A method loosely based on several established techniques for recording acoustic music and ambient sound, including:
- 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.
- The single-axis horizontally spaced, vertically coincident 4-microphone phased array known as OCCO*** popularized by Robert Sandy “Boojum” Noyes, Jim Norman and Tony Faulkner.
- STAAG, a multi-microphone setup designed by sound engineer and researcher Jamie Tagg which uses back-facing capsules to better capture ambience.
Description: Two Ambisonics subarrays are centered over each side of a round 35cm diameter baffle ∽25cm from each other, and stereo sets of microphones are placed further out at distances that produce a similar SRA (Stereo Recording Angle). Since empirical testing and calculations by Oren Levy have shown an SRA of 82° for the Jecklin effect, distances and angles default to those which deliver that same SRA. The name OVJ is short for Omni Omni Variable Jecklin.
How each pair works:
- The inner pair of Ambisonics microphones (capsules 5 to12) deliver Jecklin-affected* bilateral** signals.
Why 25cm? Since the capsules on each side of tetrahedral FOAs (Fist-Order Ambisonics) arrays are angled 109.5°, two arrays can be set to the end-fire position and rotated 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. At this distance and angle, said capsules on their own would generate a stereo signal with the same SRA as observed with OSSv2. This can be considered a good starting point for the most coherent capture when combining Ambisonics with other microphone pairs on the same bar.The resulting signals are correspondingly rotated in post once converted to Ambisonics B-Format. This method can be called “Modified for Symmetry First-Order Ambisonics” (MFS-FOA).
Why else do this? Even though in Ambisonics theory this should not be necessary, in practice the capsules probably don't exhibit textbook-like polar responses, therefore a more symmetrical layout should result in more symmetrical signals.
Conventional FOA versus MFS-FOA:
- The middle pair (1, 2) adheres to Jecklin OSSv2: omnidirectional capsules separated by about 36cm for hyper-realistic stereo.
Depending on acoustic conditions, cardioids angled ∽70.05° can be used for a similar SRA with a stronger intensity component and less ambiance.
Arguably the array could then be called OCVJ instead of OVJ.
Schoeps MK 5 are especially attractive for this application because they can be quickly switched among both polar patterns.
- The outermost pair (3, 4) separated by ∽78cm can be used for Jecklin effect mitigation if needed, while providing other spatial cues. Some front-back differentiation is also possible when using APEs for omni capsules, something the original Jecklin method notably lacks.
The OOVJ (of, for short. OVJ) array allows for unobtrusive hyper-realistic location recording from a single stand with abundant versatility when mixing.
With time-coherent processing of signals from the Ambisonics arrays and the other pairs, all of them can be used in a mix. As in OCCO, this can result in some comb filtering, which can require mitigations such as predominance by >9dB of either pair.
The array geometry allows for extra bass forward gain from the sum of all capsules on each side of the array with LPF applied to avoid comb filtering.
- Hyper-realistic multichannel results are available with modest processing. Suggested for Dolby Atmos within Apple Logic Pro:
The Audio Brewers Transcoder for converting from Ambisonics A to B format, the Audio Brewers Advanced Decoder for beam-formed signals corresponding to discrete Atmos channels. Panning each subarray >60% to each side results in very realistic immersion.
Alternatively, the Ambisonics pair can be used for surround and top channels while the middle pair (1, 2) or the outer pair (3, 4) predominate on frontal L and R.
As in STAAG, backfacing beams from Ambisonics can allow for less dependance on artificial processing and confer a more realistic sense of the space where the recording took place.
More findings:
- The Jecklin effect survives the fold-down from Atmos to stereo.
- The experience on headphones is similar to binaural, to an extent that a more natural result is possible by turning turn off frontal binaural rendering in the Dolby Atmos workflow, something which unfortunately does not work in the Apple binaural renderer at this time.
- Excellent mono-compatibility occurs when a significant portion of the signal originates from Ambisonics, unlike traditional binaural which falls horribly apart when summed to mono.
- If more portability is required or privileged over quality and options, the outer and/or inner pairs can be sacrificed and both mono, stereo and spatial signals can be obtain from the Ambisonics pairs on their own. In that case the array could be called VJ instead of OVJ.
- Baffle absorption is critical for vocals and similar sounds in which comb filtering due to reflections can be very noticeable.
Notes:
Tests took place using:
- 2x Rode / Soundfield NT-SF1 Ambisonics microphones,
- 2x Schoeps MK 5 microphones, usually in omnidirectional mode,
- 2x Earthworks QTC30 with custom 40mm APEs (Acoustic Pressure Equalizers), or 2x Immersive Soundscapes Earsight Thumb microphones.
- 35cm round Jecklin disk baffles made out of varying materials, including perforated metal, polyester fiber, Basotect, carbon fiber and acrylic.
* 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, 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.