Ambisonics: The Surround Alternative

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Ambisonics The Surround Alternative

• Richard G. Elen
• The 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 localizationonly across front
• Center Frontchallenges
• Sound sucked into speakers
• Small sweet spot
• Ambisonics can improve on this.But first
• What is it?

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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 Blumleins M-S coincident pair scheme
  into three dimensions
• B-Format
• W Omni (FBLRUD)
• 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 recording

• The 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-limitedthird channel) Good
  quality planar surround
• 2 Acceptable planar surround (stereo
  compatible)
• Decoded to drive desired number of speakers

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5.1-compatibleAmbisonics

• UHJ is stereo-compatible what is
  5.1-compatible Ambisonics?
• Vienna decoders (based on 1992 AES
  paper)support asymmetrical arrays,
  includingstandard 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 5.1

• Decoder is in the studio No extra gear at home
• G-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 (G2)
• 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_at_ambisonic.net
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