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ISO/IEC 14496-3:2005(E)
5.15.3.4 AudioFX orchestra execution
The run-time synthesis process proceeds according to the rules cited in subclause 5.7.3.3 for a standard SA
decoding process, with the following exceptions and additions:
As no access units will be received by an AudioFX process, no communication with the systems layer need be
maintained for this purpose. The only events used are those that are in the score field of the node itself. At each
time step, the AudioFX orchestra shall request from the systems layer the input audio buffers that correspond to
the child nodes. These audio buffers shall be placed on the special bus input_bus and then sent to whatever
instruments are specified in the global orchestra header.
Also, at each control-rate step, the params[] fields of the AudioFX node shall be copied into the global params[]
array of the orchestra. These fields are exposed in the scene graph so that interactive aspects of other parts of the
scene graph may be used to control the orchestra. At the end of each control cycle, the params[] array values
shall be copied back into the corresponding fields of the AudioFX node and then routed to other nodes as
specified within the scene graph. (It is not possible to give a more semantically meaningful field name than
params since the purpose of the field may vary greatly from application to application, depending on the needs of
the content).
At every point in time, the output of the orchestra becomes the output of the AudioFX node.
5.15.3.5 Speed change functionality in the AudioFX node
Speed change functionality for sounds provided from the input sources is supported in the AudioFX node. The
SAOL core opcode fx_speedc is provided for this purpose; see subclause 5.9.14.4.
5.15.4 Interactive 3-D spatial audio scenes
When an AudioSource or AudioFX node is the child of a Sound node, the spatial location, direction, and
propagation pattern of the sound subtree represented at the position of the Sound node, and the spatial location
and direction of the listener, are provided to the SAOL code in the node. In this way, subtle spatial effects such as
source directivity modelling may be written in SAOL.
The standard names position, direction, listenerPosition, listenerDirection, minFront, maxFront, minBack,
and maxBack (see subclauses 5.8.6.8.18-5.8.6.8.25) are used for this purpose.
It is not recommended that content providing 3-D spatial audio in the context of audio-visual virtual reality
applications in BIFS use the spatialize statement within SAOL to provide this functionality. In most terminals, the
scene-composition 3-D audio functionality will be able to use more information about the interaction process to
provide the best-quality audio rendering. In particular, spatial positioning and source directivity are implemented at
the end terminal with a sophistication suitable for the terminal itself (see ISO/IEC 14496-1, Sound node
specification, subclause 9.4.2.82). Content authors can use SAOL and the AudioFX node to create enhanced
spatial effects that include reverberation, environmental attributes and complex attenuation functions, and then let
the terminal-level spatial audio presentation be used to interface with the available rendering method for the
terminal. The spatialize statement in SAOL is provided for the creation of non-interactive spatial audio effects in
musical compositions, so that composers may tightly integrate the spatial presentation with other aspects of the
musical material.
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