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Audio Engineering Society
Convention Paper

Contrasting ITU 5.1 and Panor-ambiophonic 4.1 Surround Sound Recording Using OCT and Sphere Microphones
Robert E. (Robin) Miller III ©20021

1 FilmakerStudios, Bethlehem, Pennsylvania 18018, USA

Presented at the 112th Convention
2002 May 10-13 Munich, Germany

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.

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For the AES 19th International Conference on Surround Sound in Bavaria in June 2001, the author designed experiments and stereo demonstrations comparing Ambiophonic (front stage only), INA/MMA [7, 8], and OCT Optimized Cardioid Triangle [6, 9, 10, 11] of orchestra, brass quintet, and 180° "Walkabout" localization test made with an Ambiophone prototype made by the author. At the conference held at Schloss Elmau, attendees in the Ambiophonic demonstration room were able to hear recreated the nearly 120° stage width of the brass quintet session, with or without Ambisonically convolved ambience surround [12]. To demonstrate compatibility with 5.1, these recordings were also played for a large audience in the Grosser Saal (Great Hall - theater) using five-channel cinema speaker layout, and in autos with 5.1 and Logic Seven.

For the AES 111th Convention in New York City and this paper, the work was expanded to PanorAmbiophonic 360° reproduction with simultaneously made recordings of opera, guitar quintet with audience, marching bands, and a 360° Walkabout test [13]. Program material was chosen to represent a variety of musical genres in concert hall, studio, and outdoor acoustics. Recordings were demonstrated during Tech Tour 8 at the Ambiophonics Institute on both PanAmbio 4.0 and ITU 5.0 systems, along with a prototype automobile PanAmbio system.

Design and calibration of the microphones and their baffles are more critical for proper surround localization because the results can be discriminated. In the authorís forty years of professional experience with many microphone approaches, OCT and sphere derivatives offer both good localization and spatial impression -- in the past more typically an either/or choice -- plus the envelopment of surround sound. For ITU 5.1, OCT uses the directional characteristics of cardioid and supercardioid microphones to image front stage sources with unambiguous phantoms among three loudspeakers (Figures 7a, b).

Fig. 7. Contribution of idealized OCT microphones to total energy -- a) polar plot every 10° (front at bottom); b) rectangular plot. [back]

If more than the 6dB rejection of back sounds is needed, a baffle can add 7dB. Hall sound is added to LS, RS using back-facing cardioids, or to L, R, LS, & RS using a surround reverb convolver or four-channel room microphone such as side-facing figure-8s [6, 14]. 

As the basis for Ambiophonic (front only) recording, where ambience is convolved from hall impulse responses for two-channel recordings, the sphere microphone [15] - a frequency-dependent analog of the human head without pinna -- is shown idealized for mid-frequencies (Figures 8a, b). When baffled, its stereo characteristics ideally show nearly 10dB of back rejection (Figures 9a, b). The authorís prototype Ambiophone, measured every 15° with filtered pink noise in a non-anechoic studio, approaches these ideal characteristics (Figures 10a, b).

Fig. 8. 
Contribution of idealized sphere microphone to total energy at 1kHz (frequency dependent) -- a) polar and b) rectangular plots.

Fig. 9.
Idealized Ambiophone front sphere at 1kHz -- a) polar response every 10° (front at bottom); b) rectangular response.

Fig. 10.
Measured response, front PanAmbio prototype microphone in non-anechoic studio -- a) polar plot every 15°; b) rectangular plot. Cf. previous figure. Back-angle irregularities ±90° to ±180° will be masked by on-axis response of back sphere in next figure.

Fig. 11
. Measured total response, prototype PanAmbio microphone, front + back spheres, is within ±1dB around the entire 360° -- a) polar response every 15° (front at bottom); b) rectangular response.

Combining two such spheres for PanAmbio reproduces the full 360° horizontal plane within ±1dB (Figures 11a, b). For the readerís experimental verification, simultaneous recordings using both OCT and the authorís prototype PanAmbiophone are available in evaluation DTS-encoded CDs, described later, along with an early consensus of subjective opinions of each.

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