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Ambisonics: The Surround Alternative

Richard G. ElenThe Ambisonic Network

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Can your surround system do this?Pan a sound around

the room at constant speed.

Is the circle accurate, or does the sound: Jump between

the speakers? Vary in apparent

distance from the center?

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Problems with 5.1

With pairwise mixing: Good localization

only across front Center Front

challenges Sound sucked

into speakers Small sweet spot

Ambisonics can improve on this.But first…

What is it?

a

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Ambisonics Defined“Ambisonics goes further than stereo in that what it

does… is to sample the acoustic field in such a way that the combination of the signals from all the loudspeakers in the array produces… a reconstruction of the original acoustic wave field (both traveling and standing wave components).

“If a listener puts their head in the sound field, because the wavefronts are similar to the original, the perception of directionality and space should correspond to the original too. It is a ‘wavefront reconstruction’ scheme in the small. The ear signals (crosstalk and all) will be correct if the reconstructed wavefronts are correct. This is just like natural hearing.

“Increasing the number of loudspeakers in Ambisonics (each fed its correctly-decoded signal) increases the accuracy of the reconstruction and the region over which it holds up. The interaural arrival-time differences also correspond to natural hearing in Ambisonics.” —Stanley Lipshitz

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What Is Ambisonics? “Ambisonics” means “Surround Sound” Surround system developed by a team of

British researchers in the 1970s, notably the late Michael Gerzon

Designed to bring the concert hall into the living room, by accurately capturing and replaying a soundfield on practical speaker arrays

Four channels (B-Format) capture entire soundfield – including height (“Periphony”)

Transmission systems compatible with stereo (UHJ) and modern “discrete” surround (G-Format)

Ambisonic production can use special microphone (Soundfield mic) or multitrack sources

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Principles of Ambisonics Taking Blumlein’s M-S

coincident pair scheme into three dimensions

B-Format: W = Omni (F+B+L+R+U+D) X = Front – Back Y = Left – Right Z = Up – Down

These signals are symbolized in the Ambisonics logo

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The simplest method of Ambisonic recordingThe Soundfield

Microphone Tetrahedral

capsule array Control unit

generates B-Format

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Ambisonic Mixing

SFM CU(Calrec)

B-Format Converter

Pan-Rotate Transcode Decode

(Monitor)

Ambisonic Mixing System (Audio & Design)

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Ambisonic Mixing

Ambisonic mixing setup

at Cruchfield Manor

(early 1980s)

From: An Introduction

To Ambisonic Mixing

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Stereo-Compatible Transmission & Decoding B-Format can be

encoded into “UHJ”, using available channels: 4: Periphony (with height) 3: High quality planar

(horizontal) surround 2.5: (Bandwidth-limited

third channel) Good quality planar surround

2: Acceptable planar surround (stereo compatible)

Decoded to drive desired number of speakers

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“5.1-compatible”Ambisonics UHJ is stereo-compatible:

what is “5.1-compatible” Ambisonics? “Vienna” decoders (based on 1992 AES paper)

support asymmetrical arrays, including“standard” 5.1 speaker arrays

“Enhanced” B-Format (BEF) adds E & F channelsfor improved accuracy and 5.1 compatibility

Create B-Format (or BEF) mix in the studio Decode to 5.1 in the studio: “G-Format” Use standard “discrete” digital media

(DVD-A/V, SACD etc) MLP (Meridian Lossless

Packing) for DVD-A includes B-Format capability

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“G-Format” –Ambisonics for 5.1Decoder is in the studio:

No extra gear at homeG-Format is hierarchical:

B-Format can be recovered (for purists) and decoded to alternative speaker arrays

“LFE” channel can be used for height information – so can CF (Chesky)

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Working with Ambisonics Mix to BEF or B-Format using software tools

or future hardware Archive BEF or B-Format masters

May be used for future distribution formats

Decode to 5.1 “G-Format” Optionally include height

Monitor in 5.1 or with Ambisonic “virtual arrays”

Simultaneous (automatic) encode to 2-channel UHJ for 2-channel downmix (“G+2”)

SACD 2-channel area and Red Book layer DVD-A stereo area or DVD-V PCM mix

Author any current multichannel distribution format DVD-A (MLP), SACD multichannel, DTS, AC-3 etc

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5.1 and AmbisonicsCompared

Pairwise 5.1 Localization achieved

solely with level Poor inter-speaker

imaging except at front Localization varies with

listener position Small sweet spot Horizontal surround

uses six channels Speakers in special

positions (eg ITU) One-to-one channel

relationship from studio to speakers

Ambisonics Localization includes

other mechanisms Good inter-speaker

imaging all round Less variation of image

with position Larger sweet spot Even with height, only

four channels needed Decode to 5.1 or other

configurations Recover B-Format and

decode for completely flexible speaker arrays

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The Ambisonic Alternative Tried and tested technology,

mainly in the public domain Superior localization to

conventional pairwise 5.1 5.1 compatible, improves 5.1 performance Supports additional future configurations,

eg 10.x, 7.1, stacked octagons, etc Powerful soundfield capture and soundfield

synthesis (mixing) technologies Hardware and software tools in development

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Ambisonics: The Surround Alternative

For more information: www.ambisonic.net relen@ambisonic.net