The introduction of the compact
disc into widespread use during the early 1980's marked the start of a new era in the technology of sound reproduction. The ability to capture
and store acoustic signals in digital format produced a massive advance
in the quality with which sound could be reproduced. At the Institute of
Sound and Vibration Research (ISVR) at the University of Southampton,
the latest research has shown that digital technology can also be used
to produce a further vast improvement in the quality of sound reproduction
systems. The work has concentrated on improving the ability of audio systems
to produce "images" of sound sources perceived by the listener. In short,
we try to produce the illusion in a listener of being in a "virtual" acoustic
environment which is entirely different from that of the space in which
he (or she) is actually located. We are thus attempting to achieve the
long sought-after goal of making a listener in his living room hear sound
as if he were in a concert hall. The availability of modern electronic technology
for processing acoustic signals digitally has
transformed
our ability to generate this illusion, almost irrespective of the environment
(living room, office or automobile interior) which surrounds the listener.
The approach that we take is to process acoustic signals prior to their
transmission by loudspeakers. We undertake this processing in order to
generate the illusion in the listener that sound is coming from a number
of "virtual" sources in well defined spatial positions relative to the
listener. Of course, the intention of conventional "stereo" sound reproduction
by loudspeakers is to produce just such an illusion, but two channel stereophony
is capable only of producing acoustic virtual source images over a very
narrow range of spatial positions, these being restricted to positions
in the plane of, and in between, the two loudspeakers used for reproduction.
The use of modern signal processing techniques can remove this restriction,
even when only two loudspeakers are used for reproduction. A number of
approaches to "3D Audio" have been developed in recent years, but few have
correctly tackled the basic signal processing problem that has to be solved.
This is the design of a processing scheme that ensures that the correct
signals are produced at the listener's ears. In order to achieve such a
goal, the processing scheme has to account for the effect on the signal
of the loudspeakers, of the transmission path (including room reflections),
and of the effect of the listeners head and torso on the propagation of
sound to the ears. The central problem to be tackled is one of "inversion"
where all these effects have to be "turned upside down" (and thus compensated
for) before the signals are transmitted by the loudspeakers. This is a
problem with many technical subtleties, but by tackling it correctly, it's
solution can produce remarkable results.
Below we describe some of the ISVR
research in this field, much of which has been undertaken jointly with
Professor Hareo Hamada's group at Tokyo Denki University (TDU) in
Japan. In particular we highlight the development of a very specific virtual
imaging technique based on the "Stereo Dipole" which uses a pair of very
closely spaced loudspeakers to produce virtual images. The major advance
associated with this work is associated with the precise form of the acoustic
field produced by the loudspeakers which makes it an ideal means for producing
virtual images. The "Stereo Dipole" can be spectacularly convincing
and we expect this technology to come into widespread use in the not too
distant future.
Other topics addressed will include
a description of the physical form of the acoustic fields produced by such
imaging systems, the signal processing technology required and the influence
of the environment in which the imaging is undertaken. There are many factors
controlling the success of these systems, not least of which is the complex
processing of acoustic signals that occurs in the auditory system of the
listener. However, many of the features of the performance of virtual acoustic
imaging systems can be explained in terms of the known behaviour of the
human auditory system in localising acoustic sources. In addition to this,
we are very much aware of commercial developments associated with multi-channel
"surround sound" systems for "home cinema", and we are working on multi-channel
signal processing techniques which could one day greatly enhance our ability
to record and reproduce sound fields with multiple loudspeakers. We hope
you enjoy visiting our web site and that you can share some of the fascination
which makes this multi-disciplinary research such an enjoyable field in
which to work. |