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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 6

AUTO SURROUND SOUND EXPERIMENT

Today there are more high-end choices for car audio systems and a healthy aftermarket for reinstallations. Consider the traveling representative or cross-country trucker, mostly driving alone, listening to music in a fixed listening position. Consider the luxury car driver who wants the comforts of home in the cab, e.g. THX 7.1 as first certified in a Lincoln. These situations suggest a great potential market for automotive surround.

Initial models have been delivered with ITU 5.1 and 7.1 systems. These find the driver and front passenger outside the sweet area. A fifth passenger seated in the rear seat center has the best position. In addition, the small space, reflective side windows, and short distances and delay times of auto psychoacoustics significantly color the reproduced sound.

The car PanAmbio situation is promising but certainly not simpler: one or two Ambiopole pairs would be needed for each seat, so crosstalk between systems becomes a factor, not to mention issues of uncompromisingly mounting speakers behind the steering wheel, the backs of seats, etc. Electronics would need real time DSP and crosstalk cancellation algorithms unique to these speakers and this passenger compartment. For AES 111th, 2001, the author demonstrated a prototype with eight speakers - two complete PanorAmbiophonic systems for two passengers - with modest success (Figure 20). To reduce system-to-system crosstalk, speakers were placed close (0.5m) to the listeners. However, absorptive material was needed on side windows to avoid interference with the cross-talk cancellation mechanism. Bass needed augmentation due to 4in (100mm) diameter "woofers."

Still, attendees seemed impressed, possibly because the envelopment, spatial impression, and precise 360° localization of the "Walkabout" recording and width of marching bands exceeded their expectations for car sound. The systems have not been tested in motion, where road noise would be a further detriment. More work is needed in automotive surround, such as refined impulse responses and DSP, possibly unique for each listener position. However, amusement ride sound presents a workable situation for good PanAmbio if conveyances are open and carry a single rider per system.


Fig. 20
. Two passengers, each with four speakers for PanAmbio surround, demonstrated in authorís van at AES 111th Conv., 2001. [back]

PC, small TV, and Game Sound

PCs including laptops, small TV sets, and video games with speakers on either side of the monitor are well suited to Ambiophonic (front only) reproduction, requiring only crosstalk cancellation software or hardware, reduced in the future from a prototype PCB (Figure 21) to a single chip. The listener is fixed in the prerequisite position, Ambioís only ergonomic caveat. Ideally, crosstalk cancellation would be done in real time by the PC or DSP based on the binaural impulse response of the exact model speakers. A simpler albeit less exact delivery method is to produce precrosstalk cancelled generic versions of music or sound effects for games or PCs. Examples for immediate demonstration are in streaming audio form at www.filmaker.com.


Fig. 21.
Prototype real-time DSP crosstalk-canceller for one Ambiopole. The two needed for PanAmbio might be a single chip. [back]

FUTURE WORK

The Panambiophone will evolve with the next prototype, which more closely integrates spheres and baffles. A challenge is its aesthetics for use in the presence of an audience or television cameras. While the evaluation CDs include five genres of music of wide variety, many others exist which may respond more or less well to the surround interpretations. Future sessions will include organ, choir, big band, and sound effects. Auto and amusement ride applications will be further developed, as will exploring projectspecific repurposing of unused C, LFE, and CS of 6.1 formats, possibly for height if not additional surround speakers. Double-blind analysis by trained listeners has already been mentioned. Finally, the first production of an album in DVD-A, SACD, DTS-CD, or DVDV and DTV broadcast, possibly in compatible 5.0/4.0, will be proposed to a record company or broadcaster.

CONCLUSIONS

Both 5.1 and PanAmbio reproduction systems satisfy critical listeners significantly more than conventional two-channel stereo, which has been the underlying technology for vinyl, FM, and audiocassette music reproduction for five decades, film for two, and broadcast television for one. Standardized by ITU-R 775, 5.1 is positioned to replace stereo for home audio entertainment, just as it has matrix surround for film and broadcast. Sales of DVD players and home receivers prove 5.1ís popular market acceptance, even if for critical music-only reproduction it is an intentional compromise. Panor-ambiophonic 4.0 (2/2) reproduction is superior to ITU 5.1 in localization precision and therefore in directionally dependent spatial impression and envelopment for one or at most two critically positioned listeners who prefer less compromised music-only reproduction. Experiments demonstrate recording level setting and approaches for compatible production of both 5.1 and PanAmbio and for distribution using DTS-CD, DVD-A, or SACD (C unused or available for repurposing). Alternatively, Ambiophonic (front only) reproduction offers wide, precise localization with just two distribution channels -- embracing many if not most existing stereo recordings - on common PC speakers, small TV sets, games, and high-end audiophile systems that add ambience by convolution. Automotive and amusement ride PanAmbio surround has been demonstrated for one or two seats, however attention must be paid to both listening acoustics and DSP issues.

ACKNOWLEDGEMENTS

The author is grateful to Ralph Glasgal for his inspiration and wisdom and to the Ambiophonics Institute for support of this work. Thanks to J–rg Wuttke of Schoeps GMBH, Buzz Turner of Redding Audio, and Jerry Bruck of Posthorn Recordings for advice and equipment, Geoff Houser and Brad Frikkers for help fabricating prototypes, Howard Moscovitz for help recording, participating musicians, and David Del Grosso and Jeff Levison of DTS for encoding the CDs. In the authorís mind, acknowledgements must include teachers past and mentors present, including all those cited in the references, especially my colleague Angelo Farina. Trademarks are those of their owners, including DTS of Digital Theater Systems, Dolby Digital of Dolby Laboratories, SACD of Philips and Sony, Logic Seven of Lexicon, THX of Lucasfilm.

APPENDIX A -- PANOR-AMBIOPHONIC REQUIREMENTS

To fully realize PanAmbio playback requires stringent acoustics -- refer to AES Technical Document "Multichannel surround sound systems and operations" [20]. Having both ITU and PanAmbio listening requires moving four speakers (or switching nine) -- the studioís bass-manager or home theaterís subwoofer, receiver/decoder, and universal DVD player can be the same (C unused for PanAmbio). Distribution formats can be DTS-CD, DVDV, DVD-A, or SACD - the market will resolve which among them succeed - or multi-channel broadcast using AC-3 (Dolby Digital) of the DTV standard. Requirements for PanAmbio playback are:

  • Symmetrical or acoustically treated room and layout with RT less than recording venue and with one or at most two listeners seated at the center of speaker pairs directly front and back at a radius less than the acoustic room radius.

  • Universal DVD/CD player or DTV receiver with 4-channel output (2 AES/EBU or one multi-channel encoded serial digital connection, coax or optical e.g. S/PDIF -- C unused);

  • Decoder (digital stream to multi-channel analog) -- C unused -- or integrated in 5.1 home receiver. Alternative is all-digital integration of crosstalk-canceller, bass manager, and power amplifiers below;

  • Crosstalk-cancellers -- two DSPs, ideally based on impulse response of speakers used, currently in prototype form (see Figure 21) -- evaluation CD [13] has tracks pre-cancelled for "generic" speakers;

  • Amplifier/bass manager typical of available 5.1 studio units and home receivers and accommodating multi-channel inputs (S/PDIF coax or optical) and four speaker outputs (C unused) plus subwoofer, if any;

  • 4 main speakers with vertically in-line components, full range or common-woofer/satellite systems. Dual PanAmbio and ITU 5.1 requires moving four speakers (or switching nine). Subwoofer optional if main speakers are full range and producers decide the ".1" LFE channel is not needed for music;

  • Calibration of channels relative of one another within ‡dB at the listening position using SPL meter and filtered pink noise from test CD.

An alternative to PanAmbio is Ambio (front-only, 120~150° wide reproduced stage) that uses one crosstalkcancellation DSP, one main speaker pair, and any number of surround speakers that are fed ambience convolved from impulse responses of any desired hall for many existing stereo recordings. In this regard, note that the anomalies of two-speaker stereo are caused by the triangular speaker placement with respect to the listener -- they are not necessarily intrinsic to the recordings, whether made with widely spaced, closely spaced, coincident, or panned microphones.

APPENDIX B -- RECORDING CALIBRATION PROCEDURE

The method adds to either location or studio recording kit a portable (preferably battery operated) amplifier-speaker, source of filtered pink noise, and SPL meter. It is necessary to know from experience or to measure with the SPL meter the peak sound pressure level of the performance to be recorded. Then the recordist may adapt the following generalized procedure for his/her specific equipment: Beginning at a determined distance -- less than the roomís critical radius, e.g. <3m - from an OCT array, position the noise source front and center and adjust its output for some Sound Pressure Level, e.g. 72dB, measured at the microphone array. Adjust preamp C gain for a digital record level 3dB (for overload safety) lower than the point below Full Scale by the amount above 72SPL that you anticipate (or have measured) the ensemble to peak, e.g. --35FS for a small orchestra that peaks at 104SPL. Move the noise source to 3m directly left of the array and set preamp L, then 3m directly right and set preamp R, then 3m from and on common axis with any room microphone array and set preamps LS & RS. Spot microphones can be similarly calibrated at a lower FS level to allow for their closer positioning. For a PanAmbiophone, only two positions, directly front and back, are needed to set preamps LF, RF and LB, RB respectively. Change levels only in all channels equally using preamps with calibrated gain adjustment, or their attenuation controlled by ganged VCAs in the case of analog consoles, or by digital controls of digital consoles. Precision of a fraction of a dB is required to preserve localization.

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