The OVJ array is loosely based on several proven and some recent techniques for recording acoustic music and ambient sound, including:
The horizontally spaced and vertically coincident 4-microphone array known as "OCCO," popularized by Boojum, J. Norman, and T. Faulkner.
OSS (Optimal Stereo Signal / "Jecklin Disk"), by Jürg Jecklin*.
Aurally related symmetrical ambisonics, as suggested by Alper Güngörmüsler and Andrew Allen, and bilateral ambisonics, suggested by Zamir Ben-Hur, David Alon, Or Berebi, Ravish Mehra, and Boaz Rafaely.
The array is built on a horizontal bar, intersected by the Jecklin baffle. Two tetrahedral FOA (First Order Ambisonics) subarrays are added between the central and outer pairs of non-FOA microphones.
Details:
The innermost capsules (1, 2) are highly subjected to the Jecklin effect*, providing spatial cues based on time, intensity and frequency response using omnidirectional rather than cardioid capsules, while retaining the advantages of semi-coincidence as in OCCO. Cardioids can also be used if needed, which results in a more subtle Jecklin effect.
To take advantage of the natural OSS SRA of ∽81°, certain distances and angles are preferred. This same SRA occurs with cardioid capsules angled 90° at a separation of 30 cm (NOS) and for omni capsules at a separation of 79 cm, thus those are the suggested starting points for the innermost (especially if cardioids are preferred) and outermost pairs.
In a fortunate coincidence, the angle for parallel capsules within a tetrahedral FOA array is 70.5°, which happens to match the capsule angle for an SRA of 81° at a separation of 36 cm. Therefore, the two FOAs are placed at a distance and rolled at an angle such that the left pointing capsule on the left array and the right pointing capsule on the right array are 36 cm apart. We can call this the double-MFS** format.
Results:
Somewhat intense OSS binaural from capsules 1 and 2, probably because of the size of the baffle (as in OSS v2) and close distance (as in OSS v1).
OCCO-like stereo with Jecklin effect from capsules 1+3 and 2+4. additionally, capsules 1+3+5 and 2+4+12 can provide more "reach" (forward gain) when summed.
Multi-channel immersive Jecklin-affected signals in multiple formats from the FOAs.
Outputs are time-aligned in and out of Ambisonics workflows when using phase-coherent processing, such as that provided by Audio Brewers plugins within latency-compensated DAWs.
Alternatively, Jecklin-affected stereo-compatible binaural is also available by low passing the sum of all capsules on each FOA array separately for left and right, and then adding high-passed capsules 5 and 12 respectively for "focused" HF without aliasing from the inter-capsule distance.
Additional virtual microphones can be extracted from FOAs and added to the stereo mix through dominance processing or with software such as Harpex-X or Zilia Beamformer for more aesthetic control.
Preliminary listening tests in stereo and binaural suggest the OVJ array adds cues which enhance the Jecklin effect by providing some discernible position differentiation beyond the usual OSS ∽81° and also some front-to-back discrimination, especially when using APEs on the outermost capsules.
Notes:
* The "Jecklin effect" is a binaural acoustic process achieved with a baffle between matched capsules. Unlike dummy-head or current software-based binaural processes, the result is highly compatible with stereo and often quite usable in mono.
** MFS stands for "modified for symmetry": FOA microphones and the resulting B-format signals are rotated 45°.
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 mitigated by using more of the outer instead of the inner pairs if so desired.
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/
Also
available in Spanish.
