Electrical audio signal processing systems and devices – Binaural and stereophonic – Pseudo stereophonic
Reexamination Certificate
2000-10-24
2004-10-12
Harvey, Minsun (Department: 2644)
Electrical audio signal processing systems and devices
Binaural and stereophonic
Pseudo stereophonic
C381S061000, C381S001000
Reexamination Certificate
active
06804358
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a sound image localization processor for making a listener feel without using a surround loudspeaker as if a surround signal of a two-channel stereo were outputted from the surround loudspeaker using two loudspeakers located ahead of the listener.
BACKGROUND ART
FIG. 6
illustrates a conventional sound image localization processing circuit.
A surround left signal SL inputted to an input terminal P
1
is fed to a first sound image localization filter
101
and a second sound image localization filter
102
. In each of the filters
101
and
102
, filter processing corresponding to a filter coefficient of the filter is performed.
A surround right signal SR inputted to an input terminal P
2
is fed to a third sound image localization filter
103
and a fourth sound image localization filter
104
. In each of the filters
103
and
104
, filter processing corresponding to a filter coefficient of the filter is performed. The characteristics of the first sound image localization filter
101
and the characteristics of the fourth sound image localization filter
104
are the same, and the characteristics of the second sound image localization filter
102
and the characteristics of the third sound image localization filter
103
are the same.
An output of the first sound image localization filter
101
and an output of the third sound image localization filter
103
are added together in an adder
111
, and the result of the addition is outputted as L
OUT
. The output L
OUT
is fed to a left loudspeaker located at the left and ahead of a listener.
An output of the second sound image localization filter
102
and an output of the fourth sound image localization filter
104
are added together in an adder
112
, and the result of the addition is outputted as R
OUT
. The output R
OUT
is fed to a right loudspeaker located at the right and ahead of the listener.
Each of the sound image localization filters is found by a head transmission function, described below. Generally used as the sound image localization filter is an FIR (Finite Impulse Response) digital filter having several hundred taps.
Description is made of a method of calculating a sound image localization filter using a head transmission function.
As shown in
FIG. 7
, let H
LL
, H
LR
, H
RL
, and H
RR
be respectively transmission functions for each transmission path from real loudspeakers L and R arranged at the left and right and ahead of a listener
100
to the left and right ears of the listener
100
. Let W
L
and W
R
be respectively transmission functions from a virtual sound source position P where a sound is desired to be localized to the left and right ears of the listener
100
. All the transmission functions are described on the frequency axis.
In order that a voice can be heard by the listener
100
as if it were outputted from the virtual sound source position irrespective of the fact that the voice is outputted from the real loudspeakers L and R, the following equation (1) must hold, letting X be an input signal, and letting L
OUT
and R
OUT
be respectively output signals from the real loudspeakers L and R.
(
W
L
W
R
)
X
=
(
H
LL
H
LR
H
RL
H
RR
)
(
L
OUT
R
OUT
)
(
1
)
Consequently, the signals L
OUT
and R
OUT
respectively outputted from the real loudspeakers L and R are found by the following equation (2):
(
L
OUT
R
OUT
)
=
1
H
LL
H
RR
-
H
LR
H
RL
(
H
RR
-
H
LR
-
H
RL
H
LL
)
(
W
L
W
R
)
X
(
2
)
Furthermore, if it is assumed that the real loudspeakers L and R are located so as to be bilaterally symmetrical , as viewed from the listener
100
, the transmission functions which are bilaterally symmetrical are the same. Accordingly, the following equations (3) and (4) hold. The same transmission functions are respectively taken as H
THR
and H
CRS
.
H
THR
=H
LL
=H
RR
(3)
H
CRS
=H
LR
=H
RL
(4)
Consequently, the foregoing equation (2) can be rewritten to the following equation (5):
(
L
OUT
R
OUT
)
=
1
H
LL
H
RR
-
H
LR
H
RL
(
H
RR
-
H
LR
-
H
RL
H
LL
)
(
W
L
W
R
)
X
=
1
H
THR
2
-
H
CRS
2
(
H
THR
-
H
CRS
-
H
CRS
H
THR
)
(
W
L
W
R
)
X
=
(
H
THR
W
L
-
H
CRS
W
R
H
THR
2
-
H
CRS
2
H
THR
W
R
-
H
CRS
W
L
H
THR
2
-
H
CRS
2
)
X
=
(
H
1
H
2
)
X
(
H
1
=
H
THR
W
L
-
H
CRS
W
R
H
THR
2
-
H
CRS
2
H
2
=
H
THR
W
R
-
H
CRS
W
L
H
THR
2
-
H
CRS
2
)
(
5
)
As a filter in which H
1
and H
2
in the equation (5) are converted into time axes, an FIR digital filter having several hundred taps is used.
The frequency characteristics of the first sound image localization filter
101
and the fourth sound image localization filter
104
shown in
FIG. 6
correspond to H
1
in the equation 5, and the frequency characteristics of the second sound image localization filter
102
and the third sound image localization filter
103
correspond to H
2
in the equation 5.
The FIR digital filter is generally realized by a digital processor such as DSP (Digital Signal Processor). When the DSP, for example, is used for this processing, the number of processing steps required therefor is approximately the same as the number of taps of the FIR digital filter. As the overall amount of processing, therefore, processing whose amount is four times the number of taps of the FIR digital filter is required because there are four FIR digital filters.
Specifically, 1000 or more processing steps are required for the digital signal processor. Further, the FIR digital filter found by such a calculating method generally has complicated frequency characteristics. Therefore, a signal which has been subjected to FIR digital filter processing reasonably has a sharp peak dip, so that it becomes a sound which is unnatural and has an uncomfortable feeling. An example of the frequency characteristics of the FIR digital filter used for sound image localization is shown in FIG.
8
.
FIG. 9
illustrates a circuit for reproducing a multi-channel audio signal such as DolbyDigital or MPEG only on two channels utilizing the sound image localization processing technique shown in FIG.
6
. In
FIG. 9
, the same portions as those shown in
FIG. 6
are assigned the same reference numerals.
A left signal L and a right signal are added to a signal obtained by subjecting a center signal C to gain control of −3 dB by a multiplier
121
, respectively, by an adder
113
and an adder
114
.
An output of the adder
113
and the output of the adder
111
described in
FIG. 6
are added together by an adder
115
, and the result of the addition is taken as an output L
OUT
to a left loudspeaker. An output of the adder
114
and the output of the adder
112
described in
FIG. 6
are added together by an adder
116
, and the result of the addition is taken as an output R
OUT
to a right loudspeaker.
Also in such a circuit, much of the processing is processing of the FIR digital filter for sound image localization of a surround signal, so that a large burden is imposed on the DSP. Further, the FIR digital filter found by the head transmission function is used. Accordingly, the tone becomes unnatural.
An object of the present invention is to provide a sound image localization processor corresponding to a surround signal, in which the amount of processing can be reduced and a more natural tone is obtained.
DISCLOSURE OF INVENTION
In a sound image localization processor for making a listener feel without using a surround loudspeaker as if a surround signal of a two-channel stereo were outputted from the surround loudspeaker using right and left two loudspeakers which are located ahead of the listener, a first sound image localization processor according
Arent & Fox PLLC
Harvey Minsun
Sanyo Electric Co., LTD
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