In this article, kindly written for us by Dr. Helmut Wittek, CEO of Schoeps, we explore issues and techniques around recording live performances using microphone arrays designed for capturing in immersive formats like Dolby Atmos.
Navigating the Minefield that is Recording for Dolby Atmos
Mixing for Dolby Atmos and other immersive audio formats, such as Spatial Audio and MPEG-H, is the hot topic du jour. However, resources for the recording techniques to achieve a satisfying audio capture for these formats are still hard to come by. Many professional engineers are still experimenting with these formats, so the topic might appear elusive and a moving target. In this article, Dr. Helmut Wittek, Co-CEO of SCHOEPS Microphones, breaks down the range of immersive audio recording techniques for music. Drawing from SCHOEPS' rich legacy in microphone arrays and stereophonic principles, the goal is to help make your next (or first?) Atmos recording project a little less daunting.
Common Goals Across Diverse Techniques
Recording techniques vary widely, each tailored to suit specific acoustic environments and recording needs. The choice of technique impacts the final sound, with each method offering its pros and cons depending on the application.
Regardless of the technique used, the ultimate goal remains: To produce a sound that envelops the listener, offering a realistic, musical, and immersive auditory experience. This involves a careful balance between capturing direct sound from instruments and vocals while also integrating the ambient sounds and reverberations of the recording environment. Whether it's a mono, stereo, or immersive audio setup, the perceived timbre of the sound is paramount.
Reality Check
Practical considerations often dictate these choices, such as the feasibility of deploying large microphone arrays in given spaces and their associated budget. Setting up a multi-channel array with up to 11 microphones is no small feat: It requires careful planning and access to reliable mounting hardware such as tall stands, stereo bars, and positioners. In addition to traditional stand companies, microphone brands like SCHOEPS offer dedicated mounting hardware for immersive audio setups.
Furthermore, the microphones themselves must be suited for the job. Miniaturized microphones, especially when offered in a modular fashion like the SCHOEPS’ Colette series, are less obtrusive and light enough to be mounted in a large array.
Back to Basics: From Stereo to Immersive
For those looking to delve deeper into the intricacies of immersive audio recording, understanding the basic principles behind microphone array configurations is crucial. This includes the directional characteristics of microphones, the spatial arrangement of the array, and the inter-channel time and level differences that influence how sound sources are localized in the mix.
These considerations are not new: They are deeply rooted in stereophonic recording principles and still hold true for immersive audio. In fact, many, if not all, immersive audio techniques are evolutions of long-standing stereo configurations. Stereo techniques can be categorized into spaced pairs (AB), coincident pairs (XY and MS), and near-coincident pairs (ORTF and NOS). Spaced pairs are commonly created using omnidirectional microphones, while coincident and near-coincident pairs use directional microphones.
Level and time differences between the two microphones in a stereo configuration dictate the localization of a source's phantom image. We can only achieve correct imaging with sufficient separation between channels, whether achieved by level differences, time differences, or a combination of both. Consequently, the polar pattern, the distance, and the angle between the two microphones are the parameters that control the localization. These relationships directly apply to neighboring channels in a multi-channel setup.
Here are a few practical rules of thumb that take stereo concepts and apply them to immersive audio arrays:
It is only possible to create an immersive array with sufficient channel separation utilizing time differences, that is, at least some distance between the microphones.
An array built mostly around directional microphones creating level and time differences typically has realistic imaging and a natural timbre.
An array built mostly around omnidirectional microphones relying solely on time differences provides more open and impressive immersiveness, but with less precise imaging.
Directional vs. Omnidirectional Microphones
The choice between directional and omnidirectional microphones often depends on the specific requirements of the recording. Directional microphones, such as cardioids or supercardioids, are excellent for isolating sound sources and reducing background noise. In contrast, omnidirectional microphones capture sound from all around, providing a more natural ambience and expansive sound but at the risk of capturing unwanted noise and channel crosstalk. Ask two engineers, and the "Omni vs. Directional" question can quickly become a heated debate. Since the first days of stereo recording, both approaches have been continuously used, refined, and discussed.
One important consideration is that immersive audio recordings often must still be carried out as an add-on to a stereo recording rather than being "immersive first.", which may limit your options. If a record label or producer expects an omni setup for stereo, adding an entirely new array for Atmos might simply not be feasible.
Channel Formats and Immersive Recording
In immersive audio setups, like those used for Dolby Atmos or Spatial Audio, the configuration of speakers and microphones is critical. Techniques often involve complex arrays designed to maximize the spatial rendering of sound, using formats such as 5.1.4 or 7.1.4 to place sounds precisely within a three-dimensional space. This requires careful consideration of microphone placement and channel routing to ensure each sound source is accurately represented in the acoustic field.
The techniques described below assume a fully immersive setup of a roughly cuboid playback with at least eight loudspeakers. This corresponds to the channels L, R, Ls, Rs, Ltf, Rtf, Ltr, and Rtr. Many main microphone arrays also require the center channel. Adding the LFE (low-frequency extension) channel gives us the standard 5.1.4 format.
Additional surround channels may be added to achieve a 7.1.4 channel setup, and some techniques make do without the rear height channels Ltr and Rtr for 7.1.2.
Omni Arrays
Techniques such as the "Decca Tree 3D", the "Polyhymnia Pentagon," and other arrays with omnidirectional microphones have a strong foothold for recording in acoustically rich environments, such as concert halls and churches. These arrays are particularly effective in capturing expansive sounds and creating a 'larger-than-life' experience. Additional spot mics can easily be mixed in due to the diffuse nature of the main array's sound. Leading classical music producers such as Morten Lindberg (2L Productions), and Jin Choi and Stephan Cahen (sempre la musica) rely on variations of omni arrays to craft their recordings.
However, they also come with challenges, such as maintaining a specific 'sweet spot' for the listener. Since omnidirectional microphones pick up sound from all around, a source's direct sound is picked up by many more than just two microphones, which can lead to unwanted coloration, vague localization, and even echoes. Omni arrays also usually need to be positioned closer to the ensemble than an array based on directional microphones, or the recording becomes too indirect and diffuse.
An array built around omnidirectional microphones is physically larger than one with directional mics. To achieve the desired channel separation, purely based on time differences when using omnis, the microphones need to be placed at least 60 cm (or 2 feet) apart.
For a variation of the omni array, the height (and optionally the rear) channels may be replaced by cardioid or supercardioid microphones. Doing so reduces the problems with signal separation described above by using the rear rejection of the directional microphones facing away from the ensemble.
Cardioid / Supercardoid Arrays
If many microphones are used simultaneously, directional microphones can be a better choice altogether. The improved signal separation between the channels yields a balanced sound and robust localization, which is particularly important for immersive recordings. While every audio engineer and Tonmeister is free to come up with an entirely new technique, here are two standard recording techniques with directional microphones that have proven themselves in the field:
OCT-3D
This technique, developed by the author and Günther Theile, combines the well-known OCT (Optimized Cardioid Triangle) setup with rear-facing cardioids and up-facing supercardoids for a maximum of channel decorrelation, i.e., the separation between the ensemble in the front, and the diffuse reverberation and ambience. The supercardioid height microphones can be placed about 1m (or 3 feet) above the main array, but they can also be placed on the same plane for a more convenient setup with coincident pairs of up front – and up-facing microphones.
PCMA-3D
Devised by Hyunkook Lee, this array consists of five cardioid microphones for L,C,R, Ls, and Rs, plus four supercardioids for the height channels. The original version of PCMA-3D is a horizontally spaced, vertically coincident array, meaning that the L and R channels form coincident pairs with Ls and Rs, respectively. If Omni or wide cardioid microphones are used for L,C and R, it may be necessary to apply some spacing between the base and height layers. PCMA-3D v2, shown in Figures 5 and 6, uses a vertically near-coincident configuration with less than 30cm of vertical spacing.
Dedicated Room Microphone Arrays
If you’re already familiar and comfortable with the various types of stereo microphone arrays, adding a dedicated set of room microphones to an existing stereo setup might be most feasible way of getting into recording for immersive audio formats. In this case, ambience array configurations like the Hamasaki Cube intended to be used in conjunction with a main stereo array are the way to go. Their job is not to capture the ensemble but strictly to record the diffuse ambience of the room.
When a less obtrusive room microphone array is needed, the SCHOEPS ORTF-3D set, designed by the author, is a great choice. It combines eight supercardioid microphones in a compact form factor to strike the perfect compromise between immersive sound quality, imaging, and size. Due to its completely symmetrical construction and its perfectly balanced directional characteristics, it is suitable for recording situations in which direct signals are to be reproduced from all directions. It is therefore designed more for ambience recording than for recording reverberation in a concert hall.
Not least due to its indoor and outdoor set options, it has become the de facto standard as a broadcast ambience microphone for sports events. It is also regularly employed for location sound and game sound applications requiring headtracking. And it is the ideal microphone array for binauralized audio in high-profile applications such as the BBC Proms broadcast.
Folding it down
By taking the concepts we already know from recording in stereo and applying those principles to multi-channel formats up to Dolby Atmos and Spatial Audio, engineers can create immersive soundscapes that transport listeners into the heart of the music, making each listening experience unique and engaging. Every recording situation is different, so we must choose the appropriate approach by carefully considering the pros and cons of the various techniques and the particularities of the job.
At the end of the day, let your ears be the judge. It has been proven that the perceived timbre and sound character trump spatial factors in a recording. When in doubt, we might aim for a natural timbre and balanced sound rather than spectacular spaciousness. The choice of microphones and the recording's signal path are crucial to achieving a natural sound, just as much as the sound quality of the space and the performance of the ensemble to be recorded.
For a deeper dive into the topic, including more recording tips, the available equipment, and practical tips on Dolby Atmos routing, head over to the Immersive Audio resources on the SCHOEPS website.
SCHOEPS' Legacy in Microphone Arrays
SCHOEPS has been a pioneer in the development of stereophonic recording techniques since the 1950s, continually pushing the boundaries of how sound is captured and experienced. The introduction of the classic ORTF stereo microphone in the 1970s marked a significant milestone, which was followed by advancements in 5.1 surround sound techniques in the 1990s. Today, SCHOEPS remains committed to optimizing techniques for immersive audio, including the development of the ORTF-3D method, which significantly enhances spatial audio recordings for broadcast, television, and film.