U.S. patent application number 08/988115 was filed with the patent office on 2001-11-15 for method of positioning sound image with distance adjustment.
Invention is credited to MUKOJIMA, MASAHIRO.
Application Number | 20010040968 08/988115 |
Document ID | / |
Family ID | 18253856 |
Filed Date | 2001-11-15 |
United States Patent
Application |
20010040968 |
Kind Code |
A1 |
MUKOJIMA, MASAHIRO |
November 15, 2001 |
METHOD OF POSITIONING SOUND IMAGE WITH DISTANCE ADJUSTMENT
Abstract
A sound apparatus is constructed for directing a sound image of
a virtual sound source at a designated source point to a listener
in a virtual sound field. In the sound apparatus, a database
provisionally memorizes acoustic transfer characteristics of the
virtual sound field in correspondence to reference source points
distributed radially around a center point of the listener. Left
and right filters respectively filter audio signals of left and
right channels according to the acoustic transfer characteristics
loaded from the database. A processor computes a leftward acoustic
direction from the designated source point to a left ear of the
listener, and computes a rightward acoustic direction from the
designated source point to a right ear of the listener. A
controller specifies a leftward reference source point coincident
with the leftward acoustic direction to load an effective acoustic
transfer characteristic corresponding to the leftward reference
source point from the database into the left filter, and specifies
a rightward reference source point coincident with the rightward
acoustic direction to load another effective acoustic transfer
characteristic corresponding to the rightward reference source
point from the database into the right filter. A feeder feeds an
audio signal of the left channel to the left filter and feeds
another audio signal of the right channel to the right filter to
thereby direct the sound image of the virtual sound source located
at the source point to the listener positioned at the center
point.
Inventors: |
MUKOJIMA, MASAHIRO;
(HAMAMATSU-SHI, JP) |
Correspondence
Address: |
GRAHAM & JAMES
801 S FIGUEROA STREET
14TH FLOOR
LOS ANGELES
CA
900175554
|
Family ID: |
18253856 |
Appl. No.: |
08/988115 |
Filed: |
December 10, 1997 |
Current U.S.
Class: |
381/17 ; 381/18;
381/310 |
Current CPC
Class: |
H04S 2420/01 20130101;
H04S 1/002 20130101 |
Class at
Publication: |
381/17 ; 381/18;
381/310 |
International
Class: |
H04R 005/00; H04R
005/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 12, 1996 |
JP |
8-332338 |
Claims
What is claimed is:
1. A method of positioning a sound image of a virtual sound source
relative to a listener in a virtual sound field by filtering audio
signals of left and right channels through left and right filters
which simulate acoustic transfer characteristics of the virtual
sound field and by provisionally memorizing the acoustic transfer
characteristics of the virtual sound field which are distributed
radially around a center point of the listener, the method
comprising the steps of: designating a source point at which the
virtual sound source is to be located within the virtual sound
field in terms of a geometric distance and a geometric direction
relative to the center point; computing a leftward acoustic
direction from the source point to a left ear of the listener
according to the geometric distance, the geometric direction and an
offset of the left ear from the center point; computing a rightward
acoustic direction from the source point to a right ear of the
listener according to the geometric distance, the geometric
direction and an offset of the right ear from the center point;
determining an effective acoustic transfer characteristic based on
the memorized acoustic transfer characteristics according to the
leftward acoustic direction so as to enable the left filter to
simulate said effective transfer characteristic; determining
another effective acoustic transfer characteristic based on the
memorized acoustic transfer characteristics according to the
rightward acoustic direction so as to enable the right filter to
simulate said another effective transfer characteristic; and
filtering an audio signal of the left channel through the left
filter and filtering another audio signal of the right channel
through the right filter to thereby direct the sound image of the
virtual sound source located at the source point to the listener
positioned at the center point.
2. A method of positioning a sound image of a virtual sound source
to a listener in a virtual sound field by filtering audio signals
of left and right channels through left and right filters which
simulate acoustic transfer characteristics of the virtual sound
field, the method comprising the steps of: provisionally memorizing
acoustic transfer characteristics of the virtual sound field
allotted to sample points distributed radially around a center
point of the listener; designating a source point at which the
virtual sound source is to be located within the virtual sound
field in terms of a geometric distance and a geometric direction
relative to the center point; computing a leftward acoustic
direction from the source point to a left ear of the listener
according to the geometric distance, the geometric direction and an
offset of the left ear from the center point; computing a rightward
acoustic direction from the source point to a right ear of the
listener according to the geometric distance, the geometric
direction and an offset of the right ear from the center point;
selecting a leftward sample point substantially coincident with the
leftward acoustic direction to determine an effective acoustic
transfer characteristic based on the acoustic transfer
characteristic allotted to the leftward sample point so as to
enable the left filter to simulate said effective transfer
characteristic; selecting a rightward sample point substantially
coincident with the rightward acoustic direction to determine
another effective acoustic transfer characteristic based on the
acoustic transfer characteristic allotted to the rightward sample
point so as to enable the right filter to simulate said another
effective transfer characteristic; and filtering an audio signal of
the left channel through the left filter and filtering another
audio signal of the right channel through the right filter to
thereby direct the sound image of the virtual sound source located
at the source point to the listener positioned at the center
point.
3. A method according to claim 2, wherein the step of selecting a
leftward sample point comprises selecting a pair of leftward sample
points which lie oppositely relative to the leftward acoustic
direction such that said effective acoustic transfer characteristic
is determined by interpolating the acoustic transfer
characteristics allotted to the pair of the leftward sample points,
and wherein the step of selecting a rightward sample point
comprises selecting a pair of rightward sample points which lie
oppositely relative to the rightward acoustic direction such that
said another effective acoustic transfer characteristic is
determined by interpolating the acoustic transfer characteristics
allotted to the pair of the rightward sample points.
4. A method according to claim 2, wherein the step of computing a
leftward acoustic direction comprises computing an azimuth
component of the leftward acoustic direction from the source point
to the left ear of the listener in a three-dimensional space of the
virtual sound field such that the leftward sample point is selected
substantially coincident with the azimuth component of the leftward
acoustic direction, and wherein the step of computing a rightward
acoustic direction comprises computing an azimuth component of the
rightward acoustic direction from the source point to the right ear
of the listener such that the rightward sample point is selected
substantially coincident with the azimuth component of the
rightward acoustic direction, thereby directing the sound image of
the virtual sound source to the listener in an azimuth direction of
the three-dimensional space.
5. A method according to claim 4, further comprising the steps of
computing an elevation component of an acoustic direction from the
source point to the listener according to the geometric distance
and the geometric direction, and filtering the audio signal
according to the elevation component of the acoustic direction so
as to direct the sound image of the virtual sound source to the
listener in an elevation direction of the three-dimensional
space.
6. An apparatus for positioning a sound image of a virtual sound
source at a designated source point to a listener in a virtual
sound field, comprising: a database that provisionally memorizes
acoustic transfer characteristics of the virtual sound field in
correspondence to reference source points distributed radially
around a center point of the listener; left and right filters that
respectively filter audio signals of left and right channels
according to the acoustic transfer characteristics loaded from the
database; a processor that computes a leftward acoustic direction
from the designated source point to a left ear of the listener, and
that computes a rightward acoustic direction from the designated
source point to a right ear of the listener; a controller that
specifies a leftward reference source point coincident with the
leftward acoustic direction to load an effective acoustic transfer
characteristic corresponding to the leftward reference source point
from the database into the left filter, and that specifies a
rightward reference source point coincident with the rightward
acoustic direction to load another effective acoustic transfer
characteristic corresponding to the rightward reference source
point from the database into the right filter; and a feeder that
feeds an audio signal of the left channel to the left filter and
feeds another audio signal of the right channel to the right filter
to thereby direct the sound image of the virtual sound source
located at the source point to the listener positioned at the
center point.
7. An apparatus for positioning a sound image of a virtual sound
source to a listener in a virtual sound field, comprising: a
database that provisionally memorizes a pair of leftward and
rightward acoustic transfer characteristics of the virtual sound
field in correspondence to each of sample points distributed
radially around a center point of the listener at a fixed radius,
the leftward acoustic transfer characteristic simulating a path
from each sample point to a left ear of the listener and the
rightward acoustic transfer characteristic simulating another path
from each sample point to a right ear of the listener; left and
right filters that respectively filter audio signals of left and
right channels according to the left and right acoustic transfer
characteristics loaded from the database; an input that designates
a source point at which the virtual sound source is to be located
within the virtual sound field in a distance which may be different
from the fixed radius relative to the center point; a processor
that computes a leftward acoustic direction from the source point
to the left ear of the listener and that computes a rightward
acoustic direction from the source point to the right ear of the
listener; a controller that specifies a leftward sample point
substantially coincident with the leftward acoustic direction to
load the leftward transfer characteristic corresponding to the
leftward sample point from the database into the left filter, and
that specifies a rightward sample point substantially coincident
with the rightward acoustic direction to load the rightward
acoustic transfer characteristic corresponding to the rightward
sample point from the database into the right filter; and a feeder
that feeds an audio signal of the left channel to the left filter
and feeds another audio signal of the right channel to the right
filter to thereby direct the sound image of the virtual sound
source located at the source point to the listener positioned at
the center point.
8. An apparatus for positioning a sound image of a virtual sound
source relative to a listener in a virtual sound field, comprising:
a database that provisionally memorizes acoustic transfer
characteristics of the virtual sound field in correspondence to
sample points distributed radially around a center point of the
listener at a fixed radius; left and right filters that
respectively filter audio signals of left and right channels
according to the acoustic transfer characteristics loaded from the
database; an input that designates a source point at which the
virtual sound source is to be located within the virtual sound
field in terms of a geometric distance which may be different from
the fixed radius and a geometric direction relative to the center
point; a processor that computes a leftward acoustic direction from
the source point to a left ear of the listener according to the
geometric distance, the geometric direction and an offset of the
left ear from the center point, and that computes a rightward
acoustic direction from the source point to a right ear of the
listener according to the geometric distance, the geometric
direction and an offset of the right ear from the center point; a
controller that specifies a leftward sample point coincident with
the leftward acoustic direction to load an effective acoustic
transfer characteristic corresponding to the leftward sample point
from the database into the left filter, and that specifies a
rightward sample point coincident with the rightward acoustic
direction to load another effective acoustic transfer
characteristic corresponding to the rightward sample point from the
database into the right filter; and a feeder that feeds an audio
signal of the left channel to the left filter and feeds another
audio signal of the right channel to the right filter to thereby
direct the sound image of the virtual sound source located at the
source point to the listener positioned at the center point.
9. An apparatus according to claim 8, wherein the controller
comprises means for specifying a pair of leftward sample points
which lie oppositely relative to the leftward acoustic direction
such that said effective acoustic transfer characteristic is
determined by interpolating the acoustic transfer characteristics
corresponding to the pair of the leftward sample points, and for
specifying a pair of rightward sample points which lie oppositely
relative to the rightward acoustic direction such that said another
effective acoustic transfer characteristic is determined by
interpolating the acoustic transfer characteristics corresponding
to the pair of the rightward sample points.
10. An apparatus according to claim 8, wherein the processor
comprises means for computing an azimuth component of the leftward
acoustic direction from the source point to the left ear of the
listener in a three-dimensional space of the virtual sound field
such that the leftward sample point is selected substantially
coincident with the azimuth component of the leftward acoustic
direction, and for computing an azimuth component of the rightward
acoustic direction from the source point to the right ear of the
listener such that the rightward sample point is selected
substantially coincident with the azimuth component of the
rightward acoustic direction, thereby directing the sound image of
the virtual sound source to the listener in an azimuth direction of
the three-dimensional space.
11. An apparatus according to claim 10, further comprising means
for computing an elevation component of an acoustic direction from
the source point to the listener according to the geometric
distance and the geometric direction, and means for filtering the
audio signal according to the elevation component of the acoustic
direction so as to position the sound image of the virtual sound
source relative to the listener in an elevation direction of the
three-dimensional space.
12. A machine readable medium for use in an apparatus having a CPU
and positioning a sound image of a virtual sound source relative to
a listener in a virtual sound field by filtering audio signals of
left and right channels through left and right filters which
simulate acoustic transfer characteristics of the virtual sound
field, the medium containing program instructions executable by the
CPU for causing the apparatus to perform the steps of:
provisionally memorizing acoustic transfer characteristics of the
virtual sound field allotted to sample points distributed radially
around a center point of the listener; designating a source point
at which the virtual sound source is to be located within the
virtual sound field in terms of a geometric distance and a
geometric direction relative to the center point; computing a
leftward acoustic direction from the source point to a left ear of
the listener according to the geometric distance, the geometric
direction and an offset of the left ear from the center point;
computing a rightward acoustic direction from the source point to a
right ear of the listener according to the geometric distance, the
geometric direction and an offset of the right ear from the center
point; selecting a leftward sample point substantially coincident
with the leftward acoustic direction to determine an effective
acoustic transfer characteristic based on the acoustic transfer
characteristic allotted to the leftward sample point so as to
enable the left filter to simulate said effective transfer
characteristic; selecting a rightward sample point substantially
coincident with the rightward acoustic direction to determine
another effective acoustic transfer characteristic based on the
acoustic transfer characteristic allotted to the rightward sample
point so as to enable the right filter to simulate said another
effective transfer characteristic; and filtering an audio signal of
the left channel through the left filter and filtering another
audio signal of the right channel through the right filter to
thereby direct the sound image of the virtual sound source located
at the source point to the listener positioned at the center
point.
13. A machine readable medium according to claim 12, wherein the
step of selecting a leftward sample point comprises selecting a
pair of leftward sample points which lie oppositely relative to the
leftward acoustic direction such that said effective acoustic
transfer characteristic is determined by interpolating the acoustic
transfer characteristics allotted to the pair of the leftward
sample points, and wherein the step of selecting a rightward sample
point comprises selecting a pair of rightward sample points which
lie oppositely relative to the rightward acoustic direction such
that said another effective acoustic transfer characteristic is
determined by interpolating the acoustic transfer characteristics
allotted to the pair of the rightward sample points.
14. A machine readable medium according to claim 12, wherein the
step of computing a leftward acoustic direction comprises computing
an azimuth component of the leftward acoustic direction from the
source point to the left ear of the listener in a three-dimensional
space of the virtual sound field such that the leftward sample
point is selected substantially coincident with the azimuth
component of the leftward acoustic direction, and wherein the step
of computing a rightward acoustic direction comprises computing an
azimuth component of the rightward acoustic direction from the
source point to the right ear of the listener such that the
rightward sample point is selected substantially coincident with
the azimuth component of the rightward acoustic direction, thereby
positioning the sound image of the virtual sound source relative to
the listener in an azimuth direction of the three-dimensional
space.
15. A machine readable medium according to claim 14, wherein the
steps further comprise computing an elevation component of an
acoustic direction from the source point to the listener according
to the geometric distance and the geometric direction, and
filtering the audio signal according to the elevation component of
the acoustic direction so as to position the sound image of the
virtual sound source relative to the listener in an elevation
direction of the three-dimensional space.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to a method of
positioning a sound image and a sound image positioning apparatus
for use in a three-dimensional sound system or else. More
particularly, the present invention relates to a method of
simulating acoustic transfer characteristics from a virtual sound
source in a virtual sound field.
[0003] 2. Description of Related Art
[0004] In a three-dimensional virtual reality system for example, a
sound image positioning apparatus is conventionally used as a means
for enhancing presence of virtual reality experience. In such a
system, a cubic sound field is generated by creating direction
perspective and distance perspective in auditory sensation by
producing audio signals from a monaural sound source through a
plurality of channels having time difference, amplitude difference,
and frequency characteristic difference based on binaural
technique. To be more specific, an input audio signal is attenuated
in a particular frequency component by a notch filter, for example,
to create elevation. The input audio signal is also converted by a
delay circuit into left channel and right channel signals having a
time difference, and is further given acoustic transfer
characteristic from a virtual sound source by a FIR (Finite Impulse
Response) filter. A parameter of the FIR filter is given from an
HRTF database storing head-related transfer functions (HRTF)
measured with using a dummy head in advance.
[0005] In the above-mentioned conventional sound image positioning
apparatus, it is impracticable to store the HRTFs corresponding to
all virtual sound source points included in a sound field.
Normally, only the transfer characteristics at points radially away
from a listener by a certain distance, for example one meter, are
measured and stored. Therefore, if a virtual sound source is
located one meter away from a listener as shown in FIG. 5, proper
sound image positioning can be provided. However, if the virtual
sound source is located from the listener at a distance more or
less one meter, problem occurs that sound images sensed by the
right and left ears of the listener do not match each other, losing
good positioning. Especially, it is known that the human ear has an
angular resolution of .+-.3.degree. for acoustic direction.
Therefore, if a virtual sound source passes across the listener, an
error higher than .+-.3.degree. may occur, thereby causing a sense
of incongruity.
SUMMARY OF THE INVENTION
[0006] It is therefore an object of the present invention to
provide a sound image positioning method and a sound image
positioning apparatus for positioning a sound image to a correct
point even if there is a difference between a reference distance
used for measuring a head-related transfer function stored
beforehand and a setting distance of a virtual sound source.
[0007] The inventive method pans a sound image of a virtual sound
source to a listener in a virtual sound field by filtering audio
signals of left and right channels through left and right filters
which simulate acoustic transfer characteristics of the virtual
sound field. The inventive method comprises the steps of
provisionally memorizing acoustic transfer characteristics of the
virtual sound field which are distributed radially around a center
point of the listener, designating a source point at which the
virtual sound source is to be located within the virtual sound
field in terms of a geometric distance and a geometric direction
relative to the center point, computing a leftward acoustic
direction from the source point to a left ear of the listener
according to the geometric distance, the geometric direction and an
offset of the left ear from the center point, computing a rightward
acoustic direction from the source point to a right ear of the
listener according to the geometric distance, the geometric
direction and an offset of the right ear from the center point,
determining an effective acoustic transfer characteristic based on
the memorized acoustic transfer characteristics according to the
leftward acoustic direction so as to enable the left filter to
simulate said effective transfer characteristic, determining
another effective acoustic transfer characteristic based on the
memorized acoustic transfer characteristics according to the
rightward acoustic direction so as to enable the right filter to
simulate said another effective transfer characteristic, and
filtering an audio signal of the left channel through the left
filter and filtering another audio signal of the right channel
through the right filter to thereby direct the sound image of the
virtual sound source located at the source point to the listener
positioned at the center point.
[0008] Further, the inventive apparatus is constructed for
directing a sound image of a virtual sound source at a designated
source point to a listener in a virtual sound field. In the
inventive apparatus, a database provisionally memorizes acoustic
transfer characteristics of the virtual sound field in
correspondence to reference source points distributed radially
around a center point of the listener. Left and right filters
respectively filter audio signals of left and right channels
according to the acoustic transfer characteristics loaded from the
database. A processor computes a leftward acoustic direction from
the designated source point to a left ear of the listener, and
computes a rightward acoustic direction from the designated source
point to a right ear of the listener. A controller specifies a
leftward reference source point coincident with the leftward
acoustic direction to load an effective acoustic transfer
characteristic corresponding to the leftward reference source point
from the database into the left filter, and specifies a rightward
reference source point coincident with the rightward acoustic
direction to load another effective acoustic transfer
characteristic corresponding to the rightward reference source
point from the database into the right filter. A feeder feeds an
audio signal of the left channel to the left filter and feeds
another audio signal of the right channel to the right filter to
thereby direct the sound image of the virtual sound source located
at the source point to the listener positioned at the center
point.
[0009] In a different view, the inventive apparatus is arranged for
directing a sound image of a virtual sound source to a listener in
a virtual sound field. In the apparatus, a database provisionally
memorizes a pair of leftward and rightward acoustic transfer
characteristics of the virtual sound field in correspondence to
each of sample points distributed radially around a center point of
the listener at a fixed radius, the leftward acoustic transfer
characteristic simulating a path from each sample point to a left
ear of the listener and the rightward acoustic transfer
characteristic simulating another path from each sample point to a
right ear of the listener. Left and right filters respectively
filter audio signals of left and right channels according to the
left and right acoustic transfer characteristics loaded from the
database. An input designates a source point at which the virtual
sound source is to be located within the virtual sound field in a
distance which may be different from the fixed radius relative to
the center point. A processor computes a leftward acoustic
direction from the source point to the left ear of the listener,
and computes a rightward acoustic direction from the source point
to the right ear of the listener. A controller specifies a leftward
sample point substantially coincident with the leftward acoustic
direction to load the leftward transfer characteristic
corresponding to the leftward sample point from the database into
the left filter, and specifies a rightward sample point
substantially coincident with the rightward acoustic direction to
load the rightward acoustic transfer characteristic corresponding
to the rightward sample point from the database into the right
filter. A feeder feeds an audio signal of the left channel to the
left filter and feeds another audio signal of the right channel to
the right filter to thereby direct the sound image of the virtual
sound source located at the source point to the listener positioned
at the center point.
[0010] According to the present invention, as shown in FIG. 3,
based on the geometric distance r and geometric direction .theta.
to the virtual sound source point Ps and the offset 2 h between
both ears of the listener, the acoustic directions R and L from the
virtual sound source point Ps to the right and left ears of the
listener are calculated separately for the right and left channels.
Acoustic transfer characteristics of the right and left filters are
determined by these acoustic transfer directions R and L. To be
more specific, sample points PR and PL coincident with the acoustic
directions R and L are identified on a circumference having the
fixed radius r0 that is the reference distance. If the setting
distance r to the virtual sound source point Ps differs from the
reference distance r0 at which the acoustic transfer
characteristics are measured, the acoustic transfer characteristics
corresponding to the sample points PR and PL are used as the
effective acoustic transfer characteristics of both channels.
Approximating the effective or true characteristics from the object
distance r by these acoustic transfer characteristics provides high
fidelity of the sound image positioning. The above-mentioned
acoustic transfer characteristics are stored in the transfer
characteristic database beforehand at a fine angular pitch, for
example, in unit of 1.degree. on the circumference away from the
listener by the fixed reference distance r0. Alternatively, the
data may be stored at a coarse angular pitch, for example, in unit
of 90.degree. in forward, backward, rightward and leftward
directions. In the latter case, the effective acoustic transfer
characteristic corresponding to the acoustic direction concerned
may be obtained by vector compoint or composition according to the
calculated acoustic directions R and L. Consequently, according to
the present invention, an acoustic transfer characteristic of
higher fidelity can be obtained with generally the same data volume
as that used in the prior art technology. Conversely, a smaller
data volume than that conventionally used may be enough for
achieving generally the same acoustic transfer characteristic as
that of the prior art technology.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] These and other objects of the invention will be seen by
reference to the description, taken in connection with the
accompanying drawing, in which:
[0012] FIG. 1 is a block diagram illustrating constitution of a
sound image positioning apparatus practiced as one preferred
embodiment of the present invention;
[0013] FIG. 2 is a geometric diagram illustrating a point of a
virtual sound source and a point of a listener in a virtual sound
field;
[0014] FIG. 3 is a geometric diagram for describing acoustic
directions of right and left channels in the preferred embodiment
shown in FIG.
[0015] FIG. 4 is a block diagram illustrating constitution of a FIR
filter associated with another preferred embodiment of the present
invention; and
[0016] FIG. 5 is a geometric diagram for describing a problem of
the prior art.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0017] This invention will be described in further detail by way of
example with reference to the accompanying drawings. Now, referring
to FIG. 1, there is shown a block diagram illustrating a sound
image positioning apparatus, panning apparatus or localizing
apparatus practiced by a personal computer or the like as one
preferred embodiment of the present invention. An input monaural
audio signal SI is supplied to a notch filter 1 from a feeder 12 or
monaural sound source. The notch filter 1 attenuates a particular
frequency component Nt of the audio signal SI based on human
auditory characteristic to impart elevational positioning to the
input audio signal SI. An output of the notch filter 1 is delayed
by a delay circuit 2 to produce two-channel stereophonic audio
signals imparted with a sound transfer time lag T from a virtual
sound source point to both ears. These stereo signals are supplied
to FIR filters 3 and 4, respectively. The FIR filters 3 and 4
impart acoustic transfer characteristics to the audio signals of
each channel based on parameters fR(0R) and FL(0L) read from an
HRTF database 5. Outputs of the FIR filters 3 and 4 are adjusted in
right and left amplitude balance by amplifiers 6 and 7,
respectively. Outputs of the amplifiers 6 and 7 are treated by a
cross talk canceler (XTC) 8 to eliminate a cross talk that would
enter from right and left speakers (not shown) into both ears. The
outputs eliminated of cross talk are supplied to the speakers as
two-channel audio signals S0R and S0L. A central processing unit
(CPU) 9 accepts positional information r, .theta., and .phi. of a
virtual sound source designated by an input 10. Based on these
positional data, a processor in the CPU 9 calculates control
parameters for various blocks concerned, and a controller in the
CPU 9 supplies the calculated parameters to the various blocks. A
disk drive 13 is connected to the CPU 9. The disk drive 13 receives
a machine readable medium 14 such as a floppy disk or CD-ROM.
[0018] As shown in FIG. 2, let an intermediate point between both
ears of a listener 11 be a center point Po of a three-dimensional
coordinates system. Rightward, forward, and upward directions of
the listener 11 are set to X axis, Y axis, and Z axis of the
absolute coordinate system, respectively. Then, a source point Ps
of the virtual sound source is given in terms of a geometric
distance r from the center point Po to the virtual sound source, an
azimuth angle .theta. in the horizontal direction of the virtual
sound source Ps as viewed from the front side (Y-axis direction) of
the listener 11, and an elevation angle .phi. in the vertical
direction as viewed from the angle .theta. relative to the front
side of the listener 11.
[0019] It is known that, in the human ear, as the angle .phi. in
the elevation of the sound source increases, the dead band
frequency shifts to higher ranges. The CPU 9 determines the
attenuation frequency component Nt by the elevation .phi. to
control the frequency response of the notch filter 1. Also, the CPU
9 obtains the transfer time lag T of the right and left channels
based on the difference in distance from the virtual sound source
point Ps to both ears.
[0020] The CPU 9 also calculates acoustic transfer angles .theta.R
and .theta.L from the virtual sound source to both ears of the
listener 11 based on the azimuth .theta.. To be more specific, as
shown in FIG. 3, the HRTF database 5 stores acoustic transfer
characteristics from sample points or reference points distributed
along a circumference having radius r0 toward the center point Po.
The acoustic transfer characteristics are measured beforehand with
using a dummy head. The rightward acoustic transfer angle .theta.R
of a sound generated from the virtual sound source point Ps to
enter into the right ear of the listener positioned at offset
distance +h in the X-axis direction is represented by equation (1)
below.
.theta.R=cos.sup.-1(e/r0) (1)
[0021] Let straight line R that passes the right ear (x=+h) and the
virtual sound source point Ps be:
R:y=dx-dh.
[0022] Since coordinates (xs, ys) of the point Ps are
xs=r sin .theta.
ys=r cos .theta.,
[0023] d is represented by equation (2) below:
d=ys/(xs-h)=cos .theta./(sin .theta.-h/r) (2)
[0024] Further, e equivalent to Y coordinate at intersection PR
between the circumference having radius r0 and the straight line R
is represented by equation (3) below:
(e/d+h).sup.2+e.sup.2=r0.sup.2
e={-b.+-.{square root}(b.sup.2-4ac)}/2a (3)
[0025] where,
[0026] a=d.sup.2+1
[0027] b=2dh
[0028] c=d.sup.2(h.sup.2-r0.sup.2)
[0029] Therefore, the acoustic transfer angle .theta.R of the right
channel can be calculated by obtaining e by substituting equation
(2) into equation (3) and by substituting obtained e into equation
(1). The leftward acoustic transfer angle .theta.L of the left
channel can be obtained in similar manner. Consequently, the HRTF
database 5 is referenced based on the calculated transfer angles
.theta.R and .theta.L of the right and left channels obtained by
the CPU 9. To be more specific, the HRTF database 5 stores a pair
of filter parameters fR(.theta.) and fL(.theta.) at each sample
point to represent the acoustic transfer characteristics or
functions up to the right and left ears. Each sample point is
determined by the fixed distance r0 and the calculated acoustic
transfer angle .theta.. For the rightward transfer angle .theta.R,
FR(.theta.R) of the pair of filter parameters FR(.theta.R) and
FL(.theta.L) is selected. For the leftward transfer angle .theta.L,
ff(.theta.L) of the pair of filter parameters fR(.theta.R) and
ff(.theta.L) is selected. The FIR filters 3 and 4 may be operated
by these obtained filter parameters fR(.theta.R) and fL(.theta.L),
respectively, which represent effective acoustic transfer
characteristics from the virtual sound source to the listener
11.
[0030] If the sampling point pitch of the HRTF database is coarse
and therefore the effective acoustic transfer characteristic data
corresponding to the obtained right and left transfer angles
.theta.R and .theta.L does not exist in the HRTF database, vector
compoint or composition may be performed on a pair of sample points
oppositely adjacent to the calculated transfer angle so as to
obtain the effective acoustic transfer characteristic data for each
of the transfer angles .theta.R and .theta.L by interpolation.
[0031] It should be noted that the present invention is not limited
to a system that has filter parameters in the database as mentioned
above. For example, as shown in FIG. 4, the present invention is
also applicable to a system in which the head-related transfer
functions corresponding to the forward, backward, rightward, and
leftward directions of the listener are given as fixed parameters
of FIR filters 21 and 22. Directivity of the sound is imparted by
performing amplitude control through amplifiers 23 and 24 according
to the obtained transfer angles .theta.R and .theta.L on audio
signals SR and SL supplied from the FIR filters 21 and 22, and then
by adding amplified results through adders 25 and 26. It should be
noted that the FIR filter 21 is divided into sections FRF, FRB,
FRL, and FRR corresponding to the forward, backward, leftward and
rightward directions. The amplifier 23 is also divided into
sections VRF, VRB, VRL and VRR corresponding to the forward,
backward, leftward and rightward directions. This holds true with
the FIR filter 22 and the amplifier 24 of the left channel.
[0032] For summary, referring back again to FIG. 1, the inventive
apparatus is constructed for positioning a sound image of a virtual
sound source relative to a listener in a virtual sound field. In
the apparatus, the database 5 provisionally memorizes acoustic
transfer characteristics (fL, fR) of the virtual sound field in
correspondence to sample points (.theta.L, .theta.R) distributed
radially around a center point of the listener at a fixed radius.
The left and right filters 3 and 4 respectively filter audio
signals of left and right channels according to the acoustic
transfer characteristics (fL, fR) loaded from the database 5. The
input 10 designates a source point at which the virtual sound
source is to be located within the virtual sound field in terms of
a geometric distance r which may be different from the fixed radius
and a geometric direction .theta. relative to the center point. The
processor in the CPU 9 computes a leftward acoustic direction from
the source point to a left ear of the listener according to the
geometric distance r, the geometric direction .theta. and an offset
of the left ear from the center point, and computes a rightward
acoustic direction from the source point to a right ear of the
listener according to the geometric distance r, the geometric
direction .theta. and an offset of the right ear from the center
point. The controller in the CPU 9 specifies a leftward sample
point (.theta.L) coincident with the leftward acoustic direction to
load an effective acoustic transfer characteristic (fL)
corresponding to the leftward sample point (.theta.L) from the
database 5 into the left filter 3, and specifies a rightward sample
point (.theta.R) coincident with the rightward acoustic direction
to load another effective acoustic transfer characteristic (fR)
corresponding to the rightward sample point (.theta.R) from the
database 5 into the right filter 4. The feeder 12 feeds an audio
signal of the left channel to the left filter 3 and feeds another
audio signal of the right channel to the right filter 4 to thereby
direct the sound image of the virtual sound source located at the
source point to the listener positioned at the center point.
[0033] The controller may specify a pair of leftward sample points
which lie oppositely relative to the leftward acoustic direction
such that said effective acoustic transfer characteristic is
determined by interpolating the acoustic transfer characteristics
corresponding to the pair of the leftward sample points, and may
specify another pair of rightward sample points which lie
oppositely relative to the rightward acoustic direction such that
said another effective acoustic transfer characteristic is
determined by interpolating the acoustic transfer characteristics
corresponding to the pair of the rightward sample points.
[0034] The processor may compute an azimuth component of the
leftward acoustic direction from the source point to the left ear
of the listener in a three-dimensional space of the virtual sound
field such that the leftward sample point is selected substantially
coincident with the azimuth component of the leftward acoustic
direction, and may compute an azimuth component of the rightward
acoustic direction from the source point to the right ear of the
listener such that the rightward sample point is selected
substantially coincident with the azimuth component of the
rightward acoustic direction, thereby directing the sound image of
the virtual sound source to the listener in an azimuth direction of
the three-dimensional space. Further, the controller may compute an
elevation component of an acoustic direction from the source point
to the listener according to the geometric distance r and the
geometric direction .phi., and the notch filter 1 filters the audio
signal SI according to the elevation component of the acoustic
direction so as to direct the sound image of the virtual sound
source to the listener in an elevation direction of the
three-dimensional space.
[0035] The machine readable medium 14 is for use in the inventive
apparatus having the CPU 9 and positioning a sound image of a
virtual sound source relative to a listener in a virtual sound
field by filtering audio signals of left and right channels through
left and right filters 3 and 4 which simulate acoustic transfer
characteristics of the virtual sound field. As shown in FIG. 3, the
medium contains program instructions executable by the CPU for
causing the apparatus to perform the steps of provisionally
memorizing acoustic transfer characteristics of the virtual sound
field allotted to sample points (PL, PR) distributed radially
around a center point Po of the listener at a fixed distance r0,
designating a source point Ps at which the virtual sound source is
to be located within the virtual sound field in terms of a
geometric distance r and a geometric direction .theta. relative to
the center point Po, computing a leftward acoustic direction (L)
from the source point Ps to a left ear of the listener according to
the geometric distance r, the geometric direction .theta. and an
offset -h of the left ear from the center point Po, computing a
rightward acoustic direction (R) from the source point Ps to a
right ear of the listener according to the geometric distance r,
the geometric direction .theta. and an offset +h of the right ear
from the center point Po, selecting a leftward sample point PL
substantially coincident with the leftward acoustic direction L to
determine an effective acoustic transfer characteristic based on
the acoustic transfer characteristic allotted to the leftward
sample point PL so as to enable the left filter to simulate said
effective transfer characteristic, selecting a rightward sample
point PR substantially coincident with the rightward acoustic
direction R to determine another effective acoustic transfer
characteristic based on the acoustic transfer characteristic
allotted to the rightward sample point PR so as to enable the right
filter to simulate said another effective transfer characteristic,
and filtering an audio signal of the left channel through the left
filter and filtering another audio signal of the right channel
through the right filter to thereby pan the sound image of the
virtual sound source located at the source point Ps to the listener
positioned at the center point Po.
[0036] Preferably, the step of selecting a leftward sample point
comprises selecting a pair of leftward sample points which lie
oppositely relative to the leftward acoustic direction such that
said effective acoustic transfer characteristic is determined by
interpolating the acoustic transfer characteristics allotted to the
pair of the leftward sample points, and the step of selecting a
rightward sample point comprises selecting a pair of rightward
sample points which lie oppositely relative to the rightward
acoustic direction such that said another effective acoustic
transfer characteristic is determined by interpolating the acoustic
transfer characteristics allotted to the pair of the rightward
sample points.
[0037] Preferably, as shown in FIG. 2, the step of computing a
leftward acoustic direction comprises computing an azimuth
component (.phi.) of the leftward acoustic direction from the
source point Ps to the left ear of the listener in a
three-dimensional space of the virtual sound field such that the
leftward sample point is selected substantially coincident with the
azimuth component of the leftward acoustic direction, and the step
of computing a rightward acoustic direction comprises computing an
azimuth component (.phi.) of the rightward acoustic direction from
the source point Ps to the right ear of the listener such that the
rightward sample point is selected substantially coincident with
the azimuth component of the rightward acoustic direction, thereby
directing the sound image of the virtual sound source to the
listener in an azimuth direction (.theta.) of the three-dimensional
space. The inventive steps further comprise computing an elevation
component (.phi.) of an acoustic direction from the source point Ps
to the listener according to the geometric distance r and the
geometric direction, and filtering the audio signal according to
the elevation component (.phi.) of the acoustic direction so as to
direct the sound image of the virtual sound source to the listener
in an elevation direction of the three-dimensional space.
[0038] As described and according to the present invention, based
on the distance and direction from a listener to a virtual sound
source point and the distance between both ears of the listener,
the acoustic directions from the virtual sound source point to both
ears of the listener are independently calculated for the right and
left channels. The acoustic transfer characteristics of the filters
are determined by the obtained acoustic directions of the right and
left channels. Consequently, even if the setting point from the
listener to the virtual sound source point differs from the
reference distance used for measurement of the acoustic transfer
characteristics, the effective acoustic transfer characteristics
corresponding to a specified virtual sound source point can be
obtained, thereby providing good fidelity of the virtual sound
field.
[0039] While the preferred embodiments of the present invention
have been described using specific terms, such description is for
illustrative purposes only, and it is to be understood that changes
and variations may be made without departing from the spirit or
scope of the appended claims.
* * * * *