Extending SMIL with 3D audio Kari Pihkala HUT, Telecommunications Software and Multimedia Laboratory
[email protected]
Abstract This paper describes how SMIL can be extended to support 3D audio in a similar fashion than AABIFS does it for MPEG-4. The SMIL layout is extended with an extra dimension to support a 3D space. Then, audio elements are positioned in the 3D space, whilst a listener element defines a listening point. Similarly to AABIFS perceptual modeling approach, an environment element describes environmental parameters for the audio elements. As a conclusion, 3D audio enables interactive advanced audio capabilities in SMIL. In addition, any XML based rendering language could be extended with 3D audio capabilities using similar approach. 1 INTRODUCTION Stereo sound systems have been around for a long time. The recent technical advancement is to use additional channels to enhance the listener’s experience. These 5.1-channel home theaters are supported by the latest DVD players and desktop computer sound cards, such as SoundBlaster Live!. The advantage of added channels in a computer system is more immerse audio worlds in games and multimedia presentations. However, some of the recent multimedia standards have not kept up with this technical advancement. Synchronized Multimedia Integration Language 2.0 (SMIL) is the new multimedia format for the Web (Rutledge, 2002). It allows integration of media objects spatially and temporally. It is an open standard, targeted to replace proprietary multimedia standards currently used in the Web. SMIL has also been adopted as the content description language for the Multimedia Messaging Service (MMS) enabled mobile phones. However, it is a good example of a multimedia standard, which does not take full advantage of the advanced audio technology. Goose et al. (2002) have proposed a framework for three dimensional (3D) audio presentations by extending SMIL. They describe a complete system architecture to deliver 3D audio into a mobile device by preprocessing the SMIL presentation on a server into a stereo audio stream and delivering it to the mobile device. However, their approach lacks interactivity and works only for audio presentations, disregarding visual presentations all together.
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MPEG-4 is a comprehensive standard including audio and video compression, scene description, multiplexing, and synchronization. The scene description, called binary format for scenes (BIFS) is similar to SMIL, defining how the MPEG-4 objects are combined together for presentation. MPEG-4 Advanced AudioBIFS (AABIFS) describes an audio scene description, taking full advantage of audio spatialization (Scheirer et al., 1999). XMT-Ω, the textual format of MPEG-4, is based on SMIL 2.0. It includes SMIL Animation, Content Control, Media, Metainformation, Timing and Synchronization, Time Manipulations, and Transition Modules, but also extends SMIL with its own set of elements. These include means for 3D spatialization. XMT-Ω is meant to ease the authoring of MPEG-4 scenes and is compiled into MPEG-4 content before playback. Thus, it is not a presentation language and cannot be used to play back 3D audio presentations as such. (Kim et al., 2000) This paper describes how SMIL can be extended to support 3D audio. The AABIFS scene description is taken as a reference. First, the SMIL and AABIFS are briefly introduced to unveil their differences. Then, the 3D audio extension for SMIL is described. After that, the system architecture for the extension is given, while the last section draws conclusions. 2 BACKGROUND This section describes the currently available audio related features in SMIL 2.0 standard and briefs the 3D audio features in the MPEG-4 AABIFS. 2.1 SMIL One of the main design goals developing SMIL 2.0 was modularity. The SMIL 2.0 specification defines 10 functional areas, which are further divided into 45 modules. The modules define small pieces of functionality, as their name implies: Media Objects, Basic Layout, Prefetch Control, Basic Linking, and Spline Animation. The modules are combined to create language profiles, e.g., the SMIL Host Language profile contains almost all of the SMIL modules. SMIL Media Object Modules can be used to include external media objects in a presentation. The media object is referenced with a Uniform Resource Identifier (URI), which is an extension of the commonly used URL. The SMIL specification does not define which format the external media files should use. It is up to the SMIL player to support any media format it desires, however it is recommended to support at least the following MIME types: audio/basic, image/png, and image/jpeg. Typically, mono or stereo audio objects are used, but it is possible to reference a 5.1 channel audio file, too. However, this means that the 3D position information is in the external file and cannot be interactively modified within SMIL. The SMIL Layout Modules define the spatial placement of media objects in a SMIL presentation. The position of media objects is defined with absolute coordinates with the top-left and bottom-right corners in two dimensions. Also, various visual effects can be
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applied to visual objects from animation to transition effects. It is also possible to assign an audio object to a region causing the region to alter the volume of the audio. However, SMIL does not provide any other audio effects. Figure 1 depicts a short example displaying an image with a size of 100x200 pixels and playing an audio file with –6 dB of its original volume. The audio object is always played out without spatial positioning.
Figure 1: Standard SMIL 2.0 document. 2.2 AABIFS AABIFS defines 3D audio capabilities for MPEG-4. The basic AudioBIFS defines basic audio nodes in MPEG-4. These are Sound and AudioClip nodes, which define the location, direction, intensity and a reference to an audio stream. Others, AudioSource, AudioMix, AudioSwitch, AudioFX, AudioDelay, AudioBuffer, ListeningPoint, and Sound2D provide mixing, effects processing and interactive playing. AudioSource is very similar to AudioClip as it can be used to include streaming sound in MPEG-4, with fields to change the pitch and speed of the stream. The AudioMix node can be used to mix any number of input channels to any number of output channels, while AudioSwitch allows choosing any number of input channels as one output channel. The AudioFX node can be used to add sound filtering effects, expressed in Structured audio orchestra language (SAOL), which is a digital signal processing language defined in the MPEG-4 standard. The AudioDelay node adds delay to the output channel for a better control over synchronization. The AudioBuffer node is similar to the AudioClip node, but can cut clips from audio streams. The Sound2D node offers a way to place a sound in a 2D plane specifically designed for 2D applications in mind, where the viewpoint is assumed to be 1-meter outwards from the center of the 2D plane. Usually, the listener is located at the location specified at Viewpoint, however it is possible to position the listener elsewhere using the ListeningPoint node. The Advanced AudioBIFS nodes add features for virtual acoustic rendering with either physical or perceptual approach. The physical approach mimics the real physical acoustics by simulating the sound propagation in a 3D scene made of surfaces. The AcousticScene node can be used to describe the generic parameters of the scene for the physical approach. The parameters include rendering region of the sound, late reverberation, and grouping of acoustic surfaces. The AcousticMaterial node defines the acoustic properties of surfaces in a 3D scene.
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The PerceptualParameters node is used in the perceptual approach to describe how the sounds are perceived instead of trying to simulate the physical properties of the scene. This node mainly affects the reverberation of the sound nodes, dividing reverberation into four sections: direct sound, directional early reflections, diffuse early reflections, and late reverberation. Also, three frequency bands can be controlled: low-, mid-, and high-frequency bands. In addition to these, the modal density of a room response can be controlled, the effect of distance can be altered, and parameters to simulate occlusion, where a sound source is behind a source-obstructing object, can be specified. The DirectiveSound node is similar to the Sound and Sound2D node, adding better control over the directivity of the sound. It is possible to specify the attenuation of the sound in the function of its frequency. It also specifies specific control over sound propagation delay with speed of sound, while attenuation can be controlled with distance factor, and air absorption. Figure 2 gives an example of AABIFS nodes defining the listening point, one sound source and the perceptual parameters for it. Group { ViewPoint { position 0 0 2 orientation 0 1 0 0 } DirectiveSound { Source AudioSource { url 3 startTime 0 stopTime -1 } location 0 0 0 direction 0 0 1 spatialize TRUE useAirabs TRUE distance 100 roomEffect TRUE directivity [1 0.5 0.30 0.1] angles [0 1.05 2.09 3.14] perceptualParameters PerceptualParameters { sourcePresence 1.0 sourceWarmth 3.0 sourceBrilliance 0.5 roomPresence 0.5 runningReverberance 0.1 envelopment 0.1 lateReverberance 3.5 heavyness 2.0 liveness 0.5 } } }
Figure 2: Example of AABIFS nodes.
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3 ADVANCED AUDIO IN SMIL SMIL is based on XML, and thus it is extendable. A new module called Advanced Audio Markup Language (AAML) is created to support 3D audio and extended to SMIL. AABIFS nodes converted into XML elements or XMT-Ω elements cannot be used as such, because they do not take into account the layout or timing in the SMIL documents. Therefore, slightly different elements have been designed. 3.1 Extended SMIL Layout The standard SMIL 2.0 Language only supports 2D coordinate system designed for twodimensional screens. In 3D audio, as the name implies, there is also third dimension. SMIL Layout Modules define the spatial position with left, width, right, top, height, and bottom attributes. These can be extended with a similar system for third dimension by adding front, back, and depth attributes, as depicted in Figure 3. The new attributes do not affect the standard SMIL layout or media objects, they only affect the 3D audio objects. It must be noted that the new attributes are specifically designed for this purpose and most likely cannot be used as a general solution for 3D graphics in SMIL. z x top
depth
y
height
front
right
left
back
width
bottom
Figure 3: Standard SMIL coordinates extended with front, depth, and back coordinates. The proposed new attributes comply with all SMIL Layout Modules. If the depth attribute is added to the root-layout element, then percentage values can be used instead of absolute values in the new attributes, and any conflicts in them can be resolved (i.e., front value can be calculated based on back and depth values). Hierarchical Layout Module defines advanced spatial placement inside regions with subregions. In this case, the new attributes are calculated in a similar fashion as the standard coordinates. The new attributes can be animated with the SMIL Animation modules. This allows moving the region in the z-axis, too. Currently, the animateMotion element can move elements along a two-dimensional path. It is possible to create a similar animation element for 3D paths. The units used are pixels. They are converted straight into meters. If the application requires other metrics, it is possible to scale, rotate, and translate the coordinates.
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Finally, the described layout extension is not necessary, if two-dimensional positioning is considered to be adequate. In most cases, the listeners will only be able to recognize the x-axis position due to limitations in the sound reproduction systems. 3.2 Audio and Listener Elements The only controllable parameter in the standard SMIL audio object is the sound level, which is given in percentages of the original sound volume. To support spatial positioning, a new audio element is created. It places itself in the position given by the region’s coordinates. For realism, the sound attenuates in correlation with distance. The attenuation is controlled by two attributes. Audio is played at a given sound level within minimum distance (minDistance), while it is not played at all outside maximum distance (maxDistance). The sound level changes gradually between these two distances, as shown in Figure 4. Generally, media objects in a SMIL presentation do not have any orientation compared to objects in BIFS scenes, and thus audio objects are represented without orientation. Ambient sounds can be produced by setting the minimum distance to the size of the presentation. z x
max up
min
audio3d
front listener
y Figure 4: Audio and listener elements. Now, as the audio elements have a position, there has to be a listening point. For this, a listener element is defined in the head section of the document. By default, the listener is positioned at the middle of the window, a short distance backwards from it, oriented towards the z-axis, as depicted in Figure 4, to mimic the position of a real user behind the screen. Thus, audio objects on the left side of the screen will be heard from the left side, while audio objects on the right are heard from right. It is possible to modify the default position of the listener with the left, top, and front attributes associated with the listener element. In addition to the previously mentioned attributes, it is also possible to enable Doppler shift effects. Then, if the audio or listener is moved, its velocity will affect the pitch of the sound. The effect of distance attenuation and Doppler shift can be increased or decreased with the distanceFactor and dopplerFactor attributes. This will sometimes create more pleasant effects. 3.3 Audio Environment The audio environment can be modeled with perceptual parameters, defined in the head part of the SMIL document in an environment element. The parameters that can be 6
modified are similar to those in the EAX 2.0 API because of the implementation, cf. Section 4. The preset attribute in this element can be used to select one of the 25 preset environments (e.g., generic, bathroom, cave, concert hall, and underwater), while the rest of the attributes can be used to override the individual preset parameters (e.g., decayTime, reflectionsDelay, reverbDelay, and airAbsorption). Figure 5 depicts the reverberation model in EAX 2.0 API. dB 0 dB
direct reflections
reverb
time
Figure 5: Reverberation in EAX 2.0 API (Creative, 2001). 3.4 Detailed Example The example in Figure 6 shows how the advanced audio language is used in SMIL. The second line defines the namespace prefix for AAML. All AAML elements and attributes are prefixed with this prefix to distinguish them from SMIL elements. Line 4 defines that the bathroom environment with overridden decay time is used as the environment. Line 5 describes the position of the listener. The region element in line 7 is extended with the third dimension. The same region will be used to position an image and an audio object. An image in line 13 is rendered in the region. The advantage of the designed language is that the presentation can be played with a standard SMIL player, as demonstrated by lines 14-18. If the SMIL player supports AAML, then the audio object in line 15 will be played spatially in front, to left and up from the listener. If AAML is not supported, the standard SMIL audio object in line 17 will be played and the player would also have omitted the environment and listener elements in the head section. Finally, line 19 demonstrates the interactivity and dynamic nature of the new language. The animate element moves the audio and image position from left to right in 10 seconds after the image has been clicked. If the standard SMIL audio object were played back, this would not affect it. One feature of the proposed extension is that the presentation can be preprocessed on a server to an audio stream by resolving the timing and combining the audio objects. The stream can then be delivered to the client, which can play it out with a standard stereo player. Of course, interactivity is then compromised. This is similar to the approach Goose et al. used.
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1: 3: 4: 5: 6: 7: 9: 10: 11: 12: 13: 14: 15: 17: 18: 19: 21: 22: 23:
Figure 6: AAML in a SMIL document. 4 IMPLEMENTATION This section describes the overall architecture of the implementation for AAML in a client-side SMIL player. 4.1 The SMIL Player The proposed new audio module was added to the SMIL player (Pihkala et al., 2002) developed for the X-Smiles browser (Vuorimaa et al., 2002). The X-Smiles browser allows adding new markup languages and linking them to the existing languages. The extendibility of the browser made it easy to add the new module. 4.2 Rendering Architecture The audio rendering architecture is similar to that suggested by Trivi et al. (2002), as shown by Figure 7. The SMIL document contains positions for audio elements and listener, environment data, and references to audio files. These are read in by the browser, which forwards all but standard SMIL timing information to the AAML module. The AAML module renders the audio and listener objects through Microsoft’s DirectSound API, while environmental effects are achieved with Creative’s EAX 2.0 API. The timing for all this is given by the SMIL player.
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Audio Files
SMIL Doc. Browser
AAML MLFC EAX
SMIL Player
DirectSound
Hardware Abstraction environment
audio data
position data
timing
Figure 7: Architecture. The browser, SMIL player, and AAML module are implemented in Java, while DirectSound and EAX APIs are only accessible in C++. Therefore, Java Native Interface (JNI) is used in Java to access a piece of C++ code, which then calls the APIs. The implemented AAML module recognizes run-time changes in the attributes of the AAML elements, i.e., animation of coordinates or changes in the environmental parameters. The implementation samples these attributes every 50 ms and forwards any changes to the DirectSound and EAX APIs through the JNI interface. The Hardware Abstraction layer then changes the audio output accordingly. 5 APPLICATIONS Three applications were developed to evaluate the usability of the new language. In the first one, the spatial positioning is demonstrated with Doppler effects, as seen in Figure 8. The ambulance runs back and forth, while the listener is in the middle. The sound is heard either at left or right depending on the position of the ambulance. Also, the Doppler effect is audible, i.e., the frequency of the sound is higher when the ambulance is driving towards the listener.
Figure 8: Spatial positioning with Doppler effects. Another application demonstrates interaction and environmental parameters. As depicted in Figure 9, a ball bounces back and forth on the screen, causing its sound to be spatially positioned. The user can stop the ball by clicking on it. The user can also change the environmental parameters by clicking on one of the buttons.
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Figure 9: Interactivity and environmental parameters. Figure 10 depicts the third application, which is an XHTML document with AAML. The document contains a table with horizontally aligned cells. A fragment of the code is represented in Figure 11. Each of the cells has an image and an audio clip. In this case, the audio elements will be positioned according to position of the cells instead of having an absolute position as in SMIL documents. When the user scrolls to see the images, the relevant audio clip will be audible, causing the others to be silent due to distance attenuation. This demonstrates reusability of the audio module, and the use of audio in static documents.
Sound1
Sound2
Sound3
Figure 10: Screenshot of a scrollable XHTML document. ...
Figure 11: XHTML document with spatially positioned sound. 10
The demonstrations were tried out with a typical stereo audio card. It clearly showed that horizontal positioning along the x-axis was audible. However, y- or z-axis position could only be heard as attenuation of the sound. Of course, by using a 5.1-channel or other advanced audio system, the effect of spatialization would be stronger. 6 CONCLUSIONS The current SMIL 2.0 standard supports only simple audio output without any spatial control. The MPEG-4 AABIFS takes a more comprehensive approach, describing spatialized interactive audio objects with effects, such as room acoustics. A new XML based language for 3D audio was extended to the SMIL language. The new language allows spatial positioning of audio objects in three dimensions. Also, the listening point can be positioned in the same space. The perceptual environment acoustics can be altered to mimic real world acoustics. The new language can also be used with any other rendered XML language, such as XHTML or SVG. Their layout can be extended using the proposed approach, or left intact, if two-dimensional positioning is adequate. Otherwise, the new elements can just be included as media objects in the host language. In the future, better control over directivity of the sound and listener could be provided. Also, the elements and attributes could be made more similar to those in AABIFS. However, this might bring up problems with licensing, as the W3C’s standards, such as SMIL, are open standards, while MPEG-4 and AABIFS are strictly licensed by the MPEG experts group. REFERENCES Creative Technology Ltd. 2001. Environmental Audio Extensions: EAX 2.0. EAX 2.0 Extensions SDK. Referenced Dec. 2002. Available at http://developer.creative.com/. Goose, S.; Kodlahalli, S.; Pechter, W.; Hjelsvold, R. 2002. Streaming Speech: A Framework for Generating and Streaming 3D Text-To-Speech and Audio Presentations to Wireless PDAs as Specified Using Extensions to SMIL. Proceedings of the Eleventh International World Wide Web Conference. Honolulu, Hawaii, May 7-11., 2002, pp. 37-44. Kim, M.; Wood, S.; Cheok, L. 2000. Extensible MPEG-4 Textual Format (XMT). Proceedings of the 2000 ACM Workshops on Multimedia. LA, California, USA, October 30 – November 4., 2000, pp. 71-74. Pihkala, K. and Vuorimaa, P. 2002. Design of a Dynamic SMIL Player. Proceedings of the IEEE International Conference on Multimedia and Expo. Lausanne, Switzerland, August 26-29., 2002. pp. 189-192. Rutledge, L. 2002. SMIL 2.0: XML for Web multimedia. IEEE Internet Computing. Vol. 5, No. 5, pp. 78-84.
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Scheirer, E. D.; Väänänen, R.; Huopaniemi, J. 1999. AudioBIFS: Describing Audio Scenes with the MPEG-4 Multimedia Standard. IEEE Transactions on Multimedia. Vol. 1, No 3, pp. 237-250. Trivi, J.-M. and Jot, J.-M. 2002. Rendering MPEG-4 AABIFS Content Through A LowLevel Cross-Platform 3D Audio API. Proceedings of the IEEE International Conference on Multimedia and Expo. Lausanne, Switzerland, August 26-29., 2002. pp. 513-516. Vuorimaa, P.; Ropponen, T.; von Knorring, N.; Honkala, M. 2002. A Java based XML browser for consumer devices. The 17th ACM Symposium on Applied Computing. Madrid, Spain, March 10-13., 2002. pp. 1094-1099.
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APPENDIX A: ADVANCED AUDIO MARKUP LANGUAGE (AAML) Version 1.0, Kari Pihkala, Helsinki University of Technology, Dec. 2002. AAML Namespace: http://www.x-smiles.org/2002/aaml This table gives an overview of the elements and attributes supported by the AAML. The environmental attributes are mapped one-to-one to the EAX 2.0 properties. For details, see Creative’s Environmental Audio Extensions: EAX 2.0, available at http://developer.creative.com. * indicates that the value can be modified at the run-time.
rotate
scale
audio3d
Ele me nt
Attribute
Values
Default Value
Description
Function: Play 3d audio, placement by 1) region 2) left, top, front attrs 3) CSS src null URL of the audio file (currently supports only file: URL scheme) doppler* true/false false Apply doppler effect on this audio (average of 3 previous samples, required by inprecise Java) region Region describing the position for audio left* X coordinate, if region not specified top* Y coordinate, if region not specified front* Z coordinate, if region not specified loop true/false false true sets this audio sample looping coord* local/ local Coordinate system. local = relative to window, global = relative to screen global minDistance [0, 100 If listener is closer than this to audio, max * 100000] volume is played maxDistance [0, 1000 If listener is further than this to audio, outLevel * 100000] volume is played outLevel* [-100.0, 0.0 Volume outside maximumDistance, in decibels 0.0] debug true/false false If true, renders a simple rectangle into region. This is for debugging purposed to see the position of audio. Function: Scale 3D audio coordinates, child of audio3d x* 1 Scale x coord y* 1 Scale y coord z* 1 Scale z coord Function: Rotate 3D audio coordinates, by default around y coord, child of audio3d (not impl’ed) x* [0.0, 1.0] 0 X axis of rotation y* [0.0, 1.0] 1 Y axis of rotation z* [0.0, 1.0] 0 Z axis of rotation angle* [0, 360] 0 Angle to rotate
translate listener
Function: Specifies the listener position in the presentation (one per document) left* wth/2 X coordinate, if region not specified top* gth/2 Y coordinate, if region not specified front* 30 Z coordinate, if region not specified doppler* true/false false Apply doppler effect on the listener (average of three previous velocity samples, required by imprecise Java timing) distanceFac [0.0, 1.0 AAML uses meters as the default unit of tor* 10.0] distance measurements, 1m=1pixel. DistanceFactor can be used to change this (factor 1 means 1m=2pixels) – Does not work? dopplerFact [0.0, 1.0 0 means no Doppler shift, other values or* 10.0] represents a multiple of the real-world Doppler shift. 1=normal real world shift. rolloffFact [0.0, 1.0 Rolloff is the amount of attenuation that is or* 10.0] applied to sounds, based on the listener's distance from the sound source. 2 means two times the real-world rolloff. Function: Specifies the environment for audio elements (only one per document) preset* “generic” Specifies the preset environment name, one of the following: "generic", "room","bathroom","livingroom", "stoneroom","auditorium","concerthall", "cave","arena","hangar","carpetedhallway", "hallway","stonecorridor","alley","forest", "city","mountains","quarry","plain", "parkinglot","sewerpipe","underwater", "drugged","dizzy","psychotic" room* [-100.00, The master volume control for the reflected 0.0] sound (both reflections and reverberation). In decibels, 0 dB (the maximum amount) to -100 dB (no reflected sound at all). roomHF* [-100.00, Attenuation of high frequencies in the 0.0] reflected sound, see room attribute. roomRolloff [0.0, Defined the same way as listener element's * 10.0] rolloffFactor, but operates on reflected sound instead of direct-path sound. Setting the Room Rolloff Factor value to 1.0 specifies that the reflected sound will decay by 6 dB every time the distance doubles. decayTime* [0.1, The reverberation decay time. It ranges from 20.0] 0.1 (typically a small room with very dead surfaces) to 20.0 (typically a large room with very live surfaces).
environment
Function: Translate 3D audio coordinates, child of audio3d x* 0 Move x coord y* 0 Move y coord z* 0 Move z coord
decayHFRati o*
[0.1, 2.0]
-
reflections *
[-100.00, 10.0]
-
reflections Delay*
[0.0, 0.3]
-
reverb*
[-100.00, 20.00]
-
reverbDelay *
[0.0, 0.1]
-
environment Size* environment Diffusion*
[1.0, 100.0] [0.0, 1.0]
-
airAbsorpti on*
[-100.00, 0.0]
-
-
Allows setting the decay time ratio of low and high frequencies. The Decay HF Ratio value 1.0 is neutral: the decay time is equal for all frequencies. Decay HF Ratio above 1.0, means the high-frequency decay time is longer than the decay time at low frequencies. On the other hand, values below 1.0 give more natural effects. Controls the overall amount of initial reflections relative to the room attribute. Maximum of 10 dB gives maximum initial reflections, while a minimum of -100 dB gives no initial reflections at all. The amount of delay between the arrival time of the direct path from the source to the first reflection from the source. It ranges from 0 to 0.3 seconds. Controls the overall amount of later reverberation relative to the room attribute. The value of reverb ranges from a maximum of 20 dB to a minimum of -100 dB (no late reverberation at all). Defines the begin time of the late reverberation relative to the time of the initial reflection (the first of the early reflections). It ranges from 0 to 100 milliseconds. Reducing or increasing Reverb Delay is useful for simulating a smaller or larger room. The apparent size of the surrounding "room", in meters. Controls the echo density in the reverberation decay. It is set by default to 1.0, which provides the highest density. Reducing diffusion gives the reverberation a more "grainy" character that is especially noticeable with percussive sound sources. If you set a diffusion value of 0.0, the later reverberation sounds like a succession of distinct echoes. Controls the distance-dependent attenuation at high frequencies caused by the propagation medium. It applies to both the direct path and reflected sound. You can use Air Absorption HF to simulate sound transmission through foggy air, dry air, smoky atmosphere, and so on. The default value is 0.05 dB per meter, which roughly corresponds to typical condition of atmospheric humidity, temperature, and so on.
scaleDecayT ime*
true/false
false
scaleReflec tions*
true/false
false
scaleReflec tionsDelay*
true/false
false
scaleReverb *
true/false
false
scaleReverb Delay*
true/false
false
clipDecayHF *
true/false
false
A change in Environment Size value causes a proportional change of the property Decay Time. If it is FALSE, a change in Environment Size has no effect on Decay Time. If both this flag and the Reflections Delay Scale flag are TRUE, an increase in Environment Size value causes an attenuation of the property Reflections. If this flag is TRUE, a change in Environment Size value causes a proportional change of the property Reflections Delay. If this flag is TRUE, an increase in Environment Size value causes an attenuation of the property Reverb. If this flag is TRUE, a change in Environment Size value causes a proportional change of the property Reverb Delay. If this flag is TRUE, high-frequency decay time automatically stays below a limit value that’s derived from the setting of the property Air Absorption HF.
