Electrical audio signal processing systems and devices – Binaural and stereophonic – Stereo speaker arrangement
Reexamination Certificate
1998-07-09
2004-02-17
Isen, Forester W. (Department: 2644)
Electrical audio signal processing systems and devices
Binaural and stereophonic
Stereo speaker arrangement
C380S044000, C380S044000, C380S044000, C380S022000, C380S044000
Reexamination Certificate
active
06694033
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the reproduction of spatialised audio in immersive environments with non-ideal acoustic conditions.
2. Related Art
Immersive environments are expected to be an important component of future communication systems. An immersive environment is one in which the user is given the sensation of being located within an environment depicted by the system, rather than observing it from the exterior as he would with a conventional flat screen such as a television. This “immersion” allows the user to be more fully involved with the subject material. For the visual sense, an immersive environment can be created by arranging that the whole of the user's field of vision is occupied with a visual presentation giving an impression of three dimensionality and allowing the user to perceive complex geometry.
For the immersive effect to be realistic, the user must receive appropriate inputs to all the senses which contribute to the effect. In particular, the use of combined audio and video is an important aspect of most immersive environments: see for example:
ANDERSON. D. & CASEY. M. “
Virtual worlds—The sound dimension” IEEE Spectrum
1997, Vol. 34, No 3, pp 46-50:
BRAHAM. R. & COMERFORD. R. “
Sharing virtual worlds” IEEE Spectrum
1997, Vol. 34, No 3, pp 18-20
WATERS. R & BARRUS. J “The rise of
shared virtual environments” IEEE Spectrum
1997, Vol. 34, No 3, pp 20-25.
Spatialised audio, the use of two or more loudspeakers to generate an audio effect perceived by the listener as emanating from a source spaced from the loudspeakers, is well-known. In its simplest form, stereophonic effects have been used in audio systems for several decades. In this specification the term “virtual” sound source is used to mean the apparent source of a sound, as perceived by the listener, as distinct from the actual sound sources, which are the loudspeakers.
Immersive environments are being researched for use in Telepresence, teleconferencing, “flying through” architect's plans, education and medicine. The wide field of vision, combined with spatialised audio, create a feeling of “being there” which aids the communication process, and the additional sensation of size and depth can provide a powerful collaborative design space.
Several examples of immersive environment are described by D. M. Traill, J. J. Bowskill and P. J. Lawrence in “Interactive Collaborative Media Environments” (
British Telecommunications Technology Journal
Vol. 15, No. 4 (October 1997), pages 130 to 139. One example of an immersive environment is the BT/ARC VisionDome, (described on pages 135 to 136 and FIG. 7 of that article), in which the visual image is presented on a large concave screen with the users inside (see FIGS.
1
and
2
). A multi-channel spatialised audio system having eight loudspeakers is used to provide audio immersion. Further description may be found at:
A second example is the “SmartSpace” chair described on pages 134 and 135 (and FIG. 6) of the same article, which combines a wide-angle video screen, a computer terminal and spatialised audio, all arranged to move with the rotation of a swivel chair—a system currently under development by British Telecommunications plc. Rotation of the chair causes the user's orientation in the environment to change, the visual and audio inputs being modified accordingly. The SmartSpace chair uses transaural processing, as described by COOPER. D. & BAUCK. J. “
Prospects for transaural recording”, Journal of the Audio Engineering Society
1989, Vol. 37, No 1/2, pp 3-19, to provide a “sound bubble” around the user, giving him the feeling of complete audio immersion, while the wrap-around screen provides visual immersion.
Where the immersive environment is interactive, images and spatialised sound are generated in real-time (typically as a computer animation), while non-interactive material is often supplied with an ambisonic B-Format sound track, the characteristics of which are to be described later in this specification. Ambisonic coding is a popular choice for immersive audio environments as it is possible to decode any number of channels using only three or four transmission channels. However, ambisonic technology has its limitations when used in telepresence environments, as will be discussed.
Several issues regarding sound localisation in immersive environments will now be considered.
FIGS. 1 and 2
show a plan view and side cross section of the VisionDome, with eight loudspeakers (
1
,
2
,
3
,
4
,
5
,
6
,
7
,
8
), the wrap-around screen, and typical user positions marked. Multi-channel ambisonic audio tracks are typically reproduced in rectangular listening rooms. When replayed in a hemispherical dome, spatialisation is impaired by the geometry of the listening environment. Reflections within the hemisphere can destroy the sound-field recombination: although this can sometimes be minimised by treating the wall surfaces with a suitable absorptive material, this may not always be practical. The use of a hard plastic dome as a listening room creates many acoustic problems mainly caused by multiple reflections. The acoustic properties of the dome, if left untreated, cause sounds to seem as if they originate from multiple sources and thus the intended sound spatialisation effect is destroyed. One solution is to cover the inside surface of the dome with an absorbing material which reduces reflections. The material of the video screen itself is sound absorbent, so it assists in the reduction of sound reflections but it also causes considerable high-frequency attenuation to sounds originating from loudspeakers located behind the screen. This high-frequency attenuation is overcome by applying equalisation to the signals fed into the loudspeakers
1
,
2
,
3
,
7
,
8
located behind the screen.
Listening environments other than a plastic dome have their own acoustic properties and in most cases reflections will be a cause of error. As with a dome, the application of acoustic tiles will reduce the amount of reflections, thereby increasing the users' ability to accurately localise audio signals.
Most projection screens and video monitors have a flat (or nearly flat) screen. When a pre-recorded B-Format sound track is composed to match a moving video image, it is typically constructed in studios with such flat video screens. To give the correct spatial percept (perceived sound field) the B-Format coding used thus maps the audio to the flat video screen. However, when large multi-user environments, such as the VisionDome, are used, the video is replayed on a concave screen, the video image being suitably modified to appear correct to an observer. However, the geometry of the audio effect is no longer consistent with the video and a non-linear mapping is required to restore the perceptual synchronisation. In the case of interactive material, the B-Format coder locates the virtual source onto the circumference of a unit circle thus mapping the curvature of the screen.
In environments where a group of listeners are situated in a small area an ambisonic reproduction system is likely to fail to produce the desired auditory spatialisation for most of them. One reason is that the various sound fields generated by the loudspeakers only combine correctly to produce the desired effect of a “virtual” sound source at one position, known as the “sweet-spot”. Only one listener (at most) can be located in the precise sweet-spot. This is because the true sweet-spot, where in-phase and anti-phase signals reconstruct correctly to give the desired signal, is a small area and participants outside the sweet-spot receive an incorrect combination of in-phase and anti-phase signals. Indeed, for a hemispherical screen, the video projector is normally at the geometric centre of the hemisphere, and the ambisonics are generally arranged such that the “sweet spot” is also at the geometric centre of the loudspeaker array, which is arranged to be concentric with the screen. Thus, there can be no-one at the actual “sw
Hollier Michael Peter
Rimell Andrew
British Telecommunications public limited company
Grier L.
Isen Forester W.
Nixon & Vanderhye P.C.
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