[georgia]

0. INTRODUCTION
Typical production techniques involve only one or two inputs to the room simulation
system, thereby limiting the precision of source positioning to be only a matter of send
level differences and power panning. This “one source - one listener” model is not very
satisfying when producing for mono or stereo, but even worse when the reproduction
system is multichannel.
Multichannel recording and reproduction is an opportunity for the production engineer to
discriminate deliberately between scenes or instruments heard from a distance, and
sources directly engaging the listener.
For film work, engaging audio has a very pronounced effect for stimulating the viewer
emotionally, and may therefore significantly add to the illusion presented by the picture.
In the search of more authenticity in artificial room generation, long term studies of
natural early reflection patterns have led us to propose new production and algorithm
techniques. Using ray tracing in conjunction with careful adjustments by ear, we have
achieved simulation models with higher naturalness and flexibility, which is the basis of
true source positioning.
The paper will discuss two aspects of precise room simulation for multi source,
multichannel environments to cover distant and engaged listening:
l Present different production techniques
l Describe an algorithm structure to achieve the objectives
I. SINGLE SOURCE REVERB
By having only one or two inputs in a room simulator, the rendering is based upon
multiple sources sharing the same early reflection pattern, and therefore it is not really
convincing.
In the real world, all actors or instruments are not piled up on top of each other.
I. I Music Production
In many studios, one good reverb is used to render the basic environment of a particular
mix. One aux send, set at different levels on the different channels, is used to obtain depth
and some complexity in the sound image.
To obtain a sound image of a higher complexity and depth, several auxes and reverbs
have normally been used. Tuning of the levels, pans and reverb parameters in such a setup
may be very time-consuming.
For effect purposes, anything goes, but if the goal is a representation of a natural room or
a consistent rendering of a virtual room, it may be hard to achieve using conventional
reverbs .
I .2 Film and Post Production
For applications where picture is added to the sound, several psychological studies have
proven audio to be better at generating entertainment pleasure and emotions than visual
inputs. When it comes to counting neurological synapses to the brain, vision has long
been known to be our dominant input source. However, a study by Karl Ktipfmtiller [4]
has suggested, that stimulation of even our conscious mind is almost equally well
achieved from visual compared to auditive inputs.
Sense No of Synapses Conscious input, bps
Eye IO. 000.000 40
Ear 100.000 30
Skin I. 000.000 5
Smell 100.000 1
Taste 1.000 I
Stimulation of conscious mind [4]
Realism in audio is just as important when it is accompanied by picture.
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In multichannel work for film, several reverbs configured as mono in - mono out are often
used on discrete sources. By doing so, the direct sound and the diffused field are easy to
position in the surround environment. The technique is therefore especially effective for
point source distance simulation.
As an alternative, several stereo reverbs are used on the same sources to achieve a number
of de-correlated outputs routed to different reproduction channels.
With both approaches, adjustments can be very time-consuming, and a truly engaging
listening experience is difficult to achieve.
2. MULTIPLE SOURCE ROOM SIMULATION
To obtain the most natural sounding and precise room simulation, an artificial reverb
system should be based upon positioning of multiple sources in a virtual room. Each
source should have individual early reflection properties with regards to timing, direction,
filtering and level.
We have found this to be true for both stereo and multichannel presentations.
If the target format is 5.1, at least two directional configurations should exist in the room
simulator, namely for home (110 degree surround speakers) and theatre (side array
surround speakers) reproduction.
The room simulator should also be flexible enough to easily adopt to new multichannel
formats, e.g. the Dolby EX scheme.
By changing the production technique slightly, multiple sends from e.g. the Auxes, Group
busses or Direct outs of the mixing console can be used to define several discreet
positions as inputs to the room simulation system.
From a production point of view, multiple source room simulation can be configured two
ways, as described below. Any large scale console build for stereo production can adapt
to both routing schemes.
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2. I The Additive Approach
The conventional approach to reverb is additive. Dry signals are fed to the reverb system,
and wet-only signals are returned and added at the mixer.
With a multiple input room simulator, this configuration works much better than with an
single source reverb, because at least each source can be approximated to fit the nearest
position rendered. However, normal power panning still needs to be applied in the mixer.
An even more precise rendering can be achieved using the integrated approach described
below.
2.2 The Integrated Positioning Approach
The sources in a mix needing the most precise positioning and room simulation, should be
treated this way:
The source is completely positioned and rendered into a precise position by passing the
dry signal through the simulation system, from which a composite output from a number
of source positions are available.
XY positioning to any target format, stereo or multichannel, will be rendered as a best fit.
The positioning parameters (replacing conventional power panning) can be controlled
from a screen, a joystick or discrete X and Y controls.
With all positioning done in the room simulator, consoles made for stereo production may
thereby overcome some of their limitations.
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3. ALGORITHM STRUCTURE
This part of our paper describes a generic algorithm currently in use for Multichannel
Room Simulator development. It is not a description of any particular present or future
product, but rather a presentation of the framework and way-of-thinking that has produced
our latest Room Simulation products and is expected to produce more in the future.
3. I Design conditions
The overall system requirements can be stated as follows:
l The system must be able to produce a natural-sounding simulation of a number of
sources in acoustic environments ranging from “phone-booth” to “canyon”
l 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.
l The system should be able to render the simulation via a number of different
reproduction setups, e.g. 5.1,7.1, stereo etc.
l 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.
l 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/E3 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.
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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 Oth 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 [l] 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.
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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 T,(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 T,(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.
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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 107*
AES Convention in about 4 months, we will have more real life experience with the
system.
If integrated positioning is used with multi-source room simulation, our experiments have
already shown how much there is to gain in terms of realism and working speed. But even
with the less radical additive approach, virtual rooms may be rendered more convincingly
with multi-source simulators.
For applications where picture is added to the sound, the most stimulating source will be
one, where audio and video are treated with equal attention to quality and detail.
The new possibilities available from multi-source room processors may be exploited to
generate a real quality improvement at the end listener, especially when his reproduction
system is multichannel.
More convincing sound generates more convincing picture.

cheers
geo