A method 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 FOA 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 36cm and 79cm from each other.
This allows for unobtrusive hyper-realistic location recording from a single stand.
How each pair works:
- The inner pair of Ambisonics microphones (capsules 5, 6, 7, 8 and 9, 10, 11 and 12) deliver Jecklin-affected* bilateral** signals.
Why 25cm? Empirical testing has shown an SRA (Stereo Recording Angle) of ∽81° for the Jecklin effect. Under OVJ the combination of microphones used together with a Jecklin-type baffle should then preferably target that same SRA to avoid positional smearing, such as 70.05° separated by 36cm and 0° separated by 79cm.
Since the capsules on each side of tetrahedral FOAs are angled at 109.5°, they can be 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, instead of the conventional skewed layout. The resulting B-format signals are correspondingly rotated. This method can be called “modified for symmetry” (MFS).
- The middle pair (1, 2) adheres to Jecklin OSS: omnidirectional capsules separated by 36cm for hyper-realistic stereo.
Depending on acoustics, cardioids at 70.05° can be used for a similar SRA with a stronger intensity component and less ambiance.
- The outermost pair (3, 4) spaced at 79cm can provide additional spatial cues, forward gain and some front-back differentiation when using APEs.
This pic shows the OVJ array being used for an opera recording.
Main findings:
All pairs can be combined when time-coherent processing is used for the Ambisonics arrays, such as that that provided by Audio Brewers plugins, and the DAW correctly matches phase among all channels. As in OCCO, comb filtering can present itself at equal levels, so this can requiere the traditional mitigations.
Bass forward gain can be obtained by applying LPF to the sum of all capsules on each side of the array, this takes advantage of the whole array while avoiding comb filtering.
- Hyper-realistic multichannel results are available with modest processing. Suggested for Dolby Atmos within Apple Logic Pro:
The Audio Brewers Transcoder makes the most of the FOAs by performing high-quality conversion from A to B format, and the Audio Brewers Advanced Decoder uses internal upscaling to effectively evade shortcomings of DAWs which lack high-order mixing and outputs beam-formed signals corresponding to all discrete Atmos channels. Panning the output of the Ambisonics arrays ∽60% to each side results in realistic immersion.
Alternatively, Ambisonics can be used exclusively for surround and top channels while the middle pair (1, 2) can be fed directly to frontal L and R, depending on the type of microphones, content and desired amount of mono-compatibility. This is an important technical and aesthetic decision.
As in OCCO, the outer pair (3, 4) can be mixed in to add “openness”, can be used to mitigate effects of the baffle if needed, or even used predominantly for L and R, it's all up to the mixing criteria.
Reverberation based on backfacing beams or dominance can allow for less dependance on artificial processing and confer a more realistic sense of the space where the recording took place.
More findings:
- Dolby to stereo works very well. The spatial cues from the Jecklin effect survive transcoding and Atmos processing, delivering an increased impression of “being there” despite the limitations of a single pair of loudspeakers.
- The experience on headphones is similar to binaural, to an extent that some might consider binaural conversion optional when head-tracking is not required.
- Excellent mono-compatibility when a significant portion of the signal originates from Ambisonics, unlike traditional binaural which falls horribly apart when summed to mono.
- Baffle absorption is critical for vocals and similar sounds in which comb filtering can be very noticeable.
- Baffle mass seems important for the Jecklin effect. It is possible that a baffle which is is too light can act as a passive radiator, which would defeat shadowing for mid-low frequencies.
Notes:
Tests took place using:
- 2x Rode NT-SF1 Ambisonics microphones,
- 2x Schoeps MK5 microphones, usually in omnidirectional mode,
- 2x Earthworks QTC30 omnidireccional microphones with custom 40mm APEs (Acoustic Pressure Equalizers).
- 35cm round Jecklin disk baffle with polyester fiber and Basotect absortive materials.
* The “Jecklin effect” also known as Optimum Stereo Signal is 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 perfectly compatible with stereo loudspeakers.** OVJ is not by-the-book Bilateral Ambisonics, but a simplified process with FOA arrays 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 OSS works on standard stereo loudspeakers.*** Similarly, OVJ inherits from traditional OCCO the concept of the horizontally spaced, vertically coincident 4-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 come from the Neumann Recording Tools app.
This content 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, 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.