Digital Data can be shared by multiple users, distributed over network, and managed for long period time without any damage.
In contrast to these, the copyright protection problem arises, since unauthorized copying and distribution of digital data are simplified, too .
As a result, a technique called digital watermarking is introduced to protect the ownership of these contents.
AUDIO WATERMARKING Digital Audio Watermarking is a technology to hide information in an audio file without the information being audible to the listener, and without affecting in any way the audio quality of the original file. What we can define
Temporal watermarking hides watermarks directly into digital audio signals in the time domain.
The spectral audio watermarking applies certain frequency transform, such as FFT, DCT, and DWT, etc, to the data block of the audio signal, and hides the watermark information Into the transformed data block.
Here watermark is embedded by shifting the DC level of the audio signal.
An input signal is divided into frames of fixed length.
DC level of each frame =Mean of a frame-value in the frame
After that a binary watermark sequence generated randomly is introduced to the shifted signal. This is achieved by introducing a DC offset level to each frame in the signal according to the watermark bits.
If the watermark bit is 0, the signal is shifted downward.
If the watermark bit is 1, the signal is shifted upward.
level 0 ->DC level introduced when a watermark bit is 0
level 1 -> the DC level introduced when a watermark bit is 1.
To extract a watermark, a watermarked signal is first divided into frames. Then, the mean of each frame is calculated. If the mean of a frame is positive, the corresponding watermark bit is 1. Else, the watermark bit is 0.
The original signal and watermark is not needed in the extraction process. Therefore, DC level shifting is a blind watermarking scheme
Band Division Based on Quadrature Mirror Filter (QMF) Bank Cont..
Watermarking involves hiding a faint but audible sound under another louder sound (original sound), so the faint sound becomes inaudible and it acts as a watermark for the original sound.
Watermark is generated firstly by creating a pseudo-noise sequence (PN sequence) from a shift register. The shift register consists of m flip-flops with a maximum period of (2 m ) – 1.
PN sequence is filtered along with the filter that approximates the frequency characteristics of the human auditory system (HAS). Consequently, it is put through to the time domain for temporal masking.
The frequency masking algorithm is robust to coding, multiple watermarking, and resampling.
Different watermark can be embedded by this algorithm into the same signal and is easy to detect by the owner.
The detection scheme for this algorithm needs the original input signal, which is not flexible
From the extraction scheme, frequency masking is a private watermarking.
SPREAD SPECTRUM Where w i is element of watermark y is the watermarked signal A correlation detector is carried out to detect the presence of the watermark and is described by Eqn where N is vector cardinality Cont..
SPREAD SPECTRUM While x can be modelled as Gaussian random vector, the normalised correlation test is described as Eqn If p = 1, watermark is present If p=0, watermark is not present