The OVJ (Omni Variable Jecklin) microphone array

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) by Jürg Jecklin*. 

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

Description: Two Ambisonics subarrays are centered over each side of a vertical baffle known as a “Jecklin Disk” and stereo sets of small diaphragm condenser microphones are placed further out. 

This allows for hyper-realistic and versatile unobtrusive location recording from a single stand. Spot microphones are often not required thanks to beam-forming or “virtual spots”, but they can also be used if they are time-aligned to the array.

How each pair works:

• The Ambisonics microphones deliver immersive highly Jecklin-affected* bilateral** signals. 
  
  

  We prefer to use the two FOAs in a “modified for symmetry” layout with an edge-to-edge spacing of 24cm, where all front-facing capsules are aligned with the rest of the array.

  Why 24cm? After extensive testing, we empirically determined an SRA (Stereo Recording Angle) of ∽81° for the Jecklin effect. We therefore propose that any combination of microphones to be used together with a Jecklin disk should target that same SRA to avoid spatial smearing. Since the cardioid capsules on each side of tetrahedral FOAs are angled at 109.5°, we rotate and position them so the left-pointing capsule on the left FOA (5) and the right-pointing capsule on the right FOA (12) are 24cm apart and parallel to the horizon, instead of the conventional skewed layout. The resulting B-format signals are rotated in software to compensate. 

  
  

• 
  The middle pair (1, 2) adheres to traditional Jecklin OSS specs, with an inter-microphone distance of 36cm.

Omnidirectional capsules are preferred. 

 

Cardioids can also be used at 70.05° delivering a similar SRA with a stronger intensity component than standard OSS. We use Schoeps MK5 which can be quickly switched among both patterns.

 

• 
  The outermost pair (3, 4) spaced accordingly 79cm provides additional spatial cues and some front-back differentiation when using APEs.

Again, omnis are preferred but cardioids at 0° can be used if conditions are sub-optimal.

 

Preliminary findings:

All pairs can be combined within (and without) immersive workflows when phase-coherent transcoding, rotation and beam-forming are used for the Ambisonics arrays, as provided by Audio Brewers plugins within DAWs like Logic Pro. 

Additional forward gain can be obtained with each added pair of capsules, especially for LF. An interesting mixing tactic is to use auxiliary busses to add LF from all capsules on each side of the baffle to L and R, with LPF to mitigate artifacts from inter-capsule distances, a common issue with OCCO-like arrays.

Hyper-realistic immersive results are available with modest processing. Suggested for Dolby Atmos:

• Audio Brewers Advanced Decoder output from the Ambisonics arrays to all corresponding channels, -3 dB or lower to C from each side.

• Middle pair mostly to L and R replacing or mixed with the Ambisonics LR feeds – or not  depending on the type of microphones, content and desired amount of mono-compatibility. This is an important aesthetic decision, i.e.: we sometimes prefer more of the Schoeps for ensembles but more of the FOAs for vocal solos or duos.

• Outer pair mostly panned to Lss and Rss, while sending some to the front LR, Lrs and Rrs can add some openness.

Dolby fold-down to stereo works very well. The semi-binaural cues from the Jecklin effect survive transcoding and deliver an impressive sense of “being there”, realism is surprisingly preserved despite the limitations of a single pair of loudspeakers.

As expected from the Jecklin effect, the experience on headphones is similar to binaural, to such a degree that some might consider binaural conversion optional, but it can still be desirable for personalization and of course required for head-tracking.

Excellent mono-compatibility, especially when a significant portion of the signal originates from Ambisonics. Lateral components mostly cancel out, unlike traditional binaural which falls apart when summed to mono.

Pending:

Scientifically measuring the SRA of OSS, and/or the factors that determine it, in order to work at other (better?) SRA targets for inter-microphone distances and angles,

Higher-order Ambisonics microphones,

Ambisonics also on the outer pair.

Combining OSS with other microphone techniques, such as double-MS or native “Horizontal-B”.

Adding delay to decorrelate front to back.

Notes:

* The “Jecklin effect” is originally known as Optimum Stereo Signal: a binaural-like acoustic process accomplished with a circular vertical baffle between matched capsules. This delivers spatial cues from the varying intensity and frequency response depending on incidence angles, as well as time precedence caused by inter-capsule distance. The ideal diameter for the baffle is 35cm. Unlike dummy-head or current software-based binaural, this invention by Jürg Jecklin is perfectly compatible with stereo loudspeakers.

** We do not claim to be using by-the-book Bilateral Ambisonics. Ours is a simplified process with FOAs panned to either side within an Atmos or Eclipsa mix, where spatial cues from the Jecklin effect are presented to the listener somewhat separately for each ear, similar to the way traditional OSS works on standard stereo loudspeakers.

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 controlled by mixing in more of the inner or outer pairs.

Angle and distance diagrams come from the Nuemann Recording Tools app.

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: Bradford Richards, Brian Peters, Christian Amonson, Eduardo Monteiro, Eric Weber, Hudson Fair, Ian Wood, Jack Reynolds, Jake Purches, Joel Rec, Julian J. Ludwig, Lucas Guitink, Manfred Schmidt, Mario Vila, Paal Rasmussen, Paul Fee, Stefan Kießling and many others from the CMLR group on Facebook, and especially to Alejandro Cabrera, Carlos Fernández, Diego Rodríguez B., Don Booth, Esteban Zabala I., Gricelda Duarte, Helmuth Reichel-Silva, 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.