The ORTF-3D recording method
An optimal ambience arrangement for eight channels is offered by the new "ORTF-3D" system (developed by Helmut Wittek and Günter Theile). It is more or less a doubling of the "ORTF Surround" system onto two planes, i.e. there are four supercardioids on each level (upper and lower), forming rectangles with 10 and 20 cm side lengths. The two "ORTF Surround" arrangements are placed directly on top of one another, see Fig. 6.
The microphones are furthermore tilted upward or downward in order to create signal separation in the vertical plane, see Fig. 5. Thus an 8-channel arrangement is formed, with imaging in the horizontal plane that somewhat corresponds to the "ORTF Surround" system. The microphone signals are discretely routed to four channels for the lower level (L, R, LS, RS), and four for the upper level (Lh, Rh, LSh and RSh). In VR applications, virtual loudspeaker positions forming an equal-sided cube are binauralized.
Fig. 4: A prototype of the ORTF-3D system at the ICSA conference in 2015. Eight supercardioids, horizontal distance 20 cm, vertical distance 0, angle 90º.
Imaging in the vertical dimension is produced by angling the microphones into 90-degree X/Y pairs of supercardioids. Such a two-channel coincident arrangement is possible due to the high directivity of the supercardioids, and the imaging quality and diffuse-field decorrelation are both good. An even better decorrelation could be created by spacing the microphone pair. However, as found be Hyun-Kook Lee (Uni Huddersfield) the decorrelation in the diffuse field is less relevant/audible in the vertical domain.
Fig. 5: Orientation of the capsules: one vertical X/Y microphone pair for each vertical pair of loudspeakers
This results in an eight-channel array with high signal separation, optimal diffuse-field correlation, and high stability within the listening area. All requirements are optimally fulfilled, yet the array is no larger than the compact ORTF Surround system-a decisive practical advantage. Numerous test recordings have shown that the ORTF-3D approach produces very beautiful, spatially open and stable 3D recordings.
Fig. 6: ORTF-3D windcsreen with the cover removed; view from below