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Audio Concepts in Plain English: 3D and I3DL2

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Sounds attenuate (get quieter) with distance ... Minimum distance (where to start attenuating) ... Rolloff factor adjust attenuation curve relative to 'Real World' ... – PowerPoint PPT presentation

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Title: Audio Concepts in Plain English: 3D and I3DL2


1
Audio Conceptsin Plain English3D and I3DL2
  • Scott Selfon
  • Xbox Advanced Technology Group

2
Overview
  • Whats so great about 3D?
  • Simulating aspects of 3D sound
  • Audio engine implementations
  • I3DL2
  • Advanced 3D Challenges for Games
  • Q A

3
Why bother with 3D audio?
  • Screen 45-90 degree visual window
  • Whats happening in the other 270-315 degrees?
  • Sound informs player of distances, obstacles,
    environment
  • Competing with TV movie experience

4
Typical 3D Sound Presentation Priorities
  • Location
  • Distance
  • Speed
  • Geometry
  • Sound emitter orientation
  • World obstacles

5
Locating a Sound in 3D Space
  • Speaker panning
  • Cheap, can be effective
  • Doesnt 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

6
Distance in the Real World
  • Sounds attenuate (get quieter) with distance
  • Approximately half as loud (-6 dB) for every
    doubling of distance
  • Sounds muffled with distance
  • Air is a low-pass filter
  • Sound travels slowly vs. light
  • 340 m/sec (vs. 300,000 km/sec for light)
  • All values vary with humidity, altitude,
    temperature, etc.

7
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)

8
Distance Attenuation Example
9
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

10
Rolloff Factor and Attenuation
11
Minimum Distance and Attenuation
12
Rolloff Confusion
  • Maximum distance isnt 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

13
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

14
Simplifying Rolloff
  • Distance to silence
  • Trivial to calculate
  • Arbitrary user-defined point curves
  • Custom curvaturecontrol

15
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?

16
Speed Perception in the Real World
  • Doppler Effect
  • Sound/listener moving closer pitched up
  • Sound/listener moving apart pitched down

17
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 (gt1.0) or lessen (lt1.0) effect

18
Sound Geometryin 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

19
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

20
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

21
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)

22
Obstacle Simulation Obstruction
  • Objects may block the sounds path to the
    listener
  • Direct-path diffracted by or transmitted through
    obstacle
  • Reflected pathgenerally notaffected

23
Obstacle Simulation Occlusion
  • Sound is being played in another environment
  • Direct, reflectedpaths bothfiltered

24
An Introduction to I3DL2
  • Designed by Interactive Audio Special Interest
    Group (IASIG www.iasig.org)
  • Interactive 3D Audio Rendering Guidelines, Level
    2
  • Provides standard for simulating
  • 3D positioning (attenuation, doppler)
  • Environmental reverberation
  • Occlusion/obstruction materials

25
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 environments effect on this
    emitter

26
I3DL2 Environmental Reverb(Listener)
  • Decay time time for reverb to fadeto -60 dB

27
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)

28
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

29
I3DL2 Reverb and Occlusion/Obstruction Demos
30
I3DL2 Limitations
  • Reverberation flexibility restrictions
  • I3DL2 doesnt 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

31
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? (P1left, P2right)
  • Audio for complex geometry
  • Rivers
  • Non-cubical environments

32
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)

33
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/s
    oucon.html
  • http//www.gmi.edu/drussell/Demos/doppler/doppler
    .html
  • Questions, comments?
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