Decca tree

The Decca Tree is a spaced microphone array most commonly used for orchestral recording. It was originally developed as a type of stereo A–B recording method adding a center fill. The technique was developed in the early 1950s and first commercially used in 1954 by Arthur Haddy, Roy Wallace, and later refined by engineer Kenneth Ernest Wilkinson and his team at Decca Records and its recording studios, to provide a strong stereo image.
The Decca Tree setup evolved from the idea of a minimal recording technique using a pair of microphones. The first system was developed by Roy Wallace. The microphone triangle was placed about 3 to 3.6 m high above the stage level, near the conductor. The microphone system is not properly in front of the orchestra, but more "into" the orchestra.
Two more microphones can be added and placed on the sides (called "outrigger microphones"), approximately at about 2/3 of the stage width, between the conductor and the outer orchestra boundary.
A Decca Tree setup uses three omnidirectional microphones arranged in a "T" pattern outlining a triangle, often equilateral; the center microphone is mixed with the two spaced microphones to fill the "hole in the middle" in their imaging; it points the sound source. The placement can be done with three separate microphone stands or using one or more bars. In contrast to the ORTF stereo technique, the Decca Tree size is not fixed and may vary considerably; distances between the two back microphones are seen between 0.6 and 1.2 m; the front microphone is set proportionally and can be mounted slightly lower than the outside pair. The recording engineer arbitrarily adjusts these dimensions accordingly to the size of the ensemble, the dimensions of the room and the type of music. Microphone manufacturer Schoeps advises to put the three microphones at least 1.5 m apart to reduce crosstalk and excessive correlation at low frequencies (for example, setting the microphone width to 2 m and the depth to 1.5 m).
Former Decca engineer John Pellowe describes the specifics of the setup as follows:
The Decca Tree was originally used in orchestral situations, fitted on a tall boom and suspended in the air, roughly above the conductor.
The technique traditionally uses three omnidirectional microphones, traditionally of the Neumann M 50 small-diaphragm pressure transducer tube condenser type, to record in stereo. These microphones are not truly omnidirectional at the higher frequencies, but exhibit some high frequency lift and directionality which is likely to positively affect stereo imaging of the Decca Tree arrangement. Variations have been performed using a coincident pair, in X-Y, Mid/Side (M/S), or Blumlein positioning, in place of the center microphone. The Neumann M 49, KM 53, and KM 56 were also evaluated and used for early sessions by the Decca team, and later the Schoeps MK 2S were used by the team for live productions where the M 50 proved too cumbersome.
The Decca Tree is a stereo miking technique often used in large orchestral or choir performances, but it can also be used as a room miking technique. When used for room miking of drums, its wide stereo image captures the nuances of bigger environments better than most other techniques. In smaller rooms however, the Decca Tree does not work as well.