JPL's Wireless Communication Reference WebsiteChapter: Capita Selecta |
Some abiguity exists over terms used. In particular, we distinguish between watermarking, fingerprinting, posing a signature or label and transmitting over a subliminal channel
The main difference between watermarks and fingerprints is whether the identity of the transmitter or that of the recipient is embedded in the signal. In many technical solutions, this also determines where (thus, in which device) the identifier is being added to the clear signal.
The requirements for fingerprinting are contradictory. On one hand the broadcaster or copyright holder may want to easily recognize the fingerprint, preferably visually. This allows easy tracing of a decoder that is used for illegal purposes. This approach is very similar to the commonly used watermarking by means of the logo of a TV station that is continuously visible in one of the corners of the screen. On the other hand, the fingerprint should be hidden, in order not to disturb paying viewers with program-unrelated messages on their screen, or to avoid that any pirate can detect and erase the fingerprint electronically. In the latter case, it may involve specific equipment to detect and decode a fingerprint.
Embedding and extracting watermarks requires additional hardware, for example, a watermark based on masking will require image edges extraction, a watermark hidden in the frequency domain will require many fast Fourier transforms. Another problem is the prove of the legal validity of the watermark in case of court pursuits. The energy required to prove the legitimacy of the watermark is directly related to the watermark quality.
For example, a watermark can be embedded in a picture by using the following method. Bits, which represent registration numbers etc., can be hidden in the picture by modificating the luminance values inside blocks of nxn pixels. Each block can contain for example one bit of the additional information. Other methods modify the DCT coefficients generated by JPEG or MPEG coders.
A watermark can also be hidden in audio signals.
link to MP2 files Audio Engineering Society AES workshop
on copy management, May 1998 | Audio clips by
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Data hiding in audio signals provides a special challenge because the human auditory system is extremely sensitive. The pertibations in a sound file can be detected as low as one part in ten million (-80dB). Although the limit of perceptible noise increases as the noise contents of the host audio signal increases, the typical allowable noise level is very low. The human auditory system has very low sensitivity to the phase of the sound. Unfortunately, this hole has been exploited by numerous audio compression algorithms. According to experiments, 8 bits of hidden data per second can be added to host sounds with quiet backgrounds. Twice as much data can be added to host sounds with noisy background by using phase coding.
Data can also be hidden in digital document images, for example by using word and line shifting. Each document recipient (i.e. a subscriber) receives a document containing a unique set of fingerprint marks. Each mark corresponds to an imperceptible horizontal displacement of a textual object. Since the information is not observable upon casual inspection of a document, a recipient may not be aware of its presence. A recovered, unauthorized document copy can be traced to the original, authorized recipient.