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Audio Engineering Society
Convention Paper

Spatial Definition and the PanAmbiophone microphone array
for 2D surround & 3D fully periphonic recording

Robert E. (Robin) Miller III ©2004
FilmakerStudios, Bethlehem, Pennsylvania 18018, USA

Presented at the 117th Convention
2004 October 28-31 San Francisco, CA

This convention paper has been reproduced from the author's advance manuscript, without editing, corrections, or consideration by the Review Board. The AES takes no responsibility for the contents. Additional papers may be obtained by sending request and remittance to Audio Engineering Society, 60 East 42nd Street, New York, New York 10165-2520, USA; also see www.aes.org. All rights reserved. Reproduction of this paper, or any portion thereof, is not permitted without direct permission from the Journal of the Audio Engineering Society.  For a PDF version of this paper (0.5 MG), click here.

3. PANAMBIOPHONE – A MAIN MICROPHONE

A “main microphone” approach [27,28,29,22], the “PanAmbiophone” – the second part of this paper – is integral to the concept of high spatial definition. Its purpose was to:

  • Deliver 2D surround by direct recording (“PanAmbio”) and exhibit uncompromised accuracy of localization and tonality;
  • Offer compatibility with stereo and ITU 5.1 for cinema, auto sound, broadcast, home theater;
  • Simplify surround production/post-production for music, movies, VR, & training simulation using a main microphone approach that obviates the need for many spot microphones;
  • Form the basis for a 3D (with height) microphone and a multi-format-compatible system for lifelike reproduction (PerAmbio 3D).

The original PanAmbiophone design [24] married two sphere microphones, for HRTF qualities, front and back of an acoustic barrier – see Fig.4 & 5. The barrier was a sandwich of gypsum insulator between two absorbers each 2 inches (50mm) thick – and was big and heavy. Two persons were required to mount and raise the array on a substantial stand. Nevertheless, the results, using Schoeps microphone elements, were encouraging, as indicated by the subjective comments and performance curves in [24] and below. This experience evolved a new design intended for the more useful recording in acoustic spaces.

The newest PanAmbiophone, shown in Fig.9, weighs 10lb (5kg), and may be mounted on a stand or suspended. Approaching the “perfect omni,” the array may be positioned farther from sound sources than is conventional, beyond the critical radius, with results that seem more “present” than expected, obviating use of many spot microphones. Played on ordinary 5.1 home theater speakers, the results are ambient and tonally natural. Played using dual crosstalk-cancelled speaker pairs – PanAmbio as in Fig.2 – the measured results are “correct” localization in the horizontal plane. Played using PerAmbio 3D/2D speakers (see Appendix Fig.C), the results are subjectively highly lifelike, as described by auditioners at the AES 24th International Conference in Banff, Canada, June 2003 [21,30].

3.1. Directional head-shaped microphone array

The PanAmbiophone has evolved in prototype form over several generations, beginning with two sphere microphones [31] separated by an acoustical barrier, as shown in Fig.4 & 5. The original objectives [24] were:

  • Easy to use, HRTF-related main microphone
  • “Perfect omni” resp. 5Hz~30kHz around 360°
  • Directly records 360° surround (horizontal 2D)
  • Independently controllable front & back stages
  • Stable images around 360°; accurate localization
  • Lifelike tonality
  • Compatible with ITU 5.1 (center silent)
  • Basis for direct recording of 3D “full sphere”
  • Optional 3D ambience by convolution

Subjective testing [24] contrasted recordings played both on ITU 5.1 layouts and over dual stereo dipoles (PanAmbio 4.0) using crosstalk cancellation by DSP, illustrated in Fig.2. In PanAmbio, antiphonal sources, audience sounds, and reflections were heard imaging within ±5° except in the human “cone of confusion.”


Fig.2 illustrates Panor-Ambiophonic (“PanAmbio”) layout using two closely-spaced speaker pairs
for 360° horizontal (2D) surround reproduction for home theater music and movies.

The original design simulated four quasi-cardioid microphones, limited by the cutoff frequency of an acoustical barrier, as in Fig.3. Unlike real cardioid microphones, or the approach by Bruck combining omni and bi-directional elements, the frequency response using four omnis was flat to below 20Hz, along with excellent high frequency polar response around 360°.



Fig.3 – Model of the original PanAmbiophone – four ideal cardioids tangent to a head-shaped baffle.

Each pair of microphones was head-spaced at the surface of an acoustically rigid sphere. Simultaneous recordings were made for comparison using OCT [22] for 5.1 reproduction and Ambisonics for a total of 12 microphones, shown (with barrier removed) in Fig.5.



Fig.4 – The first PanAmbiophone with OCT for comparison experiment.
Barrier hides second sphere for rear stage.

Fig.5 - Original PanAmbiophone using two spheres (barrier removed),
OCT, and Soundfield microphones for comparison.

The directionality of each pair, front and back, measured with band-limited pink noise, demonstrated side-to-side separation typical of the sphere microphone – approx. 10dB. The baffle also created a front pair to back pair separation of approx. 10dB. However, compared to real directional (pressure gradient) microphones, the response was again flat to lowest frequencies and remained directional to the limit of the size of the baffle. The baffle was scaled consistent with HRTF cutoff of approx. 700Hz.

Directional measurements are plotted in Fig.6 & Fig.7. With axes of the microphone elements at ±90° directly left and right, the high frequency polar response over all 360° was more truly omni-directional, making more distant positioning possible. Still more “perfect omni” polar high frequency response would be realized in a new, directional PanAmbiophone, described in section 4.0 below.



Fig.6 - Plots side-to-side response to band-limited pink noise for one microphone pair (0° is down on polar graph).
Whether used front- or back-facing, the summed response (solid line) shows a polar characteristic that is flat ±1dB across each stage.

Fig.7 plots front-to-back response to band-limited pink noise (0° front is down on polar graph).
Summed surround response 360° (solid line) shows a polar characteristic that is flat ±1dB.

The PanAmbiophone’s benefits also are directly applicable for 2-channel stereo or Ambiophonic 2.0, whether mixed live or in post-production, or downmixed in the user’s player or receiver. The array is positioned by the recording engineer for optimum recording angle and imaging the front stage. The rear staged is then mixed in for balancing stage sound with ambience, typically at a level lower by 1½ to 3 dB relative to multi-channel use.

In a comparison test with 5.1 and stereo, in Fig.8, speech signals were recorded at 15° intervals around 360°. Except near the human cone of confusion, localization results for PanAmbio were consistent ±5°. This accuracy implies: 1) preservation of HRTF-based cues; 2) front-back discrimination that is controllable and allows more distant (natural) pickup; and 3) preservation of arrival direction necessary for correct timbre for each listener, using their own HRTF including pinna. The result using the PanAmbiophone, therefore, is not just raw, artificial spatialization, but naturally high spatial definition and lifelike reproduction.



Fig.8 – Plot of informal tests for localization of speech signals at 15° intervals around 360° comparing PanAmbio, 5.1, & stereo.
The PanAmbiophone replay consistently localized within ±5°.

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