The Omni Variable Jecklin Microphone Array v2

OVJ v2 array as seen from the back

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.

General description: Two FOA (First Order Ambisonics) subarrays are centered over each side of a “Jecklin Disk” baffle and rotated for symmetry**, and two matched stereo sets of SDCs are placed further out.

How each pair works:

The main or middle pair (1, 2) with 36cm between capsules, provides spatial cues from incidence angle dependant intensity and frequency response caused by the baffle, as well as time precedence caused by inter-capsule distance. This pair is similar to the latest version of Jecklin OSS, sometimes known as OSS 2.

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 other inter-capsule distances and angles in the array.

Omni capsules are preffered. Cardioids can also be used at 70.05° delivering a similar SRA with a slightly stronger intensity component than Jecklin OSS 2.

In either case, mixing with outer pairs can smooth the Jecklin effect if too strong, whereas mixing with output from the FOAs can increase the effect.

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

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.

Two Ambisonic arrays deliver immersive Jecklin-affected signals and beamformed virtual spots:

When using FOA, we position the arrays so the left-pointing capsule on the left (5) and the right-pointing capsule on the right (12) are 25cm apart. Higher-order Ambisonics may be used, in which case phsyical rotation might be unnecesarry.

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

Automatic equalization such as Acon Digital DeFilter can compensate for comb filtering 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.

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. This is important because there are things you can't fix in the mix.

Nevertheless, using values for coherent SRA seems to work very well, and this finding allows the default configuration of the array to be very useful for recordings when time or space for optimized positioning are unuvailable.

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

• main/middle 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 on L, C, R, Lm and Rm.

Dolby folddown to stereo works very well, realism is surprisingly preserved despite the limitations of just two loudspeakers. The semi-binaural cues from the Jecklin effect also provide an impressive experience on headpphones. Personalized binaural processing becomes optional (but is desirable and 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.

Alternatively, when using FOAs Jecklin-affected stereo is available by using capsules 5 and 12 and adding the low-passed sum of the other capsules from each array 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 a binaural-like acoustic process accomplished with a baffle between (usualy omnidirectional) matched capsules. The ideal diameter for the baflle is 35cm. Unlike dummy-head or current software-based binaural, this process invented by Jürg Jecklin is perfectly compatible with stereo loudspeaker playback.

** MFS stands for “modified for symmetry”. FOA microphones and the resulting B-format signals are rotated, resulting in a symmetrical physical layout in the left-right (front) axis instead of the conventional skewed layout… probably a good thing since human ears are usually at the same height. What were Gerzon and Craven thinking?

*** We do not claim to be using by-the-book Bilateral Ambisonics. Ours is a simplified process with FOAs panned to either side of the “bed” in an Atmos workflow, so spatial cues are presented to the listener mostly seperately for each ear, thus enhancing realism in combination with the Jecklin effect.

The OVJ name is inspired by Christian Amonson and his outside-in nomenclature. The V stands for “variable”, both to portray variability through Ambisonics and because the Jecklin effect can be heightened or softened by mixing in more of the inner or outer pairs respectively.

This content by @ignace / Ignacio Rodríguez de Rementería is open source licensed under CC BY-NC 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: Alejandro Cabrera, Bradford Richards, Brian Peters, Christian Amonson, Eduardo Monteiro, Eric Weber, Hudson Fair, Jack Reynolds, Jake Purches, Julian J. Ludwig, Lucas Guitink, Manfred Schmidt, Mario Vila, Paal Rasmussen, Paul Fee, Stefan Kießling and many others from the CMLR group on Facebook, Carlos Fernández, Claudio Quiñones, Diego Rodríguez B., Don Booth, Esteban Zabala I., Gricelda Duarte, Ian Wood, Jaime Valbuena M., Joaquín Luppi, Jorge Sacaan M., Juan P. Quezada, J. Alberto Palacios, Julio Figueroa M., Leonard Moskowitz, Pablo Saavedra 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.

Also available in Spanish.