A stereo recording is a recording produced by specific techniques which utilizes two microphones in a specific placement that will record a given sound in a manner that replicates the normal human sense of hearing that same sound.
The most basic of these is the “spaced pair” technique in which two identical mic’s are placed perpendicularly in front of the source several feet apart from each other This works fairly well but offcenter sounds can become unfocused and harder to localize inside the stereo field. By varying the type of mic and adjusting the spacing between them, it is possible to make a warm ambient recording with this technique.
Another common technique is the “X-Y” placement in which two mics are placed close together on the same plane with a 90 to 110 degree angle between them in either an x or y formation. This technique is sensitive to sounds coming from the rear and is not suitable for quite wide sound stages but works well for smaller ensembles and quiet audiences. The X-Y technique can yield good localization and depth of field. Properly done this technique will give a sharp image and more of an airy quality.
Another popular stereo microphone technique is the "M-S" or mid-side technique. This requires a directional mic which is the “mid” facing the sound source and a figure eight mic which is the “side”at 90 degrees on the same axis to the sound source. The outputs of the two mics can be processed in a matrix to yield the two signals combined together as the left side of the stereo signal and the figure eight signal subtracted from the directional signal as the right side. The matrix is accomplished by panning the mid mic signal dead center in the stereo field and splitting the side mics’ signal into two separate mixer channels . One channel is panned hard left and the other is reverse phased and panned hard right thus giving us left as the mid +side and right as mid –side. The advantage to this method is that the stereo spread can be adjusted after the recording is made by manipulating the matrixing operation. Additionally If the mix must be collapsed to mono the result will be cancellation free sound from the mid mic as would have been the case with a single mic mono recording .