43
IN
DEPTH
ROOM SIMULATION FOR MULTICHANNEL MUSIC AND FILM
In the real world, all actors or instruments are not piled up
on top of each other.
1.1 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 aux's 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.
1.2 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
Küpfmüller [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
10.000.000
40
Ear
100.000
30
Skin
1.000.000
5
Smell
100.000
1
Taste
1.000
1
Stimulation of conscious mind [4]
Realism in audio is just as important when it is
accompanied by picture.
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 aux's, 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.
2.1 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.
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.
3.1 Design conditions
The overall system requirements can be stated as follows:
• The system must be able to produce a natural-sounding
simulation of a number of sources in acoustic
