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Audio Engineering Society
Convention Paper

Transforming Ambiophonic + Ambisonic 3D Surround Sound to & from ITU 5.1/6.1
Robert E. (Robin) Miller III ę20031

Page 2 -- HYBRID APPROACH TO DUAL-FORMAT 2D/3D SURROUND SOUND - PerAmbio 6.1.10+

In the authorÝs experience, no single approach produces ideal surround sound.  In 2D surround, ITU 5.1 including 6.1 works well for cinema-style (3-2), but 5.1/6.1 does not support 360° localization that is necessary for the natural reproduction of sources in an acoustic environment [5].  Conventional widely spaced AB or Decca Tree plus room mics fail ITU 5.1/6.1 both in frontal localization and an integrated room impression.  Near-spaced directional microphones are weak in low frequencies, while OCT [4,6] suffers off-axis pick-up in the front left and right channels.  PanAmbio 4.1 [2,4], an HRTF binaural-based approach using twin baffled sphere microphones (Ambiophones), delivers accurate image and spaciousness, but confines listening to the median plane [7].  In 3D, Ambisonics, like other coincident techniques such as X-Y and M-S, fails in adequate spaciousness.  One can conclude that the cues necessary for human hearing of surround sound are not all neatly bundled in any one format! [8,9,10]

The solution is a hybrid approach, where cues lacking in one format are compensated for by another.  Considering properties above, a system that combines HRTF-based Ambiophonics with Ambisonics, called PerAmbio 6.1.10+, could, in theory, optimize 2D or 3D envelopment + frontal imaging + spatial impression.

The system employs dual-format recording, processing, and 6-channel media so that both users and entertainment providers can upgrade from 2D to 3D at lower cost.  PerAmbio combines Ambiophonics for an accurate front stage with Ambisonics for ambience.  The combination is more than the sum of its parts.  For example, there is no longer AmbioÝs limited listening area - listeners can venture freely (except very near any speaker) within both the 5.1/6.1 2D and 6.1.10+ 3D speaker layouts [2].  Surprisingly, listeners of PerAmbio 6.1.10+ in 3D can venture even outside the speaker array and enjoy plausible surround sound.

Fig.2 PerAmbio 6.1.10+ test microphone array in the concert hall.

Fig.3.  3D experimental array (from top) Soundfield microphone, OCT for 2D, and ýAmbiophoneţ with directional elements instead of baffle to reject back sound and realized using Schoeps microphones.

2D Surround Today, 3D Tomorrow from Dual-format Recordings

The system below provides manufacturers, entertainment providers, and home theater owners:

1) ITU 5.1/6.1 discrete replay in 2D (horizontal) surround sound for a standard home theater;

2) Periphonic full sphere 3D surround sound for home theater, PC/multi-media, game/virtual reality, training simulator, or amusement ride.

In horizontal 2D form, 6-channel distribution media are transformed for ITU 5.1/6.1 discrete replay directly with no special decoding.  When the user elects to upgrade to fully periphonic 3D, the same 6 channels undergo lossless matrix conversion to reconstitute signals for 10 or more speakers.

6-channel PerAmbio 6.1.10+ Microphone Array

In its simplest form, PerAmbio requires an array of six microphones (Fig 2 & 3): a Soundfield microphone or equivalent (coincident omni plus three orthogonal figure-eights) and an Ambiophone - a baffled or directional form of sphere microphone [11] that favors frontal incidence.  Six recording channels are needed.  In certain situations and as 3D sound has been refined, up to ten microphones and channels have been used for tailoring the front stage image and preserving low frequency response.

During the recording session and through mastering, it is essential to match the levels of the channels within a fraction of a dB and to track or correct them in post.  An acoustic filtered pink noise source must be recorded at several angles of incidence to the array for calibration and quality assurance.

TRANSFORM FOR DECODER-FREE 5.1/6.1

Although it would be trivial to simply deliver the six native PerAmbio channels in place of standard ITU 6.0 channels, it would repurpose these channels entirely.  This paper describes a lossless algebraic matrix that can ýencodeţ the six channels to play compatibly without any decoding, flattened to standard ITU 5.1 or 6.1 horizontal surround speaker layouts.  Or instead, it can be decoded to PerAmbio 6.1.10 full sphere 3D surround by recovering the Ambisonic B-format (W,X,Y,Z channels) [12] plus a binaural-based Ambiophonic front stage.  The B-format contribution can be sourced either from a Soundfield microphone or from 3D ambience auralization convolved during post-production or by the user from a library of 3D hall impulse responses.  For reference, Ambisonics is summarized in Appendix B.  Ambiophonics, after Glasgal et al [13,14,15], contributes a 150°-wide front stage with image accuracy of ▒5° using a closely spaced "ambiopole" speaker-pair and crosstalk cancellation.  The PerAmbio 5.1/6.1 transformation introduced here is linear and bi-directionally lossless.  Users can have full sphere 3D surround by adding a matrix decoder and 4 or 5 speakers placed as in Fig.1.

After mastering the dual format 2D/3D recording from the six PerAmbio channels, given as {Pin} in 6x1 matrix form, a 6x6 transformation matrix {S} for standard 5.1/6.1 surround is applied to obtain the 6 media channels {Sout} as follows:

{Sout} = {S} ´ {Pin}

On a standard ITU home theater surround system, a multichannel disc (6 discrete channel DVD-A, SACD, or DTS-CD) plays {Sout} directly in 6.1.  If it is 5.1, current implementations automatically add the SC information into SL and SR speaker feeds at -3dB.

When the user augments his/her system for fully periphonic 3D, a ýreconstitutionţ matrix {P} is applied, implemented in DSP, that responds to flags in the metadata to select one of six recording modes, discussed later, to output PerAmbio 6.1.10+ as follows:

{Pout} = {P} ´ {Sout}

Since matrix {P} is the inverse of matrix {S},

{Pout} = {S}-1 ´ {Sout}

As confirmation that PerAmbio 3D is reconstituted without loss, an objective of the system is that

{Pout} = {Pin}

It will be shown that the requirements are met with only slight loss of SNR (1.4 to 3dB, or less than ║ to ç bit in 24) due to a headroom factor used to avoid clipping when random phase elements are matrixed.

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