, due to your ears being slightly apart.
The difference in sound wave spectrum and arrival time at each ear is called the "head related transfer function" (HRFT). Your brain uses this difference to help you localize the position of a sound source.
It's worth noting that both how big your head is, and the thickness of your skull will certainly affect your ability to hear...
The head related transfer function relies on your having two ears. Why is this important?
Imagine a lion on the African savanna. Now imagine that you are a tasty antelope drinking at the water hole water hole. Your hear a noise! Your brain uses the head transfer function to help you locate the position of the predator so that you can turn and run the other way, and avoid becoming a snack. This is just one example of why having two ears is useful.
All stereo recording techniques boil down to the fact that you have two ears, and one goal of stereo recording is to capture a realistic, three dimensional soundfield as perceived through your two ears.
But wait, there's more... Its Playback time!
In addition to putting the mics in the right place, we've got a few other things to consider. The most important of these is playback.
After a
beautiful bird song hits your recording media, it ceases to be beautiful (as far as physics is concerned) and becomes just a collection of sine waves. While your brain can perceive these sine waves as musical, electronic equipment interprets them according to strict rules of mathematics.
Subtle timing and phase differences can cause the signal on one channel to partially cancel out the signal on the other channel when both are played back together. This is called phase cancellation, and it can give your recordings a muddy or nasal tone.
Also, converging a stereo signal to monaural is an important consideration if you are recording for broadcast - you have to consider the car radio and the TV with a single 3 inch speaker.
So, an additional requirement of stereo recording is to allow for accurate reproduction of our recording on an appropriate playback system.
Methods
If you would like to review basic stereo microphone methods,
read Stereo Microphone Techniques by Bruce Bartlett, or choose a summary below:
Coincident mic methods seek to minimizing phase cancellation is to place microphone capsules as close to each other as possible. Because of the coincident placement of the mic capsules, there is virtually no time or phase difference between the channels. The stereo field is achieved through differences in volume. A pan pot on a mixing console uses the same principle.
If you are using two microphones, the microphones are angled, forming a "V" with the narrow end pointing towards the sound source. Two separate mic stands can be used, but to minimize hardware, a mounting bracket can hold both microphones on one stand (You can build your own). Either way, a two mic setup may not be compact enough for quick, easy set up in the field.
Other stereo recording techniques seek to capture a realistic soundfield by separating the mic capsules. These techniques can yield a more satisfying recording of the stereo soundfield, but again, if you are using multiple microphones, this may not be easy to set up in the field.
One spaced-pair mic set up involves two microphones, usually omni-directional, set about 10 feet apart. This is not a compact set up, but it is common to place hydrophones in this way.
The Best of Both Worlds?
Mid-Side (M-S) techniques seem to provide the best of both worlds, but this is subjective. There are, as with most things audio, differing opinions.
M-S stereo involves the use of two mic capsules, one directional (cardioid, hypercardioid, or even omni-directional), and one figure-8. The figure-8 mic is aligned so that the pickup pattern is at a 90 degree angle to the "mid" mic. In practice this allows the microphones to be placed parallel to, and on top of each other, using a special mounting bracket. This set up lets you place both microphones inside the same windscreen.
This is more compact than most two mic set ups, and some manufacturers combine the two mic capsules into a single-body stereo mic (to be fair, you can also find single-body XY stereo mics).
Because the mic capsules are coincident, there is no phase cancellation, yet M-S recordings seem to capture a soundfield as well as spaced pair techniques. In the broadcast world, M-S stereo recordings can be played back in mono with no loss of fidelity.
To decode the signal for playback: The left channel = the middle channel plus (in phase with) the side channel. The right channel = the middle channel minus (out of phase with) the side channel. Adjusting the amplitude of the side channel varies the stereo spread. Notice that varying the stereo spread can be done after the recording has been made, which is something that can't be done with other micing techniques.
An M-S stereo mic may prove to be a good (but expensive) choice for all around use. When not recording in stereo, the "side" element can be ignored, using only the "mid" element as a single directional microphone. The downside of this is that you will be restricted to the "mid" element pickup pattern chosen by the microphone manufacturer.
Some of the drawbacks? A dedicated figure-8 pattern microphone is expensive. Fortunately, a figure-8 capsule is available for most mic systems with interchangeable capsules. You could also try converting a mic you already have. Either approach lowers the cost considerably if you want to stay with a two microphone set up. Some audio post-production people prefer X-Y stereo, so you have to consider them if you part of a larger process.
To monitor or playback an Mid-Side recording requires special electronics that may not be available on all mixing consoles or headphone amplifiers, so M-S recording may involve the adding a decoding matrix to your current set up. You can either buy one or build one.
If you just want to experiment, you could build a cable to feed an MS signal into three channels to feed into your mixing console. If you attempt to use this cable when recording, make sure that only one of the channels connected to the "side" mic is providing power.
Stereo Mics
Here are some commonly used compact stereo microphones:
Good multi-capsule mic systems are made by
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I don't know. Nobody does. There is no one-size-fits-all mic technique that works best in all situations. Don't limit yourself to just one, and don't be afraid to experiment, tinker, and consult others.
Many factors contribute to the techniques that will work best for you, and not all of the factors are sonic. Ultimately, your ears are the final authority.
Nobody ever got hurt making a stereo recording ... wait, well, actually,
that's another story ...
Continue with Part Two:
Using Your Head - Binaural Recording
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