Transcript Slide 1
Audio Concepts in Plain English: 3D and I3DL2
Scott Selfon Xbox Advanced Technology Group
Overview
• What’s so great about “3D”?
• Simulating aspects of 3D sound • Audio engine implementations • I3DL2 • Advanced 3D Challenges for Games • Q & A
Why bother with “3D” audio?
• Screen = 45-90 degree visual ‘window’ – What’s happening in the other 270-315 degrees?
• Sound informs player of distances, obstacles, environment • Competing with TV & movie ‘experience’
Typical 3D Sound Presentation Priorities • Location • Distance • Speed • Geometry – Sound emitter orientation – World obstacles
Locating a Sound in 3D Space
• Speaker panning – Cheap, can be effective – Doesn’t cover surround for 2-speaker scenarios • HRTF – Head Related Transfer Function – Simulates 3D spatialization based on shapes of ‘generic’ head and ears – Filtering (shape of ears, sound waves not directly reaching ears) – Delays (sound reaches one ear slightly before the other
Distance in the Real World
• Sounds attenuate (get quieter) with Dist. Vol.
distance 1 0 dB –
Approximately half as loud (-6 dB) for every doubling of distance
2 -6 dB 4 -12 dB • Sounds “muffled” with distance 8 -18 dB – Air is a low-pass filter 16 -24 dB • Sound travels slowly vs. light – ~340 m/sec (vs. 300,000 km/sec for light) 32 64 -30 dB -36 dB 128 -42 dB • All values vary with humidity, altitude, temperature, etc.
Distance Simulation: Volume
• Typical distance model (DSound, OpenAL): – Minimum distance (where to start attenuating) – Maximum distance (where to stop attenuating) – Rolloff (how quickly to attenuate with distance) • Others simplify to: – Minimum distance (where to start attenuating) – Distance to silence (where sound is silent; rolloff implied from here – linear or log)
Distance Attenuation Example
0 1 (min distance) -5 -6 dB (half volume) at 2 units -10 -15 10 (max distance) -20 -25
Distance (Magnitude, in units)
Distance and Rolloff
• Rolloff factor – adjust attenuation curve relative to “Real World” – .5: Less rolloff (sound louder at given distance) – 2.0: More rolloff (sound quieter at given distance) • Rolloff also influenced by minimum distance – Remember, half volume at doubled distance
Rolloff Factor and Attenuation
0 -5 -10 -15 -20 -25 -30 -35 Rolloff=1 Rolloff=2 Rolloff=3 Rolloff=.5
Distance (Magnitude, in units)
Minimum Distance and Attenuation 0 -5 -10 -15 MinDist=1 MinDist=2 MinDist=3 MinDist=4 -20 -25
Distance (Magnitude, in units)
Rolloff Confusion
• Maximum distance isn’t silence – Sound not further attenuated – Might abruptly stop voice (PC) • Distance factor unrelated to rolloff – Sets units (meters [1.0], feet [0.3048], etc.) – Used for doppler shifting
Rolloff Model Challenges
• Unintuitive calculations to determine distance to silence – spreadsheet to help • Hard for sound designer to experiment without game-exposed controls • Simplification options – Replace max dist/rolloff with “distance to silence” – Arbitrary rolloff curves
Simplifying Rolloff
• Distance to silence – Trivial to calculate • Arbitrary user defined point curves • Custom curvature control
Simulating Other Distance Cues • Filtering with distance – EAX supports – Programmable LPFs in other engines can be used for this • Delay with distance (speed of sound) – Rarely simulated by engines • Can programmatically delay sound triggers • Perceived in games as ‘lag’?
Speed Perception in the Real World • Doppler Effect – Sound/listener moving closer pitched up – Sound/listener moving apart pitched down
Doppler Effect Simulation
• Programmer updates listener/source velocities – DSound: Relative velocity used – OpenAL: Separate velocities used • Distance factor – Units do matter here (m/sec) • Doppler factor – Exaggerate (>1.0) or lessen (<1.0) effect
Sound Geometry in the Real World • Few sounds are truly omnidirectional – Most volume projected in one direction • Few sounds are truly “point” sources • Sounds interact with the environment – Environment interaction (“reverb”) – Filtered by obstacles
Sound Geometry Simulation
• Directional sound emitters – Inside/outside sound cone angles – Outer volume attenuation – 3D spatial orientation for emitter • Non-point emitter solutions – Simulate with multiple points – Simulate with relative positioning – Pre-rendered multichannel – More sophisticated engines
Environmental Simulation
• Simulation of sound propagation paths – Direct-path = signal reaches listener directly from emitter – Reflected paths = ‘echoes’ off walls, obstacles, etc.
• Some delay versus direct-path LISTENER LISTENER SOUND SOUND
Basic Reverb Simulation
• Direct-path: sound presented “intact” – Distance attenuation – Low pass filtering based on obstacles/distance • Reflected path: delays and filtering – Often less noticeable distance attenuation – Early reflections = discrete echoes – Late reflections = ‘reverberation’ (more dense, complex, decaying echoes)
Obstacle Simulation: Obstruction • Objects may ‘block’ the sound’s path to the listener – Direct-path diffracted by or transmitted through obstacle – Reflected path generally not affected
Obstacle Simulation: Occlusion • Sound is being played in another environment SOUND – Direct, reflected paths both filtered OCCLUDER LISTENER
An Introduction to I3DL2
• Designed by Interactive Audio Special Interest Group (IASIG – www.iasig.org) – “ I nteractive 3D Audio Rendering Guidelines, L evel 2 ” • Provides standard for simulating: – 3D positioning (attenuation, doppler) – Environmental reverberation – Occlusion/obstruction materials
I3DL2 Concepts
• Listener = describes environment – Timings for early/late reflections (“Room”) – Differences between high and low frequency behavior • Source = per-emitter settings – Occlusion/obstruction – Exaggerate/reduce environment’s effect on this emitter
I3DL2 Environmental Reverb (“Listener”) • Decay time = time for reverb to fade to -60 dB
I3DL2 Reverb Settings
• 30 pre-defined high level presets… – Generic environments (cave, bathroom, etc.) – ‘music’ reverbs • …or 12 low-level properties – Adjust timings: Reflections_delay, Reverb_delay, Decay_time – Set volumes: Room, Reflections, Reverb, Room_rolloff_factor – Adjust behavior of high vs. low frequencies: HF_reference, Room_HF, Decay_HF_ratio – Reverb timbre: Diffusion (“graininess”), Density (“hollowness”)
I3DL2 Sound Emitters (“Source”) • Volume adjustment – Direct-path level: Direct, Direct_HF – Room (reverb) rolloff factor and level: Room_rolloff_factor, Room, Room_HF • Occlusion/obstruction control – 8 presets supplied (doors, walls, etc.) – Volume attenuation: Obstruction, Obstruction_LF_ratio, Occlusion, Occlusion_LF_ratio
I3DL2 Reverb and Occlusion/Obstruction Demos
I3DL2 Limitations
• Reverberation flexibility restrictions – I3DL2 doesn’t handle multiple simultaneous environments (listener only) – Can be expensive (CPU, memory) – “Morphable” reverb settings are hard • No sense of game geometry – Game must test for and set occlusion/obstruction – Early reverb reflections are static, non-directional – Reverb transitions
Advanced 3D Challenges
• Multiple players, one set of speakers – Present sound relative to closest listener – Reproduce sound for all listeners • Doppler issues • Overlayed soundscapes • Speaker biasing? (P1=left, P2=right) • Audio for complex geometry – Rivers – Non-cubical environments
Advanced 3D Challenges
• 3D integration with multichannel audio – 4- vs. 5- channel 3D?
– Morphing 4.0/5.1 sounds to point sources?
• Multiple 3D sources presenting same audio (PA system?) – Sync and Doppler challenges • More sophisticated sound propagation simulation – Complex, distant environments (pathfinding?) – AI response to sound propogation (enemies can hear you)
Q & A
• Resources – OpenAL: http://www.openal.org/ – DirectSound: http://msdn.microsoft.com/ – I3DL2, IA-SIG: http://www.iasig.org/ – http://www.midi.org/ • Audio physics resources – http://hyperphysics.phy astr.gsu.edu/hbase/sound/soucon.html
– http://www.gmi.edu/~drussell/Demos/doppler/doppler.
html • Questions, comments?