The Decca Tree, September 2003—It’s Not Just for Stereo Anymore

IT'S NOT JUST FOR STEREO ANYMORE 5/18/2004 8:00 AM Eastern

Ever since the 1930s, when early experiments in stereo transmission techniques were conducted on both sides of the Atlantic, two basic but radically different approaches to stereophonic recording—coincident microphones vs. spaced microphones—have coexisted. Each has its own camp of followers who champion its particular attributes and often denigrate those of the other.

One of these techniques employs coincident microphones to create a stereophonic pickup based entirely on the intensity differences generated between a pair of microphones as the soundwave passes by. Its claim to fame relies on the strong stability and clear articulation of the stereophonic image that it creates. Its primary drawback, however, is a somewhat constricted image width and what some people consider to be a tendency to sound “dry” or “sterile.” The other approach uses two (or sometimes three) spaced microphones to capture and reproduce both the soundwave's intensity and time-of-arrival cues as it passes by the microphone array. A natural result of these time-of-arrival cues is a greater sense of “spaciousness” than can be achieved with solely intensity-derived techniques. The drawback, however, is a lack of articulation across the stereophonic image and what many listeners consider ambiguity in the center imaging.

Thus, these two camps have staunchly opposed one another: coincident microphones vs. spaced microphones and articulation vs. spaciousness. And in the age of multichannel, surround sound production, listeners demand both an articulate image and spatial envelopment from the soundtrack. The logical solution, therefore, is to employ techniques that combine the best attributes of both coincident and spaced microphones. Choose your compromises wisely.

In the early 1930s—when early experiments in spaced-microphone (left-center-right) stereo were being conducted in the U.S. by the engineers at Bell Laboratories—British scientist Alan Blumlein, working for EMI on the other side of the Atlantic, was developing the concepts of coincident microphone techniques. His pioneering work was codified in a patent that was issued in 1933, in which he defined and described a technique to create a stable and articulate stereophonic image by using just two crossed bidirectional microphones, a configuration that has come to bear his name: the Blumlein technique.

Blumlein realized that by using the unique cosine pattern of bidirectional (figure-8) microphones, the principal pickup axis of one could be precisely co-aligned with the axis of minimal pickup (the null-plane) of another, resulting in a very stable, extremely accurate and well-articulated stereophonic image, one that relies entirely on the differences in the intensity cues as the sound reaches each of the two microphones.

In the same patent, Blumlein also described a mathematical transformation of these crossed bidirectionals, which he termed the Mid/Side Technique. This also employs a bidirectional microphone—the Side microphone, which is oriented laterally with the null-axis aimed directly at the sound source—to provide the essential directional contribution to the stereophonic imaging. The Mid microphone provides the overall pickup. Its principal pickup axis is aimed directly at the sound source; again, co-aligned with the null axis of the bidirectional microphone. Although not yet a conventional stereophonic pickup, when the signals of these two microphones are combined via a sum-and-difference matrix system, left and right stereophonic signals result: Mid + Side = Left; Mid - Side = Right.

It is important to note that although convention depicts the Mid microphone as cardioid, it may be any polar pattern, from omnidirectional to bidirectional. At the same time, the ratio of Mid-to-Side signals introduced into the matrix can vary. By virtue of these two variables (Mid-mic polar pattern and Mid-to-Side ratio), an infinite variety of “virtual stereo pairs” can be created using this technique.

Since its inception, the Decca Tree has been widely used for large-scale recordings and is a favorite among film scoring mixers because of its ability to maintain excellent imaging and separation even through the various matrix-encoding systems used to distribute film soundtracks.

We can trace its origins to March of 1954, when engineers Roy Wallace and Arthur Haddy at the Decca Studios in London prepared for a recording session with the Mantovani Orchestra. Always experimenting in the then-new medium of stereo, Wallace assembled a T-shape steel array and attached Neumann M49 microphones to each of the three ends. (The left and right microphones were “hard-assigned” to their respective channels, and the center mic was assigned equally to the two channels but at a somewhat lower level to avoid “center buildup.”) He then suspended the entire array from a large studio boom, above and slightly behind the conductor's podium. Wallace recalls: “It was a crude attempt to re-create the artificial head that I spent about a year making.” When Haddy first saw the array, he remarked: “It looks like a bloody Christmas Tree!” The name stuck.

In later revisions, Wallace and Haddy used Neumann KM56 microphones, sometimes experimenting with a “Blumlein shuffler” (custom EQ employed to augment the low-frequency content of the difference information in a stereo signal). Further refinements by Decca engineers Ken Wilkinson and Stan Goodall evolved the classic “Decca Tree” as we know it today: three Neumann M50 omnidirectional microphones arrayed.

Because the sound arrives at the center of the tree—forward microphone slightly earlier than the left/right pair—the Law of the First Wavefront guarantees that this central image will be strongly focused and clear. This results in a significant improvement over previous spaced-microphone configurations, which were criticized for presenting poor or diffused central imaging.

A frequent criticism of the coincident Blumlein and Mid/Side pickups is that their stereo imaging tends to be constricted and lacks the spaciousness provided by near-coincident or spaced arrays. Conversely, spaced-microphone techniques are criticized for not providing the same clear, articulate image that can be obtained with coincident configurations. An easy and obvious solution to this controversy is to combine the desirable attributes of both techniques.

Long an advocate of Mid/Side recording, I began experimenting in the late 1980s with a variation of the Decca Tree that used a M/S microphone pair—rather than a single omni—as the front-center pickup. My goal was to maintain the vitality and articulation from the M/S pickup, while “broadening” it slightly and providing the spaciousness that could be had only from the separated, flanking microphones. To preserve the sonic integrity of the entire array, in my initial experiments, the center-stereo microphone was an AKG C426 and the flanks were AKG C414s: all large-diaphragm condensers using the same capsule design. Various spacings were tried, both front-to-back and side-to-side, which were determined by the size of the performing ensemble and of the performance space. I created configurations that ranged in size: from a “mini tree” that was one meter wide and a half-meter deep, to the “full-size” tree with the standard spacing of two meters by one meter.

Building on my experiments with the Decca Tree, I have developed an expanded surround sound configuration: a Mid/Side microphone—or, for a more complete surround experience, the SoundField Mk-V microphone—as the front/center pickup and two pairs of flanking microphones on the rear bar. One of these pairs is aimed forward (toward the sound source) and serves to “flank” the center-stereo pickup in the front left and right channels, as described above. The second pair is aimed at the rear and provides the essential signals for the surround channels. This microphone configuration can combine several discrete stereophonic pairs into a complex and widely variable complement of conventional frontal stereo and/or surround pickups.

An important attribute of this array is that all of the microphones are relatively closely spaced so that minimal phasing anomalies or cancellations result if/when these signals are mixed down to “smaller” formats, such as conventional stereo or even mono. Remember, with all spaced-microphone techniques, phase cancellations are unavoidable and some comb filtering will inevitably result. However, due to the relatively close proximity among the microphones on the Decca Tree, these effects will be less obvious than with more widely spaced arrays.

For a conventional stereophonic (2-channel) recording, the surround microphones can be mixed with the other mics' signals to add ambience and/or natural reverberation. This will also be reasonably coherent because it is well within the “fusion zone” of the primary stereo signals. An ideal choice for these surround microphones is a pair of hypercardioids with a good off-axis response (such as the Schoeps MK41 or Neumann AK50 capsules). Because the rear lobes of these hypercardioids tend to “cross” the channels of the front stereophonic image (i.e., the left microphone picks up the right side and vice versa), a “purist” might prefer to use cardioids. To me, this cross-channel effect actually increases the sense of envelopment in the total sonic perspective, particularly if the null angle of each of these surround mics is aimed directly at its opposite; very much akin to the crossing of the nulls in a conventional Blumlein pair.

When a SoundField Mk-V microphone system is employed as the front/center pickup, additional options for surround sound recording are available, because this unique microphone inherently provides a surround sound pickup in its own right. With the SoundField SP451 Surround Sound Processor, the system produces a full 5.1 surround array with complete variability of the balance and sonic perspective. By combining the SoundField's coherent surround signals with the 2&3 and 4&5 spaced-microphone pairs, it is possible to create an even broader spectrum of stereophonic and/or surround sound images, while at the same time, satisfying the desire for both articulation and accuracy of the sonic image, as well as breadth and spacious envelopment of the listener: the best of both worlds.

It is possible, of course, to configure a surround sound Decca Tree array by placing (or suspending) each of the microphones individually in the appropriate relationships to each other. This can, however, be cumbersome and time-consuming, because it requires five stands or hanging rigs. A more convenient method is to mount all of the microphones onto a common Decca Tree fixture and then support the entire array from above or below as appropriate. Stand-mounting provides the most ease and flexibility of placement, but the array can be “flown,” as well.

A SoundField Mk-V serves as the front/center microphone pickup; two subcardioid microphones are the 2&3 pair; and a pair of hypercardioid microphones is the 4&5. Of course, any microphones can be used. Experiment for yourself and have fun. After all, creativity is the essence of the recording experience.

The basic loudspeaker arrangement for 5.1 surround systems has been defined as a front pair, a center and a surround pair, plus subwoofer. Many advocates of surround sound systems, however, urge even more channels and loudspeakers. Many home-theater systems now offer a 7.1 configuration, adding two loudspeakers directly to the side of the listener. These additions enhance the sense of envelopment by providing increased early-lateral (reflection) signals. When the surround sound Decca Tree employs the SoundField Mk-V microphone as its front/center pickup, the array can provide sufficient discrete directional information to generate a 7.1 surround system, because either the SoundField or the rear-facing microphones (or a combination of both) can be used to derive the side or surround signals, or vice versa. If the surround sound Decca Tree is combined with a second pair of ambience (or additional surround) mics, then the surround depth and/or the number of channels can be expanded geometrically.

Combining the various elements of the surround sound Decca Tree is a matter of personal and/or professional taste. If a more articulated image is desired, then the coherent or coincident components should dominate the mix. To achieve a more “spacious” sound, the L/R2 and L/R3 pairs may be increased. Additional microphones can also be added to the mix to highlight individual sections or soloists and/or to augment the surround experience. Technology, like creativity, knows no bounds.

The author wishes to thank Roy Wallace, Michael Gray and Tony Faulkner for their kind correspondences that provided unique personal insights into the historical background of the Decca Tree development.

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