Transcript PowerPoint Presentation - Ambisonics – The Surround

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:

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
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?
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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.”
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—Stanley Lipshitz
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:

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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 recording
 The
Soundfield
Microphone
 Tetrahedral
capsule array
 Control unit
generates
B-Format
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Ambisonic Mixing

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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:

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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?

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“Vienna” decoders (based on 1992 AES paper)
support asymmetrical arrays, including
“standard” 5.1 speaker arrays
“Enhanced” B-Format (BEF) adds E & F channels
for 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

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Mix to BEF or B-Format using software tools
or future hardware
Archive BEF or B-Format masters
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Decode to 5.1 “G-Format”
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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”)
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May be used for future distribution formats
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 Ambisonics
Compared

Pairwise 5.1
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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
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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
[email protected]
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