Electrical audio signal processing systems and devices – Binaural and stereophonic – Pseudo stereophonic
Reexamination Certificate
1998-11-03
2003-10-21
Nguyen, Duc (Department: 2643)
Electrical audio signal processing systems and devices
Binaural and stereophonic
Pseudo stereophonic
C381S017000, C381S001000
Reexamination Certificate
active
06636608
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a pseudo-stereo circuit for converting monophonic audio signals into stereophonic audio signals.
2. Prior Art
FIG. 1
shows an example of a conventional pseudo-stereo circuit. The pseudo-stereo circuit is principally comprised of L-channel phase-shift circuit
100
L and R-channel phase-shift circuit
100
R for shifting the phase of a monophonic audio signal Min, to generate respective output signals, and a stereo coordination circuit
200
that receives the output signals of the phase-shift circuits
100
L and
100
R, and produces stereophonic audio signals carried by two channels, i.e., L and R channels.
The L-channel phase-shift circuit
100
L includes, for example, three all-pass filters
101
L,
102
L and
103
L that are cascade-connected in this order. Similarly, the R-channel phase-shift circuit
100
R includes three all-pass filters
101
R,
102
R, and
103
R similar in structure to the filters
101
L,
102
L and
103
L, that are cascade-connected in this order. Each of these all-pass filters will be described in detail below.
The all-pass filter
101
L is comprised of an operational amplifier
301
, resistors
302
-
304
, and a capacitor
305
, that are connected in the manner as shown in FIG.
1
. The resistors
303
and
304
have the same resistance value. Accordingly, the input voltage Vn of the inverting input terminal (−) of the operational amplifier
301
is given by the following expression (1):
Vn=
(Min+
Vo
)/2 (1)
where Vo is the output voltage of the operational amplifier
301
.
On the other hand, the input voltage Vp of the noninverting input terminal (+) of the operational amplifier
301
is given by the following expression (2):
Vp=
Min/(1+
j&ohgr;C
1
R
1
) (2)
where R
1
represents the resistance value of the resistor
302
, C
1
the capacitance value of the capacitor
305
, and &ohgr; the angular frequency of the input monophonic signal Min.
In the circuit arrangement shown in
FIG. 1
, since the inverting input terminal (−) and noninverting input terminal (+) of the operational amplifier
301
are virtually short-circuited to each other due to negative-feedback operation of the circuit, the input voltage Vp becomes equal to the input voltage Vn, and the following expression (3) is established:
(Min+
Vo
)/2=Min/(1+
j&ohgr;C
1
R
1
) (3)
By transforming the above expression (3), the transfer function of the all-pass filter
101
L is obtained as follows:
H
=
Vo
/
Min
=
(
1
-
j
ω
C1R1
)
/
(
1
+
j
ω
C1R1
)
(
4
)
Thus, the gain G of the all-pass filter
101
L with respect to the input monophonic signal Min is obtained from the above expression (4), and expressed by:
G
=
&LeftBracketingBar;
H
&RightBracketingBar;
=
&LeftBracketingBar;
(
1
-
j
ω
C1R1
)
/
(
1
+
j
ω
C1R1
)
&RightBracketingBar;
=
&LeftBracketingBar;
(
1
-
j
ω
C1R1
)
&RightBracketingBar;
/
&LeftBracketingBar;
(
1
+
j
ω
C1R1
)
&RightBracketingBar;
=
1
(
5
)
Accordingly, the input monophonic signal Min of any level of frequency passes through the all-pass filter
101
L while keeping its amplitude at the same value.
The phase of the input monophonic signal Min is shifted when the signal passes through the all-pass filter
101
L. The phase shift amount or phase angle &thgr; is determined depending upon the frequency of the input signal Min, as shown in the following expression (6):
θ
=
arg
(
H
)
=
-
2
tan
-
1
(
ω
C1R1
)
(
6
)
The all-pass filter
101
L has the above described construction and frequency characteristics.
The other all-pass filters
102
L and
103
L subsequent to the all-pass filter
101
L have exactly the same structure as the all-pass filter
101
L. As is apparent from the above expression (6), the phase shift amount given by each of the all-pass filters
101
L-
103
L to the input monophonic signal Min varies from 0 to −&pgr;, as the frequency f=&ohgr;/2&pgr; changes. Accordingly, the phase shift amount given by the L-channel phase-shift circuit
100
L as a whole to the input signal Min varies from 0 to −3&pgr; as the frequency f of the input signal changes. The phase shift amount &thgr;L given by the whole L-channel phase-shift circuit
100
L is illustrated in FIG.
2
.
The R-channel phase-shift circuit
100
R has basically the same structure as the L-channel phase-shift circuit
100
L as explained above, but the resistance value of the resistor
302
and the capacitance value of the capacitor
305
of each of the all-pass filters
101
R-
103
R are different from the values R
1
and C
1
of the all-pass filters
101
L-
103
L, such that, as shown in
FIG. 2
, the curve representing the frequency characteristic of the phase shift amount &thgr;R of the R-channel phase-shift circuit
100
R as a whole is shifted with respect to the curve representing the frequency characteristic of the phase shift amount &thgr;L of the L-channel phase-shift circuit
100
L in the direction of the X-axis representing the frequency of the input signal.
By appropriately selecting the resistance value of the resistor
302
and the capacitance value of the capacitor
305
in each of the L-channel phase-shift circuit
100
L and R-channel phase-shift circuit
100
R, a difference (&thgr;L−&thgr;R) between the phase shift amounts of these circuits
100
L,
100
R can be controlled to approximately &pgr;/2 over almost the entire audio frequency band, as shown in FIG.
2
. In the circuit shown in
FIG. 1
, the resistance and capacitance values are suitably selected so that the above requirement is satisfied.
In the circuit arrangement shown in
FIG. 1
, therefore, the L-channel phase-shift circuit
100
L and the R-channel phase-shift circuit
100
R output respective audio signals whose phases are shifted with respect to the phase of the input monophonic signal Min and are different from each other by &pgr;/2.
The stereo coordination circuit
200
functions to produce stereophonic audio signals based on the respective output signals of the L-channel phase-shift circuit
100
L and R-channel phase-shift circuit
100
R as explained above. The stereo coordination circuit
200
is comprised of a subtracter
201
, a filter
202
, an adder
203
and a subtracter
204
. In the thus constructed stereo coordination circuit
200
, the subtracter
201
produces a signal corresponding to a difference between the output signals of the L-channel phase-shift circuit
100
L and the R-channel phase-shift circuit
100
R, and the filter
202
limits the frequency range of the output signal of the subtracter
201
. The adder
203
performs addition of the output signal of the filter
202
and the output signal of the L-channel phase-shift circuit
100
L. The subtracter
204
performs subtraction between the output signal of the filter
202
and the output signal of the R-channel phase-shift circuit
100
R. The adder
203
and the subtracter
204
then generate stereophonic audio signals carried by two channels, or L and R channels, so as to produce sound that affords the listener a sense of the spatial distribution of the sound sources.
When the above-described pseudo-stereo circuit is produced as an integrated circuit or IC, the resulting IC chip has a relatively large area since the circuit requires a large number of constituent components, such as operational amplifiers. Also, the known pseudo-stereo circuit requires six capacitors only in the phase-shift circuits for the L and R channels, and these capacitors are generally required to have large capacitance values. It is, therefore, difficult to form these capacitors on the IC board, in view of the limitation of the chip area, and the capaci
Kishii Tatsuya
Noro Masao
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