The OVJ Microphone Array (v3)

A method loosely based on several established techniques for recording acoustic music and ambient sound, including:

• Optimal Stereo Signal* by Jürg Jecklin.

• Bilateral Ambisonics** by Z. Ben-Hur, D. Alon, O. Berebi, R. Mehra, and B. Rafaely.

• OCCO*** popularized by Robert Sandy “Boojum” Noyes, Jim Norman and Tony Faulkner.

• STAAG, by Jamie Tagg.

Description: Two Ambisonics subarrays are centered over each side of a 35cm diameter round baffle, and stereo sets of microphones are placed beside them. OVJ is short for Omni Variable Jecklin, because at least one of those sets is usually comprised of two omnidirectional microphones.

How each pair works:

• The pair of Ambisonics arrays separated by ∽36cm delivers Jecklin-affected* bilateral** signals.

  If using FOAs, they are both each physically rolled 45° so the leftmost and rightmost capsules are opposite to each other. Resulting signals are correspondingly rotated after converting to Ambisonics B-Format. We can be call this Modified for Symmetry First-Order Ambisonics (MFS-FOA).

• The inner non-Ambisonics pair (1, 2) adheres to OSSv2: omnidirectional capsules separated by ∽36cm, as close as possible to the Ambisonics sub-arrays to facilitate W-signal replacement:

• Since calculations by Oren Levy and empirical testing have shown an SRA (Stereo Recording Angle) of 82° for Jecklin OSS, distances and angles for other stereo sets within this array default to that same SRA as a starting point. The outer pair (3, 4) is therefore separated by ∽78cm. This pair can be used for Jecklin effect mitigation if needed, while providing other spatial cues. If omnis are used the extra bass capture is useful. If APE spheres are added there will be some additional front-back discrimination. If acoustic conditions are adverse this pair can be switched to cardioids, at 0° the same SRA will be maintained.

 

Main findings:

• The array allows for unobtrusive hyper-realistic recording with significant versatility when mixing.

• As in OCCO, the geometry allows for extra forward gain from the sum of all capsules on each side, and the extra distance between outer capsules can add spatial cues for LF. LPF can be used to mitigate comb filtering.

• When using FOAs, capsules 4 (FOA left 4) and 5 (FOA right 1) can provide adequate stereo preview monitoring without the need for Ambisonics processing.

• Similarly, if dedicated Ambisonics plugins are not available for some reason, simple summed busses can be used to create signals for "quick and dirty" discrete multichannel monitoring.

• Hyper-realistic multichannel spatial results are available with modest processing. Suggested for Dolby Atmos within Apple Logic Pro:

• Audio Brewers Transcoder for Ambisonics A to B format conversion and W-channel replacement.

• Audio Brewers AB Rotator for compensating MSF FOAs in B-format.

• Audio Brewers Advanced Decoder for conversion from B format to beamformed signals corresponding to discrete Atmos channels.

• Panning the Ambisonics subarrays to each side is not required for realistic immersion, as long as frontal mid is attenuated. There has been debate among bilateral Ambisonics users about how to pan, mask or apply dominance to each side, the Jecklin baffle provides a possible solution.

• As in STAAG, backfacing beams from Ambisonics allow for less dependance on artificial processing and confer a more natural sense of the space where the recording took place. If more reverb is required, a useful approach is applying processing to said beams only.

More findings: 

• The “Jecklin effect” survives fold-down from Atmos to stereo.

• The experience on headphones is similar to binaural, to an extent that a more natural result is possible by turning off frontal binaural rendering in the Dolby Atmos workflow, something which unfortunately does not seem possible with the Apple binaural renderer at this time.

• W-signal replacement causes some beam shifting in the general direction of the replacement signal related to the Ambisonic center, therefore compensating with some rotation might be needed.

• Excellent mono-compatibility occurs because a significant portion of the signal originates from Ambisonics, unlike traditional binaural which falls apart when summed.

• Baffle absorption is critical for vocals and similar sources on which comb filtering due to reflections can be very noticeable. Basotect is a winner for absorption.

• Baffle core rigidity seems important for LF performance, probably because flexing turns it into a passive radiator which passes on LF. Acrylic and carbon fiber work very well.

Notes:

Tests took place using: 

• 2x Rode / Soundfield NT-SF1 Ambisonics microphones, 

• 2x Schoeps MK 5 microphones, usually in omnidirectional mode, 

• 2x Earthworks QTC30, sometimes with custom 40mm APEs (Acoustic Pressure Equalizers),  or 2x Immersive Soundscapes Earsight Thumb microphones.

• 35cm round "baffles made out of varying materials, including perforated metal, polyester fiber, Basotect, carbon fiber and acrylic.

 * The “Jecklin effect” is the result of the Optimum Stereo Signal, an acoustic process accomplished with a round vertical baffle between matched capsules. OSS delivers spatial cues from varying intensity and frequency response depending on incidence angles, as well as time precedence caused by inter-capsule distance. Unlike dummy-head or current software-based binaural, this method is highly compatible with stereo loudspeakers.

** OVJ is not by-the-book Bilateral Ambisonics, but a simplified process where spatial cues from the Jecklin effect are presented to the listener somewhat separately for each ear, similar to the way OSS works. 

 *** OVJ inherits from traditional OCCO the concept of the single-axis horizontally spaced, vertically coincident multi-microphone phased array, but not the practice of setting the inner pair to a different SRA than the outer pair.

 The OVJ name is inspired by Christian Amonson and his outside-in nomenclature. 

SRA prediction diagrams come from the Neumann Recording Tools app.

All original content on this page by @ignace / Ignacio Rodríguez de Rementería is open source licensed under CC BY-NC-SA 4.0 https://creativecommons.org/licenses/by-nc-sa/4.0/

Many thanks to the following friends and teachers for their help in alphabetical order: Benjamin Maas, Bradford Richards, Brian Peters, Christian Amonson, Darío Bustos, Eduardo Monteiro, Eric Weber, Francisco González, E., Hudson Fair, Ian Wood, Jack Reynolds, Jake Purches, Joel Rec, John Cone, Julian J. Ludwig, Lucas Guitink, Manfred Schmidt, Mario Vila, Oren Levy, Paal Rasmussen, Paul Fee, Stefan Kießling and many others from the excellent CMLR group on Facebook, and especially to Alejandro Cabrera, Carlos Fernández, Diego Rodríguez B., Don Booth, Esteban Zabala I., Gricelda Duarte, Helmuth Reichel-Silva, Iván Órdenes, Jaime Valbuena M., Joaquín Luppi, Jorge Montesi, Jorge Sacaan M., Juan P. Quezada, J. Alberto Palacios, Julio Figueroa M., Leonard Moskowitz, Mauricio Landeros, Pablo Saavedra, Ricardo Henríquez, Romualdo Castro, Sebastián Errázuriz, Xavier León and last but not least my lovely wife Patricia Reichel for putting up with me during more than a year of research, design and testing.