592 Chapter 19
transcription. ” In the meantime, composers and sound designers will use special content
creation tools to directly create SA bit streams. This is not considered to be a fundamental
obstacle to the use of MPEG-4 structured audio because these tools are very similar to
the ones that contemporary composers and editors use already; all that is required is to
make their tools capable of producing MPEG-4 output bit streams. There is an interesting
corollary here with MPEG-4 for video. For, while we are not yet capable of integrating
and coding real-world images and sounds, there are immediate applications for directly
synthesized programs. MPEG-4 audio also foresees the use of text-to-speech conversion
systems.
19.7.3 Audio Scenes
Just as video scenes are made from visual objects, audio scenes may be usefully
described as the spatiotemporal combination of audio objects. An “ audio object ” is a
single audio stream coded using one of the MPEG-4 coding tools, such as structured
audio. Audio objects are related to each other by mixing, effects processing, switching,
and delaying them, and may be panned to a particular three-dimensional location. The
effects processing is described abstractly in terms of a signal-processing language—the
same language used for SA.
19.8 Digital Audio Production ......................................................................................
We’ve already looked at the technical advantages of digital signal processing and
recording over its older analogue counterpart. We now come to consider the operational
impact of this technology, where it has brought with it a raft of new tools and some new
problems.