Periodic-combined-envelope-sequence generation device, periodic-combined-envelope-sequence generation method, periodic-combined-envelope-sequence generation program and recording medium
View Patent ↗An envelope sequence is provided that can improve approximation accuracy near peaks caused by the pitch period of an audio signal. A periodic-combined-envelope-sequence generation device according to the present invention takes, as an input audio signal, a time-domain audio digital signal in each frame, which is a predetermined time segment, and generates a periodic combined envelope sequence as an envelope sequence. The periodic-combined-envelope-sequence generation device according to the present invention comprises at least a spectral-envelope-sequence calculating part and a periodic-combined-envelope generating part. The spectral-envelope-sequence calculating part calculates a spectral envelope sequence of the input audio signal on the basis of time-domain linear prediction of the input audio signal. The periodic-combined-envelope generating part transforms an amplitude spectral envelope sequence to a periodic combined envelope sequence on the basis of a periodic component of the input audio signal in the frequency domain.
1. An encoder comprising:
processing circuitry configured to:
execute a spectral-envelope-sequence calculating processing which takes, as an input audio signal, a time-domain audio digital signal in each frame which is a predetermined time segment, and calculates a spectral envelope sequence of the input audio signal on the basis of time-domain linear prediction of the input audio signal;
execute a periodic-envelope-sequence generating processing which obtains a periodic envelope sequence P[1], . . . , P[N] as
P
[
n
]
=
{
h
·
exp
(
-
(
n
-
(
floor
(
(
U
×
T
′
)
/
2
L
)
)
)
2
2
PD
2
)
}
,
or
P
[
n
]
=
{
h
·
exp
(
-
(
n
-
(
Round
(
U
×
T
)
)
)
2
2
PD
2
)
}
where,
h= 2.8·(1.125−exp(−0.07 ·T′/ 2 L )),
PD= 0.5·(2.6−exp(−0.05 ·T′/ 2 L ))
for an integer n in the range of
( U×T ′)/2 L −v≤n≤n ( U×T ′)/2 L +v
where N and U are positive integers, T is an interval between occurrences of a periodic component in a frequency-domain coefficient string X[1], . . . , X[N] derived from the input audio signal, L is a number of decimals of the interval T, v is an integer greater than or equal to 1, floor(*) is a function that drops a fractional part of a value and returns an integer value, Round(*) is a function that rounds off a value to the nearest integer and returns an integer value, T′=T×2 L , and W[1], . . . , W[N] is a spectral envelope sequence;
execute a periodic-combined-envelope generating processing which transforms the spectral envelope sequence to a periodic combined envelope sequence on the basis of a periodic component of the input audio signal in the frequency domain;
execute a variable-length-coding-parameter calculating processing which calculates a variable-length coding parameter r n dependent on an amplitude value from the periodic combined envelope sequence; and
execute a variable-length coding processing which uses the variable-length coding parameter r n to encode a frequency-domain sequence derived from the input time-domain audio signal by variable-length coding and to output a variable-length code;
wherein the periodic-combined-envelope generating processing obtains the periodic combined envelope sequence W M [1], . . . , W M [N] as
W M [n]=W[n ]·(1 +δ·P[n ])
where a value δ is selected from among a plurality of candidates for δ such that minimizes a value E defined by
E
=
∑
n
=
1
N
(
X
[
n
]
·
W
~
M
[
n
]
)
4
where values ˜ W M [n] and G are defined by,
W
~
M
[
n
]
=
1
W
M
[
n
]
·
G
G
=
∑
n
=
1
N
X
[
n
]
·
1
W
M
[
n
]
.
2. A coding method executing:
a spectral-envelope-sequence calculating step for taking, as an input audio signal, a time-domain audio digital signal in each frame which is a predetermined time segment, and calculating a spectral envelope sequence of the input audio signal on the basis of time-domain linear prediction of the input audio signal;
a periodic-envelope-sequence generating step of obtaining a periodic envelope sequence P[1], . . . , P[N] as
P
[
n
]
=
{
h
·
exp
(
-
(
n
-
(
floor
(
(
U
×
T
′
)
/
2
L
)
)
)
2
2
PD
2
)
}
,
or
P
[
n
]
=
{
h
·
exp
(
-
(
n
-
(
Round
(
U
×
T
)
)
)
2
2
PD
2
)
}
where,
h= 2.8·(1.125−exp(−0.07 ·T′/ 2 L )),
PD= 0.5·(2.6−exp(−0.05 ·T′/ 2 L ))
for an integer n in the range of
( U×T ′)/2 L −v≤n ≤( U×T ′)/2 L +v
where N and U are positive integers, T is an interval between occurrences of a periodic component in a frequency-domain coefficient string X[1], . . . , X[N] derived from the input audio signal, L is a number of decimals of the interval T, v is an integer greater than or equal to 1, floor(*) is a function that drops a fractional part of a value and returns an integer value, Round(*) is a function that rounds off a value to the nearest integer and returns an integer value, T′=T×2 L , and W[1], . . . , W[N] is a spectral envelope sequence;
a periodic-combined-envelope generating step of transforming the spectral envelope sequence to a periodic combined envelope sequence on the basis of a periodic component of the input audio signal in the frequency domain;
a variable-length-coding-parameter calculating step of calculating a variable-length coding parameter r n dependent on an amplitude value from the periodic combined envelope sequence; and
a variable-length coding step of using the variable-length coding parameter r n , to encode a frequency-domain sequence derived from the input time-domain audio signal by variable-length coding and to output a variable-length code;
wherein the periodic-combined-envelope generating step obtains the periodic combined envelope sequence W M [1], . . . , W M [N] as
W M [n]=W[n ]·(1 +δ·P[n ])
where a value δ is selected from among a plurality of candidates for δ such that minimizes a value E defined by
E
=
∑
n
=
1
N
(
X
[
n
]
·
W
~
M
[
n
]
)
4
where values ˜ W M [n] and G are defined by,
W
~
M
[
n
]
=
1
W
M
[
n
]
·
G
G
=
∑
n
=
1
N
X
[
n
]
·
1
W
M
[
n
]
.
3. A non-transitory computer-readable recording medium on which the coding program for causing a computer to function as the encoder according to claim 1 is recorded.