Multidirectional audio decoding

Details Multidirectional audio decoding Multidirectional audio decoding

1997-03-14

2002-09-10

Isen, Forester W. (Department: 2747)

Electrical audio signal processing systems and devices

Binaural and stereophonic

Reexamination Certificate

active

06449368

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to multidirectional audio decoding. More particularly, the invention relates to a computer-software-implemented acoustic-crossfeed canceller using very low processing resources of a personal computer for use in a multidirectional audio decoding and presentation system.
BACKGROUND OF THE INVENTION
Multichannel audio for personal computer-based multimedia video games, CD ROMs, Internet audio and the like (often referred to as “multimedia audio”) has emerged as a new application for the Dolby Surround and Dolby Digital multichannel sound encoding and decoding systems.
Dolby Surround, based on the use of a 4:2:4 amplitude-phase matrix, has heretofore become well known as a system for encoding four audio channels (left, right, center and surround) on two channel audio media (cassettes and compact discs), radio transmissions and the audio portions of video recordings (video tapes and laser discs), and television broadcasts, and for decoding therefrom. Dolby Surround (and Dolby Surround Pro Logic, which employs an active surround decoder to enhance channel separation) is widely used in home theatre systems, typically requiring a minimum of three loudspeakers (left and right loudspeakers positioned adjacent to the picture display and one surround loudspeaker, behind the audience) and preferably four loudspeakers (two surround loudspeakers instead of one, located at each side of the audience). Ideally, even a fifth loudspeaker is used, to provide a “hard” center channel reproduction.
Dolby Digital employs the Dolby AC-3 digital audio coding technology in which 5.1 audio channels (left, center, right, left surround, right surround and a limited-bandwidth subwoofer channel) are encoded on a bit-rate reduced data stream. Dolby Digital, a newer technology than Dolby Surround, is already widely used in home theatre systems and has been chosen as the audio standard for the digital video disc (DVD) and high definition television (HDTV) in the United States. In a home theatre environment, Dolby Digital requires a minimum of four loudspeakers because it renders two surround channels instead of one.
In the personal computer “multimedia” environment, typically only two loudspeakers are employed, left and right speakers located adjacent to or near the computer monitor (and, optionally, a subwoofer, which may be remotely located, such as on the floor—in the present discussion, the subwoofer is ignored). When presented over the left and right speakers via conventional means, stereo material generally produces sonic images that are constrained to the speakers themselves and the space between them. This effect results from the crossfeed of the acoustic signal from each speaker to the far ear of a listener positioned in front of the computer monitor. Acoustic cancellation and arbitrary source position rendering are aspects of the same common process.
To reproduce Dolby Surround encoded material in a computer environment, certain prior art arrangements employ multiple loudspeaker drivers within a single enclosure in order to simulate the use of multiple loudspeakers. See, for example, U.S. Pat. No. 5,553,149, which is hereby incorporated by reference in its entirety.
Other prior art arrangements have proposed the use of sound image processing employing acoustic-crossfeed cancellation to render the perception that the surround sound information is coming from virtual loudspeaker locations behind or to the side of a listener when only two forward-located loudspeakers are employed. See, for example, published European Patent Application EP 0 637 191 A2 and published International Application WO 96/96515. The origin of the acoustic-crossfeed canceller is generally attributed to B.S. Atal and Manfred Schroeder of Bell Telephone Laboratories (see, for example, U.S. Pat. No. 3,236,949, which is hereby incorporated by reference in its entirety). As originally described by Schroeder and Atal, the acoustic crossfeed effect can be mitigated by introducing an appropriate cancellation signal from the opposite speaker. Since the cancellation signal itself will crossfeed acoustically, it too must be canceled by an appropriate signal from the originally-emitting speaker, and so on.
The present invention is directed to an acoustic crossfeed canceller which may be implemented using very low processing resources of a personal computer particularly for use in a multidirectional audio decoding and presentation system such as a computer multimedia system having only two main loudspeakers.
SUMMARY OF THE INVENTION
In accordance with the present invention, an acoustic crossfeed canceller is provided, intended for implementation in software, such that when run in real time on a personal computer, the canceller has very low mips requirements and uses a small fraction of available CPU cycles. Thus, for example, the program could be included with video games, CD ROMs, Internet audio and the like, rendering surround sound images outside the space between left and right computer multimedia loudspeakers when the audio from such sources is reproduced.
In an ideal reproduction system, if a source recording has M channels, each having an associated source direction, the listener should perceive these M channels reproduced from their respective M source directions. In practical reproduction systems, the M source channels are reproduced by N presentation channels or loudspeakers, each having a position with respect to the original source directions and with respect to one or more listeners (each stationary listener having a listening position P at each ear). The overall system may be expressed as:
M
[C]
N
[R]
P
where [C] is an M×N port filter network C which processes or maps the M source channels to the N presentation channels (i.e., linear, time-invariant mapping) and [R] is an N×P port filter network R which processes or maps the N presentation channels to P listening positions (also linear, time-invariant mapping).
The filter network R may be represented by a room matrix R of filter responses or transfer functions (in practice, head related transfer functions or HRTFs), determined by measuring or estimating the transfer function from each of the N presentation channels to each of the P listening positions, forming an N×P matrix of transfer functions, each of which may include the effects of loudspeaker response deviations, room acoustics, delays, echoes, possible head shadow, etc.:
R
≡
[
r
11
r
12
…
r
1
⁢
p
r
21
r
22
…
r
2
⁢
p
⋯
⋯
⋯
⋯
r
n1
r
n2
…
r
np
]
,
where the matrix elements r
11
. . . r
np
are individual filter responses representing the transfer function from each presentation channel to each listening position. If the matrix elements r
11
. . . r
np
are frequency domain transfer functions expressed, for example, as fast fourier transforms (FFTs), standard matrix operations (addition, multiplication, etc.) may be accomplished with the matrix. In accordance with the present invention, the room matrix may be simplified by ignoring all but the time delay and frequency dependent attenuation in the direct acoustic path between each presentation channel and each listening position and by smoothing the attenuation response throughout at least a substantial portion of the audio sound spectrum intended to be reproduced by said presentation channels.
The filter network C constitutes an acoustic crossfeed canceller and may be represented by a cancellation matrix C of filter responses or transfer functions:
C
≡
[
c
11
c
12
…
c
1
⁢
n
c
21
c
22
…
c
2
⁢
n
⋯
⋯
⋯
⋯
c
m1
c
m2
…
c
m
⁢
 
⁢
n
]
,
where the matrix elements c
11
. . . c
mn
are individual filter responses. If the matrix elements c
11
. . . c
mn
are frequency domain transfer functions expressed, for example, as fast fourier transforms (FFTs), standard matrix operations (addition, multiplication, etc.) may be accomplished with the matrix.
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