The Decca Tree is a technique of recording that grew out of Decca's research and development into stereo which started in 1954 with Arthur Haddy, Kenneth Wilkinson and their team. The first Decca stereo products (60 records and 4 reproducers) came in 1958.
Decca has a long tradition of developing their own methods and technology, and so they set out to develop their own method of recording stereo as well as developing their own proprietory designs of console and other recording equipment.
The criteria that were applied to making a good recording, as reported by producer James Locke (quoted in: Jim Betteridge, 'Keyed In To Opera', in 'Studio Sound', April 1987) are, in order of importance:
The use of the three microphone technique that has come to be known as the "Decca Tree" grew out of the desire to find a compromise between the use of a minimal pair and the use of a multi-miking technique, in order to maximize the clarity and depth of opera and orchestral recordings.
The actual "tree" and the original stereo mixer were designed by Roy Wallace. A triangle of microphones is placed roughly ten to twelve feet above the stage level, just behind the conductor. There are usually two additional flanking microphones used as well, anywhere from either edge of the orchestra, to about one third of the hall's width in. The mixer is adjusted so that the centre mike goes equally to both channels of a two track recorder, with the right tree mike and right outrigger going to the right channel, and both left mikes to the left.
When this technique was first used in 1954, the microphones used were Neumann KM 56s, tilted 30 degrees toward the orchestra. Other microphones were tried including the cardioid M 49 (in baffles), omnidirectional KM 53, and finally settling on the omnidirectional M50 under Kenneth Wilkinson in 1955. The baffles used with the cardioid microphones were four large boards fanned out from the central point in such a way that each microphone was positioned within its own pie wedge. No baffles are used with the M 50 omnidirectional mikes.
The outriggers, mentioned above (additional to the Tree), also developed to the point where the mic of choice is the M 50, facing diagonally across the orchestra. These mic's signals increase the breadth and spaciousness of the stereo image. Soloists with the orchestra were and are usually spot miked.
The use of the tree has remained virtually unchanged since the '60s, although Decca engineers have made modifications to the microphones. In a typical Decca recording session, every effort is taken to find a suitable recording venue with desired reverberant characteristics, which are then tailored by the addition of large hanging drapery or the placement of large sheets of wood in the hall the adjust the quality of the reflected sound.
Kenneth Wilkinson is quoted as saying (in Michael H. Gray's, 'The Birth of Decca Stereo', in the Association for Recorded Sound Collections, November 1987, vol. 8, no. 1, page 7):
"You set up the Tree just slightly in front of the orchestra. The two outriggers, again, one in front of the first violins, that's facing the whole orchestra, and one over the cellos. We used to have two mikes on the woodwind section -- they were directional mikes, 56's in the early days. You'd see a mike on the tympani, just to give it that little bit of clarity, and one behind the horns. If we had a harp, we'd have a mike trained on the harp. Basically, we never used too many microphones. I think they're using too many these days."
As for the spacing of the three mikes themselves, this varies with the venue used and with the size of the ensemble. For instance, when the Montreal Symphony under Charles Dutoit is recorded in Ste. Eustache, the centre mike is dispensed with entirely. In chamber recordings, the outriggers are frequently not used. The size of the triangle itself varies with the amount of width and spaciousness desired.
Another good source for information on this subject is an article by Michael Gray, entitled 'From the Golden Age', that appears in The Absolute Sound magazine, Vol. 11, No. 42, pp.103 - 110