canal.cl / grabación: The Omni Variable Jecklin Microphone Array

Loosely based on several proven and some recent techniques for recording acoustic music and ambient sound, including:

The horizontally spaced, vertically coincident 4-microphone phased array known as OCCO, created / 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 in MFS format** 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 30cm between capsule provides intense spatial cues based mostly on direction-dependant intensity and frequency response caused by the baffle, as well as some time precedence.

We have empirically determined an SRA (stereo recording angle) of aproximately ∽81°.

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 it is 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 also be used if the acoustic conditions are sub-optimal.

This traditional AB-like pair can add 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:

By lucky coincidence, the angle for parallel capsules within a tetrahedral FOA array is 70.5°, matching the angle for an SRA of 81° at 36cm, which just happens to be the recommended larger separation for later-stage evolved Jecklin OSS.

We can take advantage of this by positioning the FOA arrays so the left-pointing capsule on the left (5) and the right-pointing capsule on the right (12) are 36cm apart, what we call the “double-MFS” format.

Dominance processing or post-transcode panning can be used to control left-right seperation and increase front-back discrimination if needed.

♥ Better together:

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

Additional gain in the forward direction can be obtained with each added pair of capsules, especially for LF.

As usual with multiple microphone pairs, an important caveat applies: 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 just can't fix in the mix. Nevertheless, using values for coherent SRA seems to work well.

Very 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, with -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 of course needed for head-tracking).

Furthermore, thanks to a significant portion of the signal originatating from transcoded Ambisonics, the result is very mono-compatible, unlike traditional binaural.

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.

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.

** . FOA microphones and the resulting B-format signals are rotated 45°, resulting in a symmetrical layout in the left-right (front) axis… 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 actually using by-the-book Bilateral Ambisonics. Ours is a simplified process where we pan each FOA towards either side of the “bed” inside an Atmos workflow, so spatial cues are presented to the listener mostly seperately for each side. We have empirically found this to enhance realism in combination with the Jecklin effect.

The OVJ name is inspired by Christian Amonson and his outside-in nomenclature, with the V being used for “variable”, both to portray the variability introduced by Ambisonics and because the Jecklin effect can be heightened or softened by using 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, 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., 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.