Stereo microphone recording techniques
Mono recording - i.e. with a single microphone - is very straightforward. However, when we need to record a bidimensional sound, such as in a stereo recording, things may become complicate. For this reason, many techniques have been devised.
XY (Coincident Pair): Two directional microphones are placed very closely together, but not touching. The diaphragms are almost in the same position, but angled left and right in order to obtain a stereo image. Sometimes, the microphones are placed just above each other, in order to align diaphragms. The angle between diaphragms is generally around 90 degrees but can vary up to 135 degrees in order to widen the stereo image.
Spaced pair (AB): Two identical omnidirectional microphones are placed spaced apart. This configuration is used mostly for capturing ensembles. The distance from the source should be a few feet in the front, facing directly forward. The greater the space between the microphones is and the wider the stereo image will be. If the microphones are too far apart, the stereo image will sound excessive. Conversely, if they are too close, the stereo image will tend to sound as a mono recording. The best placement follows the 3:1 rule - i.e. the microphones are placed three times apart their distance from the source.
Near-Coincident Pair (ORTF - Office de Radiodiffusion Télévision Française): It is similar to the XY technique, but the microphones are spaced 17 cm apart. The spacing is meant to provide an accurate representation of sounds as the microphones are placed similarly to our ears.
Middle-Side (coincident pair): It needs one directional (or omnidirectional) and one figure of eight microphone. The directional microphone ius placed in front of the source pointing at it. The figure of eight microphone is placed perpendicular to it, so that it faces the sides. The diaphragms need to be as close as possible. This technique suffers from phasing issues and usually needs some retouch during the mixing stage. In order to have the two sides of the stereo image, the figure of eight track must be duplicated and one of the tracks be put out of phase.
Baffled-Omni Pair (binaural pair): Two omnidirectional microphones are separated a few inches by a baffle between them. The baffle is a hard disk covered with absorbent foam. Otherwise, the baffle may also be a hard sphere with the microphones flush-mounted on opposite sides. Sometimes, a dummy head is used, to replicate the way our ears capture sounds. Those sounds must then be directly replayed into the corresponding ears via headphones, to preserve staging. This setup needs some equalization (a broad dip around 3 kHz) and has sever issues with phasing, making this type of recording quite unsuitable for ordinary speakers.
Blumlein Pair: It consists of an array of two matched microphones that have a bidirectional (figure of eight) pickup pattern. They are positioned 90° from each other. This is the usual setup with ribbon microphones, which allows the creation of recordings that, upon replaying through headphones or loudspeakers, recreate the spatial characteristics of the recorded signal. The Blumlein pair produces an exceptionally realistic stereo image, but the quality of recordings is highly dependent on the acoustics of the room and the size of the sound source.
Decca Tree: It was originally developed as a sort of stereo AB recording method adding a center fill to provide a strong stereo image. A Decca Tree setup uses three omnidirectional microphones, three arranged in a "T" pattern. The stem of the T faces the orchestra, and the microphones on the crossbar of the T are placed about 5 feet apart. The centre microphone is placed 2.5 feet out.
Decca Tree with surround sound: It like a Decca Tree plus another two onmidirectional outrigged microphones further left and right, to capture ambient sound.