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the watermark band. With the narrower band, the embedded data was extracted with-
out error at MP3 bit rate of 64 kbps per sound channel. When using BPSK, error-free
extraction was achieved for all the 5 tested host signals even at the MP3 bit rate of 56
kbps and with a wider watermarking band.
Table 3.
Robustness against MP3: BER(%) at different MP3 bit rates. Left and right BER
values were obtained with watermark bands [W/4, W/2] and [W/4, 3W/8] respectively
MP3 bit rate
128 kbps
112 kbps
96 kbps
80 kbps
64 kbps
56 kbps
I
0/0
0/0
0/0
0.52/0
2.06/0
5.67/1.55
II
0/0
0/0
0/0
0/0
2.06/0
1.55/0
III
0/0
0/0
0/0
0/0
3.09/0
3.61/1.03
IV
0/0
0/0
0/0
0/0
0.52/0
2.58/0.52
V
0/0
0/0
0/0
0/0
0.5 /0
2.06/0
4
Conclusions
Using a frequency domain dithering technique, a substantial amount of information
can be embedded into a digital audio signal. In this technique, a data sequence is en-
coded and inserted into the spectrum of short frames of the signal. A high degree of
imperceptibility is achieved by utilizing the HAS both in the time domain and in the
frequency domain. With a large quantization step, the system is sufficiently robust
against additive white Gaussian noise and MP3 compression coding.
When the quantization step becomes small, better transparency but less robustness,
results. This is considered to be suitable for covert communication applications, and
should be subject to both perceptive and statistic analysis. It has been found that, with
a small quantization step, say, max(|
C
n
|)/8, the modifications to the waveform of the
affected frame as shown in Fig.4 is in fact well beyond several least significant bits.
Therefore, LSB based steganalytic techniques cannot be used to detect the presence of
the data embedding. Moreover, since the frames are sparsely scattered, locating signal
segments that likely contain secrete information without the knowledge of the syn-
chronization pilot is extremely difficult. Further study in this aspect is required.
A number of parameters can be varied to meet different requirements. For exam-
ple, choosing a short frame length and a narrow watermark band toward lower fre-
quencies can make the watermark more robust. The embedded data can be repeated in
the candidate frames over the host signal if only a few data are to be embedded. On
the other hand, if data capacity is important, a longer frame should be used, and a
more efficient modulation scheme such as 16QAM (
D
=4) can be chosen. Error cor-
rection techniques may also be introduced with a moderate reduction of payload.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No.
60072030), and Key Disciplinary Development Program of Shanghai (2001-44).
