The OVJ (Omni Variable Jecklin) microphone array

Loosely based on several established techniques for recording acoustic music and ambient sound, including:

• The binaural-like stereo-compatible OSS (Optimal Stereo Signal)* by Jürg Jecklin.

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

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

Description: Two Ambisonics subarrays are centered over each side of a baffle, and stereo sets of small diaphragm condenser microphones are placed further out. 

This allows for unobtrusive hyper-realistic location recording from a single stand. Spot microphones are often not required thanks to beam-forming or “virtual spots”.

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 25cm, where all front-facing capsules are aligned with the rest of the array.

  Why 25cm? We empirically determined through testing an SRA (Stereo Recording Angle) of ∽81° for the Jecklin effect. We therefore believe any combination of microphones used together with a Jecklin-type baffle should preferrably target that SRA to avoid positional smearing. Since the capsules on each side of tetrahedral FOAs are angled at 109.5°, we rotate the FOAs so the left-pointing capsule on the left (5) and the right-pointing capsule on the right (12) are 25cm apart and parallel to the horizon, instead of the conventional skewed layout. The resulting B-format signals are correspondingly rotated in software.

  Arguably, the position of each FOA should be determined considering the central point between all capsules as reference, but we prefer to use the outer capsules as references for the benefit of directly usable outputs from them for real-time stereo monitoring or broadcasting, within the desired SRA.

  
  

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

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) is accordingly spaced 79cm and provides additional spatial cues, forward gain 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 immersive workflows when phase-coherent transcoding, rotation and beam-forming are used for the Ambisonics arrays, as provided by Audio Brewers plugins.

LF forward gain can be obtained with each added pair of capsules. An interesting tactic is to use auxiliary busses to add LF from all capsules on each side of the baffle to L and R. 

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

• Audio Brewers Advanced Decoder output from the Ambisonics arrays to all corresponding channels. The internal upscaling of this decoder effectively evades shortcomings of DAWs which lack high-order mixing.

• Middle pair mostly to L and R 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 technical and aesthetic decision, i.e.: we sometimes prefer more of the Schoeps for ensembles but more of the FOAs for vocal solos.

• The outer pair can add some “openness” while mitigating effects of the baffle if needed when summed to LR, Lss and Rss.
  


• Immersive reverberation based on the Ambisonic channels allows for less dependance on artificial processing.

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

The stereo experience on headphones is similar to binaural, to an extent 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 horribly 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.

Replacing or mixing in W signal from high-pedigree omnis to mitigate shortcomings of low-priced FOAs.

Higher-order Ambisonics microphones and processing.

Ambisonics also on the outer pair.

Replacing the FOAs for double-MS or native “Horizontal-B” microphones.

Adding delay to surround channels in order to de-correlate front to back.

Notes:

* The “Jecklin effect” also known as Optimum Stereo Signal is a binaural-like acoustic process accomplished with a 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. Unlike dummy-head or current software-based binaural, this method is perfectly compatible with stereo loudspeakers. Although the baffled recording method is often attributed to Jürg Jecklin it was actually invented by Alan Blumlein in the 1930s, however Jecklin researched multiple sizes and distance combinations and came up with a winner: 36cm inter-capsule and a 35cm diameter round baffle.

** We do not claim to be using by-the-book Bilateral Ambisonics. Ours is currently a simplified process with FOAs panned to either side within an Atmos workflow, 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.

*** Similarly, we inherit from traditional OCCO the concept of the horizontally spaced, vertically coincident 4-microphone phased array, but not the common 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 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, John Cone, Julian J. Ludwig, Lucas Guitink, Manfred Schmidt, Mario Vila, 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, 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.