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Future Proof Surround Sound Mixing using Ambisonics


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A paper delivered at the forum for innovation in music production and composition in 2009.

Published in: Technology, Business

Future Proof Surround Sound Mixing using Ambisonics

  1. 1. 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]
  2. 2. Introduction <ul><li>Surround sound is now commonplace, with the standard presentation format being 5.1 </li></ul><ul><li>Most Digital Audio Workstations (DAWs) will mix for and create outputs for this system. </li></ul><ul><li>However, why would we want to be tied to a single speaker array. </li></ul><ul><li>It would be very useful to mix using a system that allowed: </li></ul><ul><ul><li>More speakers to be used for presenting the work to large groups of people </li></ul></ul><ul><ul><li>Less speakers to be used for single listener mixing (including headphones). </li></ul></ul>
  3. 3. Ambisonics <ul><li>Ambisonics has always been a system capable of single mix, multiple reproduction . </li></ul><ul><li>The audio is mixed to an intermediate format. </li></ul><ul><ul><li>This format is hierarchical and extendable, and so could be 2, 3, 4 or more channels depending on desired spatial acuity. </li></ul></ul><ul><li>This mix can then be rendered over any, practical, arrangement of loudspeakers or headphones exhibiting (with some caveats): </li></ul><ul><ul><li>Same relative volume per source </li></ul></ul><ul><ul><li>Same source positions </li></ul></ul><ul><ul><li>Same spatial impression </li></ul></ul>
  4. 4. Ambisonics - What is it, and Why isn’t it Used? <ul><li>What is Ambisonics? </li></ul><ul><ul><li>A system where the spatial encoding of the audio is separate from the decoding of that audio to a set of speakers. </li></ul></ul><ul><ul><li>A set of rules and equations that help to optimise and quantify the performance of a multi-speaker audio presentation. </li></ul></ul><ul><ul><li>A system based on coincident recording principles. </li></ul></ul><ul><ul><li>An extendable system based on Spherical Harmonics. </li></ul></ul><ul><ul><li>A future proof audio format. </li></ul></ul><ul><li>Why isn’t it widely used? </li></ul><ul><ul><li>Ambisonic Hardware isn’t widely available </li></ul></ul><ul><ul><li>Digital Audio Workstations are so predicated on the standard, stereo mixing desk, that working in Ambisonics is often difficult, or impossible. </li></ul></ul>
  5. 5. Ambisonics - Definition <ul><li>Ambisonics represents a set of rules used for the design of a decoder/reproduction system. </li></ul><ul><li>An Ambisonic system is one where (taken from patent): </li></ul><ul><ul><li>The decoded velocity and energy vector angles agree and are substantially unchanged with frequency. </li></ul></ul><ul><ul><li>At low frequencies (below around 400 Hz) the low frequency velocity vector magnitude is equal to 1 for all reproduced azimuths. </li></ul></ul><ul><ul><li>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. </li></ul></ul>
  6. 6. Ambisonic Encoding – B Format <ul><li>The encoding, or recorded format is known as B-Format, which consists of 4 channels, W, X, Y & Z. </li></ul><ul><li>It is made up of 0 th and 1 st degree spherical harmonics (which equate to standard pressure and pressure gradient polar patterns). </li></ul>
  7. 7. Ambisonic Decoding <ul><li>Once the input signals are known, a linear combination of these, sent to speakers, can be used to satisfy the velocity/energy vector equations. </li></ul><ul><li>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. </li></ul>
  8. 8. Regular 1 st Order Decoders Cardioid Decode Optimised Decode
  9. 9. Decoders for irregular speaker arrays <ul><li>Irregular decoders cannot be optimised so easily. </li></ul><ul><li>For left/right symmetrical systems: </li></ul><ul><ul><li>Amplitude </li></ul></ul><ul><ul><li>Polar pattern </li></ul></ul><ul><ul><li>Angular spread </li></ul></ul><ul><li>Must all be optimised, per speaker pair. </li></ul><ul><li>This means solving a set of non-linear simultaneous equations. </li></ul><ul><li>Heuristic methods can be used to solve this problem. </li></ul>
  10. 10. Speaker Array Limitations <ul><li>If too few speakers are used: </li></ul><ul><ul><li>For a horizontal decoder, you need more speakers than channels </li></ul></ul><ul><ul><li>Horizonal 1 st order uses three channels (W, X and Y) </li></ul></ul><ul><li>The Speaker detent effect will occur: </li></ul><ul><ul><li>Speakers will ‘stick out’ in the mix. </li></ul></ul><ul><ul><li>Reproduced angles will become distorted. </li></ul></ul>
  11. 11. Higher Order Spherical Harmonics
  12. 12. Effect of Higher Order Components <ul><li>Higher Order Components give two main benefits: </li></ul><ul><ul><li>More focused polar patters </li></ul></ul><ul><ul><ul><li>Gives more ‘focused’ localisation </li></ul></ul></ul><ul><ul><ul><li>Needs more speakers </li></ul></ul></ul><ul><ul><li>Allows asymmetrical decode patterns to be used </li></ul></ul><ul><ul><ul><li>Ideal for irregular speaker arrays (ITU….) </li></ul></ul></ul>
  13. 13. Ambisonics – How? <ul><li>Several recent events have finally made creating, storing and listening to Ambisonics possible: </li></ul><ul><ul><li>Wigware Ambisonic Plug-ins </li></ul></ul><ul><ul><ul><li>DirectX and VST plug-ins </li></ul></ul></ul><ul><ul><ul><li>Panning (encoding) to 1 st , 2 nd , 3 rd and 4 th order Ambisonics </li></ul></ul></ul><ul><ul><ul><li>Playback (decoding) to 4, 5, 6, 8, 12, 15, and 24 2 and 3 dimensional speaker arrays (at present) </li></ul></ul></ul><ul><ul><ul><ul><li>Including multiple decoders for ITU 5 speaker layout </li></ul></ul></ul></ul><ul><ul><ul><li>1 st order, three dimensional reverb. </li></ul></ul></ul><ul><ul><li>Reaper Audio Sequencing and Creation Software </li></ul></ul><ul><ul><li>Richard Dobsons .AMB file format and associated command line tools </li></ul></ul><ul><ul><li> free hosting of Ambisonic content. </li></ul></ul>
  14. 14. Ambisonic Bus Structure <ul><li>Reaper is one of the few DAWs that can realise the routing shown below and extend it up to 64 channels per track. </li></ul>25 9 4 th 16 7 3 rd 9 5 2 nd 4 3 1 st 3D 2D Number of Channels Order
  15. 15. Wigware Plug-ins <ul><li>Freely available from: </li></ul><ul><ul><li> </li></ul></ul>
  16. 16. Reaper Mix Window ( http:// )
  17. 17. Routing Matrix
  18. 18. Audio Track Routing Examples Audio Track to Reverb and Dry Buss Total Buss to Decoders
  19. 19. Archiving Work - .AMB File Format <ul><li>In order to archive the work, the B-Format audio channels must be kept. </li></ul><ul><ul><li>An Ambisonic file format is in use to allow easy playback of Ambisonic material. </li></ul></ul><ul><ul><li>Proposed by Richard Dobson </li></ul></ul><ul><ul><li>Interleaved WAV File with a unique GUID to signify Ambisonic content. </li></ul></ul><ul><ul><ul><li>To make sure the encoded Ambisonic material isn’t just routed to speakers. </li></ul></ul></ul><ul><ul><li>Command line tools to create and extract audio from and to .AMB files can be found: </li></ul></ul><ul><ul><ul><li>http:// </li></ul></ul></ul><ul><li>Using the Wigware Direct Show filter plug-in, Windows Media Player will deocde .AMB files to a number of standard speaker layouts. </li></ul>
  20. 20. Ambisonia Website –
  21. 21. Conclusions <ul><li>Finally, the tools are available to create, distribute and listen to future-proof Ambisonic content. </li></ul><ul><li>This allows for much more flexibility in terms of the final speaker target for distributed, performed or mixed audio. </li></ul><ul><li>Most of these tools are free! </li></ul><ul><ul><li>Non-commercial license of Reaper is $50 </li></ul></ul><ul><li>Screencasts demonstrating how to do this can be found at: </li></ul><ul><ul><li> </li></ul></ul>