Information Technology Reference
In-Depth Information
2.4
Structure of the Coded Signal Spectrum
The watermarked frame is composed of the preserved audio components
s
'(
k
), the
embedded mark
m
(
k
), and a synchronization pilot
p
(
k
). The preserved audio contains
most of the important frequency contents essential for imperceptibility, and the rest
carries both the stego-data and the pilot. The spectrum of the composite signal is
[
{
]
}
{
[
]
}
X
(
n
)
=
S
(
n
)
W
(
n
)
+
M
(
n
)
+
Q
S
(
n
)
W
(
n
)
+
P
(
n
)
+
Q
S
(
n
)
W
(
n
)
(5)
S
M
P
where
S
(
n
),
M
(
n
), and
P
(
n
) are the signal spectrum, the mark symbols, and the pilot,
respectively, and 0
1. Windows for the mark, the pilot and the preserved
audio signal are defined, respectively, by
≤
n
≤
N
−
1
(
N
/
4
)
+
L
≤
n
≤
(
N
/
2
−
1
W
M
(
n
)
=
(6)
0
0
≤
n
≤
(
N
/
4
)
+
L
−
1
or
(
N
/
2
≤
n
≤
N
−
1
,
1
(
N
/
4
≤
n
≤
(
N
/
4
)
+
L
−
1
W
P
(
n
)
=
(7)
0
0
≤
n
≤
(
N
/
4
−
1
or
(
N
/
4
)
+
L
≤
n
≤
N
−
1
,
and
W
(
n
)
=
1
−
W
(
n
)
−
W
(
n
)
0
≤
n
≤
N
−
1
.
(8)
S
M
P
The mark, the pilot, and the quantization operator
Q
[
.
] are designed such that
[
]
[
]
{
}
Q
M
(
n
)
+
Q
S
(
n
)
=
Q
S
(
n
)
,
(9)
and
[
]
[
]
{
}
Q
P
(
n
)
+
Q
S
(
n
)
=
Q
S
(
n
)
.
(10)
L
) symbols, and each com-
plex symbol represents
D
bits (
D
=2 when using QAM), the number of stego-bits is
(
N
/4
Since the mark window can accommodate (
N
/2
−
N
/4
−
−
L
)
D
. In the above example where the sampling frequency
f
s
44.1kHz,
N
2
23.2ms, the watermark band can accommodate a total of
N
2
/8 =
N
/4
= 128 symbols including the mark and pilot when
B
=1/4. With QAM and
L
= 31, for
example, the data capacity is 194 bits representing 27 ASCII characters.
1024, and
T
2.5
Watermark Detection
The first step in watermark detection is to locate the encoded frame. This can be done
by cross-correlating the pilot sequence with the spectral lines in
W
P
(
n
) for each frame
of the audio waveform. A correlation peak indicates that synchronization is achieved.
To speed up the search process, a replacement scheme may be used for the pilot
sequence instead of dither modulation, and the search is carried out in the time do-
main. The price paid is a slight increase of distortion. In this method, a candidate
frame is band-pass filtered to suppress spectral contents outside the embedded band,
and then correlated with a locally generated pilot waveform that is the time domain
representation of the pilot. A two-step search procedure is adopted: A coarse search is