In my experience the most common way to capture the stereo field is with the stereo pair or ‘XY’ recording technique. Of which has been explained in fluid detail by fellow Pro Audio Web Blog writer Will Anderson! (Here is a link to the article). The normal XY pair microphone arrangement is placed specifically to replace our ears, each mic mimics the human ear in the placement. This is a brilliant way of recording and can be used for outstanding effect. However an alternate way to capture a stereo image is to use the Mid and side technique of which, I will endeavorer to explain in a simplistic and practical manner, so you can go and try this yourself!
Firstly the mic placement. You use two microphones for this technique, unlike the X,Y placement the microphones don’ t have to be matched pairs. I have used two different microphones when recording mid and side. The mid microphone is placed at the center of the sound source, this is normally a cardioid/hyper cardioid pattern. The second mic has to be a figure of 8 polar pattern and is placed off axis at 90 degrees from the sound source. The two microphones need to be placed as close together as possible for the best outcome.
So you know have two signals from your sound source. You have mid and a side signal of your sound source. Now comes the clever bit! When you record your sound source you are capturing the mid signal and ONE side recording. You mid is the like a center channel, and the side adds the audio which creates the ‘space’, it captures the reflections of the room and the ambient qualities of your recording environment. After you have recorded your audio, you will have a mid and one side recording. Because of the figure of 8 pattern of the side mic, the two sides of the mic’s capsule are 180 degrees out of phase.
To create the stereo field you will need, in your DAW, to create 3 tracks. One with the mid recording and then you copy the side recording to create to side channels. Now pan each of the side recordings, one hard left and one hard right, and then flip the phase of one of the side recordings. In this process of flipping the phase and panning the side channels left and right you are effectively recreating the figure of 8 pattern from the side mic. Now if you listen back to you should have a stereo signal when all 3 tracks are playing. However if you solo the mid channel the signal will revert to a mono audio signal.
This style of stereo capture makes it brilliant if you need to sum to mono when mixing, however you can achieve this with X,Y recording phase cancellation is more likely to be a problem. The advantage of Mid and Side recording is that you can vary the amount of side level to widen or reduce the stereo spread of your mix. This can give you excellent control of the stereo image when mixing.