Information Technology Reference
In-Depth Information
3
Experimental Results and Performance Study
3.1
Experimental System
A block diagram of the experimental system is shown in Fig.3. The OFDM subcarri-
ers consist of the dither modulated spectral components and the unmodified signal
spectral lines outside the embedding band. The complex watermark stream is obtained
from a binary sequence using QAM. A 31-bit
m
-sequence is used as the pilot. After
IFFT, a frame of marked waveform replaces the selected frame in the host.
Complex
symbols
QAM
Binary sequence
to be embedded
Candidate
frame
Dither
modulation
FFT
IFFT
PN
generator
Pilot
Mixer
Marked
audio
Host audio
Marked audio
Pilot
extractor
Cross-
correlator
Key
Local pilot
Encoded
frame
Bandpass
filter
FFT
Extracted
m
ark
+
Mark
decoder
Quantizer
Fig. 3.
Block diagram of the experimental system
At the receiver, search is performed either in the frequency domain or in the time
domain. In the time domain approach, signal components in the known band is ex-
tracted with a 5th-order type I Chebyshev band-pass filter, and then cross-correlated
with a locally generated pilot waveform. In order to obtain an accurate alignment, a
dual-direction filter is used in the fine search to preserve the phase.
3.2
Embedding Capacity and Imperceptibility
Embedding capacity is a function of watermark bandwidth
B
, frame length
N
2
, bit
number represented by each symbol,
D
, and length of the pilot,
L
m
, as given by
