The Decca Tree - microphone system - or Decca triangle - is a type of stereo recording technique that was empirically found during recordings and that Arthur Haddy, Kenneth Ernest Wilkinson and their recording teams tried out as early as 1954 .
The first real stereo recordings by Decca Records from London were released in 1958 by four producers and comprised 60 records. Decca can look back on a long tradition in researching special methods in the field of recording technology. In addition to her microphone recording method, she also developed her own stereo mixers and other special recording devices.
The producer and sound engineer James Lock (Jimmy) reported in the article by Jim Betteridge, "Keyed In To Opera", Studio Sound, April 1987, about the necessary criteria for a good recording. The order of their importance is as follows:
- The musical work (compositional quality)
- The type of presentation (quality of presentation)
- The technical quality (acoustic-electrical recording quality)
The application of the three-microphone technique, which is well known in specialist circles as the “Decca-Tree” or “Decca-Triangle”, developed out of the idea of the minimal recording technique with only two microphones and the multi-microphone technique to find a compromise in order to bring out the clarity and the gradation of depth in opera and orchestral recordings.
The first “tree system” as well as the first stereo mixer were developed by Roy Wallace. The microphone triangle is set up around the conductor at a height of about 3.00 m to 3.60 m (11 feet) above the stage. This can be done with three separate microphone stands or with a pole frame. The microphone system is not in front of the orchestra, as you often see, but more in the orchestra. Usually two additional cantilever microphones (outriggers) are set up on the sides, which are roughly in the middle between the conductor and the outer orchestra boundary up to about 2/3 the width of the stage. The panpot of the center microphone goes to both stereo tracks in the center of the stereo recording. The left “triangular microphone” and the left “boom” go panpot-wise fully to the left channel and the right microphone and the right boom go to the right channel.
When this technique was first tried out in 1954, the Neumann KM 56 microphones were used, which had an inclination of 30 ° towards the orchestra. Other Neumann microphones were also tried out, such as the M 49 with a cardioid polar pattern. The latter was a large diaphragm microphone . Additional partition walls were also set up between the microphones and the microphones with omnidirectional characteristics KM 53 and finally the omnidirectional microphones M 50 (special small diaphragm (!) - pressure receiver on a 4 cm diameter plexiglass ball the size of a golf ball) were tried out. It is said again and again that the M 50 are large diaphragm microphones, which is not correct. The membrane itself is "small".
The partition walls, which were initially tried out with the M 49 cardioid microphones, consisted of four boards that were angled outwards from a center point so that each microphone was positioned in the middle of its 60 ° "pie corner". Since the use of the M 50 omnidirectional microphones, no partitions have been used between the microphones.
The additional “cantilevers” developed so that M 50 microphones were also used for this purpose, pointing diagonally or straight over the orchestra. These microphone signals enlarge the image width and the impression of spatiality of the stereo recording. Usually soloists have to be supported with additional microphones.
The “Tree” arrangement remained generally unchanged for a long time, even if the Decca engineers kept making minor changes to the microphone setups, which are always to be expected due to the differences in room, orchestra and score.
This type of recording technique, which cannot be calculated physically and mathematically, was not taken into account by the scientific acoustics institutes for precisely these reasons - indeed it was actually avoided.
In a typical Decca recording session, every effort is made to find a suitable recording room for the composition and cast that has the right reverberation characteristics. The necessary quality of the reflected sound is further optimized by hanging up cloths or by laying large wooden panels over the upholstered seats in the concert hall.
Note: In contrast to the established ORTF microphone system , the dimensions of the microphone system "Decca-Tree" (Decca triangle) are completely free. The T-arrangements offered for this with the dimensions 1.25 m and 0.7 m, as well as 2.50 m and 1.25 m are only non-binding suggestions for the Decca Tree and are mostly too small. Each sound engineer sets his own dimensions individually, depending on the type of music, the size of the orchestra and the dimensions of the room.
As a rule of thumb, the distance between the three microphones should never be less than a whole meter (3 feet). If the distance between the microphones is less, there is no longer a Decca triangle in the effect.
The imaginative surround sound microphone structure Atmos 5.1 (with a front microphone distance of less than 25 cm) is completely wrongly called the Decca triangle for advertising reasons. All other triangles with a side length of around 30 cm are also nice and small and practical, but acoustically always turn out to be wrong and really have nothing to do with the idea of a Decca triangle.
If you have to lower the level of the microphone in the center by more than 3 dB compared to the other microphones, then something may be wrong with the microphone spacing. This three-way system is not a large AB system with "auxiliary support" for the center.
Especially since surround sound recordings have existed, there is no longer any way around the Decca Tree recording technique, even if this largely eludes a precise mathematical-acoustic calculation.
Helmut Krüger, who made many stereo test recordings on magnetic tape for the Reichsrundfunkgesellschaft during the Second World War , used similar constellations. Few of these have survived. The best known is the recording of the 5th Piano Concerto in E flat major by Ludwig van Beethoven , played by Walter Gieseking, Arthur Rother and the orchestra of the Reichssender Berlin . The last movement of Anton Bruckner's 8th Symphony , played by the Prussian State Orchestra under the direction of Herbert von Karajan , was recorded in stereo in 1944. Despite a strong background noise, both recordings offer open acoustics. The space in the studio, hall No. 1 in the Haus des Rundfunks , will also play a part in this. The hall is still used for recordings today.
- Thomas Görne: Sound engineering. 1st edition, Carl Hanser Verlag, Leipzig, 2006, ISBN 3-446-40198-9 .
- Thomas Görne: Microphones in theory and practice. 8th edition, Elektor-Verlag, Aachen, 2007, ISBN 978-3-89576-189-8 .
- John Borwick: Sound Recording Practice , 1976, Oxford University Press, London, 1st edition only: Trygg Tryggvason, Classical Music , pp. 210–228.
- Decca Tree Recording with Neumann pressure receivers M 50 (PDF; 136 kB)
- Decca-Tree - yesterday and today - essay (PDF; 472 kB)
- The Decca Tree - It's Not Just for Stereo Anymore (English)
- Andreas Gernemann: "Decca-Tree" - yesterday and today - video stream ( RAM ; 0 kB)
- Image of a Decca tree assembly (English)