TC Electronic Broadcast 6000 - Manual (Page 52)

52
paGe Head rOOm sImulaTIOn fOr mulTICHannel musIC and fIlm
• The system should not be limited to simulating natural
acoustics: Often quite unnatural reverb effects are
desired, e.g. for pop music or science fiction film effects.
• The system should be able to render the simulation via a
number of different reproduction setups, e.g. 5.1, 7.1,
stereo etc.
• The system should be modular so that new rooms, new
source positions in existing rooms, new source types or
new target reproduction setups can be added with
minimal change to existing elements.
• The system should be easily tuneable: In our experience,
no semi-automatic physical modeling scheme, however
elaborate, is likely to produce subjective results as good
as those obtained by skilled people tuning a user-friendly,
interactive development prototype by ear.
Fortunately there are a few factors that make the job easier
for us:
• There are no strict requirements for simulation accuracy:
Certainly not physical accuracy (the sound field around
the listener’s head), and not even perceptual accuracy
(the listener’s mental image of the simulated event and
environment). The listener has no way of A/B switching
between the simulation and the real thing, so only
credibility and predictability counts: The simulation must
not in any way sound artificial, unless intended to, and
the perceived room geometries and source positions
should be relatively, but not absolutely, accurate.
• Moore’s Law is with us. The continual exponential growth
in memory and calculation capacity available within a
given budget frame has two effects: It constantly
expands the practical limits for algorithm complexity, and
it makes it increasingly feasible to trade in a bit of code
overhead for improved modularity, tuneability, etc.
• There are physical modeling systems readily available,
which may provide a starting point for the simulation.
3.2 Block diagram
The overall block diagram of the Room Simulator is shown
in fig. 1. As often seen, the system is divided into two main
paths: An early reflections synthesis system consisting
of a so-called Early Pattern Generator (EPG) for each
source and a common Direction Rendering Unit (DRU)
that renders the early reflections through the chosen
reproduction setup. And a Reverb system producing the
late, diffuse part of the sound field. Note that - contrary to
what is normally the case - there is no direct signal path.
The dry source signals are merely 0th order reflections
produced by their respective EPGs. In the following, a
more detailed description of the individual blocks is given.
3.3 Early Pattern Generators
Each EPG takes one dry source input and produces a
large set of early reflections, including the direct signal,
sorted and processed in the following “dimensions”
• Level
• Delay
• Diffusion
• Color
• Direction
The Level and Delay dimensions are easily implemented
with high precision, the other 3 dimensions are each
quantized into a number of predefined steps, for instance
12 different directions. Normally, the direct signal will not
be subjected to Diffusion or Color. The quantization and
step definition of the Direction dimension must be the same
for all sources, because it is implemented in the common
Direction Rendering Unit. Physical modeling programs such
as Odeon [1] may provide an initial setting of the EPG.
3.4 Direction Rendering Unit
The purpose of this unit is to render a number of inputs
to an equal number of different, predefined subjective
directions-of-arrival at the listening position via the chosen
reproduction setup, typically a 5-channel speaker system.
Thus, the DRU may be a simple, general panning matrix,
a VBAP [2] system or an HRTF- or Ambisonics-based [3]
system.
3.5 Reverb Feed Matrix
The reverb feed matrix determines each source’s
contribution to each Reverberator input channel. Besides
gain and delay controls, some filtering may also be
beneficial here.
3.6 Reverberator
To ensure maximum de-correlation between output
channels, each has its own independent reverb “tail”
generator. Controllable parameters include:
Reverberation time as a function of frequency Tr(f)
• Diffusion
• Modulation
• Smoothness
We take particular pride in the fact that our “tail” can
achieve such smoothness in both time and frequency, and
that modulation may be omitted entirely. This eliminates the
risk of pitch distortion and even the slightest Doppler effect,
which tends to destroy focus of the individual sources in a
multichannel room simulator.
Again, an initial setting of Tr(f) may be obtained from
Odeon.
3.7 Speaker Control
This block is by default just a direct connection from input
to output. But it may also be used to check the stereo- and
mono compatibility of the final simulation result by applying
a down-mixing to these formats. Also it provides delay- and
gain compensation for non-uniform loudspeaker setups,
which may also - as a rough approximation - be used the
other way around to emulate non-uniform or misplaced
setups and thus check the simulation’s robustness to such
imperfections.
4. CONCLUSION
The system described above is evidently a very open
system under continual development. At the time of writing
these words, our test system is running in real time on a
multiprocessor SGI server with an 18-window graphical
user interface providing interactive access to approximately
2000 low- and higher-level parameters. However, this is
not the time or place to go into more details. When this
paper is presented at the 107th AES Convention in about