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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.

Page 2

Derived from cinema, 5.1 localization is more precise than two-speaker stereo within the front 60° where it is best (and where trained listeners can localize sources on the order of 1°). The listening area is enlarged for sound from the center (front) speaker, benefiting, when utilized, cinema dialogue and music solos. A frontal speaker also preserves the proper tonal color compared to stereoís phantom "virtual images," especially onerous in the center due to ITD comb filtering of two identical but spaced sources and the pinna-determined source angle discrepancy. Yet while film mixers embrace this tool, some music recording engineers ignore the center channel for "artist" reasons.

For professional audio engineers who practice "surround without accompanying picture" such as music, 5.1 is an intentional compromise -- but some measure of it would suggest how acceptable the compromise is. A subjective comparison with a technology that is superior in one or more ways would be useful in that determination, leading to techniques that work within its compromises. One such approach is termed Panor-ambiophonics (Figure 3). Using two closely-spaced speaker pairs in front and back and requiring four monaural transmission channels (two stereo), "PanAmbio" - essentially two Ambiophonic systems - is in the authorís experience superior to 5.1 in accurate 360° localization, spatial impression, and envelopment with uncompromised frontal tone color (no comb filtering or pinna confusion). Bass management to redirect low frequencies from main channels, plus a ".1" LFE channel if used, are applicable to both ITU and PanAmbio, hence the 5.1 and 4.1 designations here -- although these refer more precisely to the number of transmission channels, not speakers. PanAmbioís disadvantages, aside from limited popular acceptance and not being recognized by a standard, are its need for crosstalk cancellation and that it works for only one or at most two listeners, not a group. Still, regarding 5.1ís qualities, PanorAmbiophonics is at least a benchmark of excellence, if not an alternative for high-quality music listening.


Fig. 3
. PanorAmbiophonic 4.1 (2/2) speaker placement turns stereo "inside out," creating accurate images outside pairs of transducers. It can serve as a benchmark of quality for, or alternative to, ITU 5.1. [back]

This paper describes experimental recordings to evaluate subjectively each of these reproduction systems in the light of the other. The objective will be to put each to its highest use. In addition, we will explore compatibilities for producing recordings that play well on both systems and a method of critical multi-channel recording level calibration. Applications are not limited to music only, but include music and natural sounds for film and broadcast.

PANOR-AMBIOPHONICS DESCRIBED

5.1 has been described elsewhere and is relatively well understood [2], so this paper will dwell more on Panorambiophonics. "PanAmbio" is two Ambiophonic systems, one for the front 180° and a second for the rear 180° as in Figure 3. (Note that Ambiophonics is not Ambisonics, a surround approach that uses coincident omni and figure-8s after Gerzon.) Each Ambiophonic system is two closely spaced speakers - an Ambiopole or stereo dipole - with crosstalk-cancellation provided by digital processing. Each Ambiopole more precisely reproduces recording angles up to 150° with reduced "angular distortion" [3, 4], which is characteristic of phantom images in stereo and 5.1, where instruments "relocate" toward one speaker or another when the listener is off the central plane. With one Ambiophonic dipole, instruments are localized more precisely, within ±5° where listening acoustics permit, failing due to pinna confusion (in this instance sounds intended for the sides coming from the front) only as they approach the extremes of a 180° wide stage. Contrast this single Ambiophonic system with conventional stereo, where all sounds are heard within 60°. Why and how Ambiophonics works -- even for many existing stereo recordings - is the subject of Glasgal's papers available in AES publications [5] and at www.ambiophonics.org. Discussed here are its uses, limitations, recording techniques, comparison to, and compatibility with 5.1.

For surround reproduction, a second speaker dipole is added in back, and full 360° PanAmbiophonic reproduction has been demonstrated by Ralph Glasgal and the author at the 111th AES Convention, December 2001. The experimental result is precise (±5°) localization of sources around 360° (Figure 4), virtually duplicating the recording layout, although with some coloration and soft focus of voices near ±90° directly left and right. While anomalies in these two side regions are within the cone of confusion of human hearing and might be considered negligible, a PanAmbio listener is able to turn his/her head to confirm direction and tone color, just as in normal living, so the author considers these anomalies near ±90° disadvantageous. PanAmbiophony works best when reproduced in a symmetrical, "dry" (cf. recording) acoustic and with speakers at less than the critical distance (room radius) of the listening environment. With four well-matched speakers and calibrated levels, the degree of precision possible can reveal subtle errors in recording -- so PanAmbio is useful also for monitoring.


Fig. 4
. Panor-Ambiophonic 4.1 (2/2) reproduction localizes sources accurately within ±5°, virtually duplicating the recording session directions above. A guitar quintet and fans are placed as shown for experiments that contrasts two 360° reproduction methods. Multichannel surround sound is more "realistic" by localizing both front stage instruments and sounds from around and behind, including antiphonal voices, audience participation, and ambience. [back]

In contrast, the ITU 5.1 (3/2) studio and home theater speaker layout "redirects" sounds as in Figure 5, but accommodates audiences of more than two, as the "sweet area" for "important" sounds from the center speaker is increased to 1.5m2 [6]. Both ITU 3/2 and PanAmbio 2/2 remove the 60° confines of two-speaker stereo (Figure 6). However, PanAmbio offers a discriminating listener, or at most two, superior realism for critical music appreciation at home or automobile.


Fig. 5. Contrasted with PanAmbio, ITU 5.1 "relocates" quintet+fans by angular distortion (although less than two-speaker stereo). Original angles indicate sounds recorded at ±75° are heard at ±30° and are superimposed within the band. If precision localization is not essential to a recording, ITU 5.1 may be quite acceptable. [back]


Fig. 6. Downmixing from PanAmbio or ITU 5.1 surround to two speakers by panning C to a phantom center and "folding" in back channels degrades to stereoís "hole in the middle" and all sounds in front. Multi-channel surround offers great improvement for music and digital television, as it has for the cinema. [back]

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