Ambisonics: The Surround Alternative

1. Ambisonics:The Surround Alternative Richard G. Elen The Ambisonic Network

2. 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?

3. 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?

4. 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

5. 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

6. 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

7. The simplest method of Ambisonic recording • The Soundfield Microphone • Tetrahedral capsule array • Control unit generates B-Format

8. Ambisonic Mixing • SFM CU(Calrec) • B-Format Converter • Pan-Rotate • Transcode • Decode(Monitor) • Ambisonic Mixing System (Audio & Design)

9. Ambisonic Mixing Ambisonic mixing setup at Cruchfield Manor (early 1980s) From: An Introduction To Ambisonic Mixing

10. 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

11. “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

12. “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)

13. 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

14. 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 5.1 and AmbisonicsCompared

15. 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

16. Ambisonics:The Surround Alternative For more information: www.ambisonic.net relen@ambisonic.net