Schoeps ORTF-3D Outdoor Set Ambisonic Microphone System

The ORTF-3D Outdoor Set is a complete set including windshield with suspension and fur, 8 microphones, two multicore cables, two breakout cables; additional rain protection available as an option. The ORTF-3D was developed aiming at optimal imaging characteristics, similar to those of the ORTF Stereo and ORTF Surround setups. It features very good 360 degree imaging with a pleasant, natural-sounding spatial impression and a large listening area.

The ORTF-3D setup uses 8 supercardioid studio quality microphones: 4 * CCM 41 + 4 * CCM 41V. Applications include: virtual reality audio capture, immerisive audio, ambience for sports events, film sound, etc.

Features:

• Outdoor plug and play solution for 3D Ambience
• Wind and weatherproof with internal heating
• 8 discrete recording channels for L, R, Ls, Rs and Lh, Rh, LSh, RSh. If a Center channel is needed, add the downmix L+R at a low level.

Built for 3D Audio

The new approaches included in “3D Audio” reproduce sound from all spatial directions. This includes the Dolby Atmos and Auro3D stereophonic systems; binaural / virtual reality (“VR”) systems; and soundfield synthesis approaches such as Ambisonics and wavefield synthesis systems. 3D Audio can give distinctly better spatial perceptions than 5.1. Not only is the elevation of sound sources reproduced, but noticeable improvements can also be achieved with regard to envelopment, naturalness, and accuracy of tone color. The listening area can also be greater; listeners can move more freely within the playback room without hearing the image collapse into the nearest loudspeaker.

Recording engineers who work with 3D sound face a difficult task when choosing a suitable recording technique. The number of channels is greater than with playback systems that operate only in the horizontal plane, so the complexity increases as well.

When a customer demands 3D audio rather than conventional 5.1 surround it may be tempting to apply solutions that are overly simple. But when a 3D recording has been made well, using a suitable recording technique, the advantages are impressively audible.

One recording method for all 3D formats?

There are various 3D audio playback systems, so the recording techniques that work best for each of them will naturally be different. For soundfield synthesis systems, multichannel microphone arrays can be a solution, while for 3D stereo, stereophonic miking techniques are the norm.

For binaural reproduction in the simplest case, a dummy head can be used.

But all these systems share one requirement when recording complex, sustained sound sources such as ambient sound: Stereophonic techniques must be used, because they alone offer both highquality sound and high channel efficiency. It is impossible or inefficient to reproduce in high quality the sound of a large chorus, for example, or the complex, ambient sound of a city street, by compiling single point sources recorded with separate microphones.

In the same way, multichannel microphone arrays for soundfield synthesis, such as higher-order Ambisonics (“HOA”) or wavefield synthesis, fall short in practice because their channel efficiency and sonic quality are too low. If on the other hand the number of channels is reduced, e.g. with firstorder Ambisonics, the spatial quality becomes burdened with compromise.

For binaural playback, the dummy head technique is clearly the simplest solution-but it does not, in itself, produce results compatible with virtual reality glasses, in which the binaural signals must respond to user’s head motions. This is possible only through the “binauralization” of a stereophonic array-a technique that is already well established in audio for games.

Is first-order Ambisonics adequate for 3D?

There is a common assumption that Ambisonics would be the method of choice for 3D and VR. The professional recording engineer would do well to examine the situation more closely.

Ambisonics, which has existed for a long time by now, is a technology for representing and reproducing the sound field at a given point. But just as with wavefield synthesis, it functions only at a certain spatial resolution or “order”. For this reason, we generally distinguish today between “first-order” Ambisonics and “higher-order” Ambisonics (“HOA”).

First-order Ambisonics cannot achieve error-free audio reproduction, since the mathematics on which it is based are valid only for a listening space the size of a tennis ball. Thus the laws of stereophony apply here-a microphone for first-order Ambisonics is nothing other than a coincident microphone with the well-known advantages (simplicity; small number of recording channels; flexibility) and disadvantages (very wide, imprecise phantom sound sources; deficient spatial quality) of that approach in general.

Creation of a Ambisonics studio microphone with high spatial resolution is an unsolved problem so far. Existing Ambisonics studio microphones are all first-order, so their resolution is just adequate for 5.1 surround but too low for 3D audio. This becomes evident in their low inter-channel signal separation as well as the insufficient quality of their reproduced spatiality.

The original first-order Ambisonics microphone was the Soundfield microphone, built the same way as for example the Tetramic or the new Sennheiser VR microphone. The Schoeps “Double M/S System” works in similar fashion, but without the height channel.

Ambisonics is very well suited as a storage format for all kinds of spatial signals, but again, only if the order is high enough. A storage format with only four channels (first-order Ambisonics calls them W, X, Y, Z) makes a soup out of any 3D recording, since the mixdown to four channels destroys the signal separation of the 3D setup.

Ambisonics offers a simple, flexible storage and recording format for interactive 360° videos, e.g. on YouTube. In order to rotate the perspective, only the values of the Ambisonics variables need be adjusted. Together with the previously mentioned small first-order Ambisonics microphones, 360° videos are very easily made using small, portable cameras.

For virtual reality the situation is different, however. The acoustical background signal of a scene is generally produced by “binauralizing” the output of a virtual loudspeaker setup, e.g. a cube-shaped arrangement of eight virtual loudspeakers. The signals for this setup are static; turning one’s head should not cause the room to spin. Instead, head tracking causes the corresponding HRTFs to be dynamically exchanged, just as with any other audio object in the VR scene.

As a result, most of the advantages of first-order Ambisonics do not come into play in VR. On the contrary, its disadvantages (poor spatial quality, crosstalk among virtual loudspeaker signals) only become more prominent.

If practical conditions allow for a slightly larger microphone arrangement, an ORTF-3D setup would be optimal instead as an ambience microphone for VR.