canal.cl / grabaciones: The OVJ Microphone Array (v1)

Please be aware there is a newer version of this post: The OVJ array v2.

The OVJ microphone array is loosely based on several proven techniques for recording acoustic music and ambient sound, including:

The horizontally spaced, vertically coincident 4-microphone phased array known as OCCO, popularized by Robert Sandy “Boojum” Noyes, Jim Norman and Tony Faulkner.

The binaural-like OSS (Optimal Stereo Signal) “Jecklin Disk”, by Jürg Jecklin*.

Bilateral Ambisonics***, by Zamir Ben-Hur, David Alon, Or Berebi, Ravish Mehra, and Boaz Rafaely.

OVJ array viewed from the back

Two tetrahedral FOA (First Order Ambisonics) subarrays are centered over each side of a baffle, and two matched stereo sets of SDCs are placed closer in and further out.

How each pair works:

The innermost pair (1, 2) with an inter-microphone distance of 30cm or less between provides intense spatial cues based mostly on direction-dependant intensity and frequency response caused by the baffle, as well as some time precedence. This is known as the Jecklin effect.

We have empirically determined an SRA (stereo recording angle) of aproximately ∽81° for the Jecklin effect, therefore that is the target SRA suggested as a starting point for all inter-microphone distances and angles.

Omni capsules are preffered. Cardioids can also be used, becoming “baffled NOS” with similar SRA consisting of a stronger intensity component than either standard NOS or Jecklin OSS.

In either case, panning and mixing with outer pairs can smooth the effect if too strong, and automatic equalization such as Acon Digital DeFilter can compensate for comb filtering, should reflections from the baffle be too noticable.

The outermost pair (3, 4) can be spaced accordingly at 79cm for a similar time-based SRA.

Again, omni capsules are preffered. Cardioids can be used if acoustic conditions are sub-optimal.

This pair adds position cues beyond the limits of OSS and some front-back differentiation when using APEs or cardioids.

Pulling this pair in a little closer can deliver a Tony Faulkner-styled sound, where the outer pair has a wider SRA than the inner pair.

The FOAs deliver immersive Jecklin-affected signals:

We position the FOA arrays so the left-pointing capsule on the left (5) and the right-pointing capsule on the right (12) are 24 to 36cm apart.

Ambisonics dominance processing and post-transcode panning can control left-right separation and increase front-back discrimination if needed.

♥ Better together:

All pairs can be combined within (and without) immersive workflows when phase-coherent transcoding and rotation are used, as provided by Audio Brewers plugins.

Some additional forward gain can be obtained with each added pair of capsules, especially for LF.

As usual with multiple microphone pairs, an important caveat: SRA doesn't tell the whole story. On-location monitoring and adjustment of distances and angles might be warranted.

Nevertheless, values for coherent SRA seem to work very well, therefore the default configuration of the array can be useful when time or space for optimized positioning are unavailable.

Realistic immersive results are possible with modest processing. Suggested for Dolby Atmos:

inner pair to L and R,
outer pair -12dB to L, R, Lm and Rm ,
transcoded signals from the FOAs to all corresponding channels, except -12dB or more attenuation on L, C, R, Lm and Rm.

Dolby folddown to stereo works very well, realism is surprisingly preserved despite the limitations of a sigle pair loudspeakers. The binaural-like cues from the Jecklin effect also provide an impressive experience on headpphones. Personalized binaural processing becomes optional (but is of course needed for head-tracking).

Furthermore, thanks to a significant portion of the signal originatating from Ambisonics, the result is very mono-compatible, unlike traditional binaural, beacuse lateral components are cancelled out.

Alternatively, Jecklin-affected stereo is available by using capsules 5 and 12 and adding the low-passed sum of the other capsules from each FOA for left and right respectively. This avoids aliasing from the inter-capsule distance and can be useful when low latency, reduced visibility or increased portability are needed, and multichannel immersive is not.

Notes:
* The “Jecklin effect” is the result of the Optimum Stereo Signal, a binaural-like acoustic process accomplished with a round vertical baffle between matched capsules. OSS delivers spatial cues from the 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 with Ambisonics arrays panned to each side within a multichannel workflow, where spatial cues from the Jecklin effect are presented to the listener somewhat separately for each ear, similar to the way OSS works over traditional stereo loudspeakers.

*** 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. The V stands for “variable” to portray variability through Ambisonics.

Angle and distance diagrams for SRA prediction 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, 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.