Electrical audio signal processing systems and devices – Binaural and stereophonic – Stereo speaker arrangement
Reexamination Certificate
2000-12-01
2004-03-23
Mei, Xu (Department: 2644)
Electrical audio signal processing systems and devices
Binaural and stereophonic
Stereo speaker arrangement
C381S307000, C381S022000
Reexamination Certificate
active
06711270
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an audio reproducing apparatus for reproducing a single-channel or a multichannel audio signal by means of headphones.
As a multichannel audio signal, there is known a 5.1-channel digital audio signal employed in the AC-3 (so-called Dolby digital) system. The 5.1-channel digital audio signal S
5
.
1
is generated by encoding the following signals into a piece of serial data.
SLF: audio signal for a channel at the left front of the listener
SRF: audio signal for a channel at the right front of the listener
SCF: audio signal for a channel at the center front of the listener
SLB: audio signal for a channel at the left back (or on the left side) of the listener
SRB: audio signal for a channel at the right back (or on the right side) of the listener
SLFE: audio signal representing a component in a low frequency range below 120 Hz, for example.
The digital audio signal S
5
.
1
recorded on, for example, a DVD together with a video signal provides effects such that when the image of the video signal is reproduced, the position of a sound source in the image coincides with that of a sound image that is actually heard, and also a naturally spreading sound field is created.
The digital audio signal S
5
.
1
is basically intended for reproduction by means of speakers that are disposed around the listener. However, there has been contrived an audio reproducing apparatus that makes it possible to reproduce a similar reproduction sound field by means of headphones.
Specifically, in
FIG. 3
, reference numeral
200
denotes an example of such an audio reproducing apparatus. Reference numeral
100
denotes a DVD player.
Now, a 5.1-channel digital audio signal S
5
.
1
is extracted from the DVD player
100
. The audio signal S
5
.
1
is supplied to a decoder circuit
10
of the audio reproducing apparatus
200
, where digital audio signals SLF to SLFE for respective channels are decoded. Then, the decoded and extracted audio signals SLF and SRF are supplied via adding circuits
21
and
22
to a converting circuit
3
F, which will be described below, while the audio signals SLB and SRB are supplied via adding circuits
23
and
24
to a converting circuit
3
B.
Furthermore, the audio signal SCF is basically a signal to be supplied to a speaker disposed at the center front of the listener; however, a similar effect can be obtained even when the signal SCF is supplied to speakers disposed respectively at the left front and at the right front of the listener. Therefore, the signal SCF is supplied to the adding circuits
21
and
22
to be added to the audio signals SLF and SRF. In addition, the audio signal SLFE is a signal representing a low frequency range below 120 Hz, and its reproduced sound does not provide a directional feeling. Thus, the audio signal SLFE is supplied to the adding circuits
21
to
24
to be added to each of the signals SLF to SRB. Signals SLF
0
to SRB
0
are extracted from the adding circuits
21
to
24
.
The converting circuit
3
F is provided to convert audio signals SLF
0
and SRF
0
into audio signals that can provide a reproduction sound field similar to that obtained in speaker reproduction even when the audio signals SLF
0
and SRF
0
are to be reproduced by means of headphones. Signal processing by the converting circuit
3
F will hereinafter be referred to as virtual sound image localization processing.
Specifically, the converting circuit
3
F processes the original audio signals SLF
0
and SRF
0
in such a way that when output audio signals SLFX and SRFX of the converting circuit
3
F are supplied to the headphones, a reproduction sound field similar to that obtained when the audio signals SLF
0
and SRF
0
before the conversion are supplied to speakers disposed respectively at the left front and at the right front of the listener can be obtained.
Similarly, the converting circuit
3
B processes the original audio signals SLB
0
and SRB
0
in such a way that when output audio signals SLBX and SRBX of the converting circuit
3
B are supplied to the headphones, a reproduction sound field similar to that obtained when the audio signals SLB
0
and SRB
0
before the conversion are supplied to speakers disposed respectively at the left back and at the right back of the listener can be obtained. Incidentally, specific configurations of the converting circuits
3
F and
3
B will be described later.
Then, an audio signal SLFX from the converting circuit
3
F and an audio signal SLBX from the converting circuit
3
B are added together at an adding circuit
4
L to be converted into a digital audio signal SL for a left channel. Also, an audio signal SRFX from the converting circuit
3
F and an audio signal SRBX from the converting circuit
3
B are added together at an adding circuit
4
R to be converted into a digital audio signal SR for a right channel.
Then, these audio signals SL and SR are supplied to D/A converter circuits
5
L and
5
R, where the audio signals SL and SR are subjected to D/A conversion to become analog audio signals SL and SR. The resulting audio signals SL and SR are supplied via amplifiers
6
L and
6
R to acoustic units (electroacoustic transducer devices)
7
L and
7
R of the headphones
7
for a left and a right channel.
FIG. 4
shows a specific example of the converting circuit
3
F. The converting circuit
3
F is provided to realize virtual sound image localization processing for the audio signals SLF
0
and SRF
0
by convolving respective transfer functions for ranges from speakers at the left front and at the right front of the listener to both ears of the listener in the audio signals SLF
0
and SRF
0
using digital filters or by signal processing substantially similar to the above method.
Specifically, the audio signals SLF
0
and SRF
0
from the adding circuits
21
and
22
are supplied to an adding circuit
31
L and a subtracting circuit
31
R to form a sum signal and a difference signal. The sum signal and the difference signal are supplied to digital filters
32
L and
32
R, respectively.
The digital filters
32
L and
32
R are each provided with a delay circuit
321
having a plurality of stages connected in a cascade manner, a plurality of coefficient circuits
322
and adding circuits
323
. The digital filters
32
L and
32
R are of a FIR type. An input signal is supplied to a first stage of the delay circuit
321
to be sequentially delayed, and an output signal from each stage of the delay circuit
321
is supplied to a coefficient circuit
322
to be multiplied by a specified coefficient. The signals resulting from the multiplication are added to each other by an adding circuit
323
, and the result is extracted as a filter output.
Then, output signals from the digital filters
32
L and
32
R are supplied to a subtracting circuit
33
L and an adding circuit
33
R to form a difference signal and a sum signal. The difference signal and the sum signal are extracted as output signals SLFX and SRFX of the converting circuit
3
F.
Here, respective head related transfer functions for ranges from speakers at the left front and at the right front of the listener to both ears of the listener are convolved in the audio signals SLF
0
and SRF
0
by providing specified characteristics to the digital filters
32
L and
32
R. Incidentally, the converting circuit
3
B can be configured in the same manner as the converting circuit
3
F, and therefore description of the converting circuit
3
B will be omitted.
As described above, according to the audio reproducing apparatus
200
of
FIG. 3
, a reproduction sound field similar to that obtained by means of five speakers disposed at the left front, at the center front, at the right front, in the left rear, and in the right rear of the listener and a speaker for a low frequency range can be reproduced by means of the headphones
7
.
However, it has been found that in the case of the audio reproducing apparatus
200
of
FIG. 3
, reproduced sound outputted from the headphones
7
lacks low frequency sound that should be obtained from th
Maioli Jay H.
Mei Xu
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