Future Proof Surround Sound Mixing using Ambisonics

Forum for Innovation in Music Production and Composition Dr Bruce Wiggins Signal Processing Applications Research Group University of Derby Future Proof Surround Sound Mixing using Ambisonics [email_address] http://www.derby.ac.uk/staff-search/dr-bruce-wiggins

Introduction
Surround sound is now commonplace, with the standard presentation format being 5.1
Most Digital Audio Workstations (DAWs) will mix for and create outputs for this system.
However, why would we want to be tied to a single speaker array.
It would be very useful to mix using a system that allowed:
More speakers to be used for presenting the work to large groups of people
Less speakers to be used for single listener mixing (including headphones).

Ambisonics
Ambisonics has always been a system capable of single mix, multiple reproduction .
The audio is mixed to an intermediate format.
This format is hierarchical and extendable, and so could be 2, 3, 4 or more channels depending on desired spatial acuity.
This mix can then be rendered over any, practical, arrangement of loudspeakers or headphones exhibiting (with some caveats):
Same relative volume per source
Same source positions
Same spatial impression

Ambisonics - What is it, and Why isn’t it Used?
What is Ambisonics?
A system where the spatial encoding of the audio is separate from the decoding of that audio to a set of speakers.
A set of rules and equations that help to optimise and quantify the performance of a multi-speaker audio presentation.
A system based on coincident recording principles.
An extendable system based on Spherical Harmonics.
A future proof audio format.
Why isn’t it widely used?
Ambisonic Hardware isn’t widely available
Digital Audio Workstations are so predicated on the standard, stereo mixing desk, that working in Ambisonics is often difficult, or impossible.

Ambisonics - Definition
Ambisonics represents a set of rules used for the design of a decoder/reproduction system.
An Ambisonic system is one where (taken from patent):
The decoded velocity and energy vector angles agree and are substantially unchanged with frequency.
At low frequencies (below around 400 Hz) the low frequency velocity vector magnitude is equal to 1 for all reproduced azimuths.
At mid/high frequencies (between around 700 Hz and 4 kHz) the energy vector magnitude is substantially maximised across as large a part of the 360 0 sound stage as possible.

Ambisonic Encoding – B Format
The encoding, or recorded format is known as B-Format, which consists of 4 channels, W, X, Y & Z.
It is made up of 0 th and 1 st degree spherical harmonics (which equate to standard pressure and pressure gradient polar patterns).

Ambisonic Decoding
Once the input signals are known, a linear combination of these, sent to speakers, can be used to satisfy the velocity/energy vector equations.
1 st order Example – using a mixture of a 0 th (omni) and 1 st (figure of 8) mic, any 1 st order mic pattern can be created.

Regular 1 st Order Decoders Cardioid Decode Optimised Decode

Decoders for irregular speaker arrays
Irregular decoders cannot be optimised so easily.
For left/right symmetrical systems:
Amplitude
Polar pattern
Angular spread
Must all be optimised, per speaker pair.
This means solving a set of non-linear simultaneous equations.
Heuristic methods can be used to solve this problem.

Speaker Array Limitations
If too few speakers are used:
For a horizontal decoder, you need more speakers than channels
Horizonal 1 st order uses three channels (W, X and Y)
The Speaker detent effect will occur:
Speakers will ‘stick out’ in the mix.
Reproduced angles will become distorted.

Higher Order Spherical Harmonics

Effect of Higher Order Components
Higher Order Components give two main benefits:
More focused polar patters
Gives more ‘focused’ localisation
Needs more speakers
Allows asymmetrical decode patterns to be used
Ideal for irregular speaker arrays (ITU….)

Ambisonics – How?
Several recent events have finally made creating, storing and listening to Ambisonics possible:
Wigware Ambisonic Plug-ins
DirectX and VST plug-ins
Panning (encoding) to 1 st , 2 nd , 3 rd and 4 th order Ambisonics
Playback (decoding) to 4, 5, 6, 8, 12, 15, and 24 2 and 3 dimensional speaker arrays (at present)
Including multiple decoders for ITU 5 speaker layout
1 st order, three dimensional reverb.
Reaper Audio Sequencing and Creation Software
Richard Dobsons .AMB file format and associated command line tools
www.Ambisonia.com free hosting of Ambisonic content.

Ambisonic Bus Structure
Reaper is one of the few DAWs that can realise the routing shown below and extend it up to 64 channels per track.
25 9 4 th 16 7 3 rd 9 5 2 nd 4 3 1 st 3D 2D Number of Channels Order

Wigware Plug-ins
Freely available from:
http://www.derby.ac.uk/staff-search/dr-bruce-wiggins

Reaper Mix Window ( http:// reaper.fm )

Routing Matrix

Audio Track Routing Examples Audio Track to Reverb and Dry Buss Total Buss to Decoders

Archiving Work - .AMB File Format
In order to archive the work, the B-Format audio channels must be kept.
An Ambisonic file format is in use to allow easy playback of Ambisonic material.
Proposed by Richard Dobson
Interleaved WAV File with a unique GUID to signify Ambisonic content.
To make sure the encoded Ambisonic material isn’t just routed to speakers.
Command line tools to create and extract audio from and to .AMB files can be found:
http:// people.bath.ac.uk/masrwd/mctools.html
Using the Wigware Direct Show filter plug-in, Windows Media Player will deocde .AMB files to a number of standard speaker layouts.

Ambisonia Website – www.ambisonia.com

Conclusions
Finally, the tools are available to create, distribute and listen to future-proof Ambisonic content.
This allows for much more flexibility in terms of the final speaker target for distributed, performed or mixed audio.
Most of these tools are free!
Non-commercial license of Reaper is $50
Screencasts demonstrating how to do this can be found at:
http://ambisonic.ning.com

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Future Proof Surround Sound Mixing using Ambisonics

by Bruce Wiggins on May 28, 2010

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