Electrical computers and digital processing systems: support – Multiple computer communication using cryptography – Particular communication authentication technique
Reexamination Certificate
1998-10-14
2001-03-27
Hayes, Gail O. (Department: 2766)
Electrical computers and digital processing systems: support
Multiple computer communication using cryptography
Particular communication authentication technique
C399S366000
Reexamination Certificate
active
06209094
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to digital signal processing and, in particular, to a particularly robust watermark mechanism by which identifying data can be encoded into digital signals such as audio or video signals such that the identifying data are not perceptible to a human viewer of the substantive content of the digital signals yet are retrievable and are sufficiently robust to survive other digital signal processing.
BACKGROUND OF THE INVENTION
Video and audio data have traditionally been recorded and delivered as analog signals. However, digital signals are becoming the transmission medium of choice for video, audio, audiovisual, and multimedia information. Digital audio and video signals are currently delivered widely through digital satellites, digital cable, and computer networks such as local area networks and wide area networks, e.g., the Internet. In addition, digital audio and video signals are currently available in the form of digitally recorded material such as audio compact discs, digital audio tape (DAT), minidisc, and laserdisc and digital video disc (DVD) video media. As used herein, a digitized signal refers to a digital signal whose substantive content is generally analog in nature, i.e., can be represented by an analog signal. For example, digital video and digital audio signals are digitized signals since video images and audio content can be represented by analog signals.
The current tremendous growth of digitally stored and delivered audio and video is that digital copies which have exactly the same quality of the original digitized signal can easily be made and distributed without authorization notwithstanding illegality of such copying. The substantive content of digitized signals can have significant proprietary value which is susceptible to considerable diminution as a result of unauthorized duplication.
It is therefore desirable to include identifying data in digitized signals having valuable content such that duplication of the digitized signals also duplicates the identifying data and the source of such duplication can be identified. The identifying data should not result in humanly perceptible changes to the substantive content of the digitized signal when the substantive content is presented to a human viewer as audio and/or video. Since substantial value is in the substantive content itself and in its quality, any humanly perceptible degradation of the substantive content substantially diminishes the value of the digitized signal. Such imperceptible identifying data included in a digitized signal is generally known as a watermark.
Such watermarks should be robust in that signal processing of a digitized signal which affects the substantive content of the digitized signal to a limited, generally imperceptible degree should not affect the watermark so as to make the watermark unreadable. For example, simple conversion of the digital signal to an analog signal and conversion of the analog signal to a new digital signal should not erode the watermark substantially or, at least, should not render the watermark irretrievable. Conventional watermarks which hide identifying data in unused bits of a digitized signal can be defeated in such a digital-analog-digital conversion. In addition, simple inversion of each digitized amplitude, which results in a different digitized signal of equivalent substantive content when the content is audio, should not render the watermark unreadable. Similarly, addition or removal of a number of samples at the beginning of a digitized signal should not render a watermark unreadable. For example, prefixing a digitized audio signal with a one-tenth-second period of silence should not substantially affect ability to recognize and/or retrieve the watermark. Similarly, addition of an extra scanline or an extra pixel or two at the beginning of each scanline of a graphical image should not render any watermark of the graphical image unrecognizable and/or irretrievable.
Digitized signals are often compressed for various reasons, including delivery through a communications or storage medium of limited bandwidth and archival. Such compression can be lossy in that some of the signal of the substantive content is lost during such compression. In general, the object of such lossy compression is to limit loss of signal to levels which are not perceptible to a human viewer or listener of the substantive content when the compressed signal is subsequently reconstructed and played for the viewer or listener. A watermark should survive such lossy compression as well as other types of lossy signal processing and should remain readable within in the reconstructed digitized signal.
In addition to being robust, the watermark should be relatively difficult to detect without specific knowledge regarding the manner in which the watermark is added to the digitized signal. Consider, for example, an owner of a watermarked digitized signal, e.g., a watermarked digitized music signal on a compact disc. If the owner can detect the watermark, the owner may be able to fashion a filter which can remove the watermark or render the watermark unreadable without introducing any perceptible effects to the substantive content of the digitized signal. Accordingly, the value of the substantive content would be preserved and the owner could make unauthorized copies of the digitized signal in a manner in which the watermark cannot identify the owner as the source of the copies. Accordingly, watermarks should be secure and generally undetectable without special knowledge with respect to the specific encoding of such watermarks.
What is needed is a watermark system in which identifying data can be securely and robustly included in a digitized signal such that the source of such a digitized signal can be determined notwithstanding lossy and non-lossy signal processing of the digitized signal.
SUMMARY OF THE INVENTION
In accordance with the present invention, watermark data is encoded in a basis signal by division of the basis signal into segments and inverting the basis signal in segments corresponding to watermark data bits with a first logical value and not inverting the basis signal in segment corresponding to watermark data bits with a different logical value. In particular, the basis signal is generated independently of the watermark data. Such facilitates detection of a watermark in a watermarked digitized signal notwithstanding a substantial degree of lossy processing of the digitized signal even if it is not known beforehand what watermark data to expect.
At segment boundaries, abrupt change from an inverted signal to a non-inverted signal, or from a non-inverted signal to an inverted signal, can produce a discontinuity which is perceptible beyond the perceptibility of the original basis signal. Accordingly, the basis signal is smoothed at segment boundaries to eliminate any such discontinuities. Insufficient smoothing can result in perceptible artifacts in the watermark signal while excessive smoothing reduces the amount of watermark signal used to represent watermark data. Good results are achieved when scaling the basis signal by a cube-root of the positive half of a sine function which is aligned with segment boundaries such that the cube-root sine function tapers to zero at segment boundaries. The results are smooth transitions across segment boundaries and a substantial amount of signal energy used to represent the watermark data.
Further in accordance with the present invention, a watermark is decoded from a watermarked signal by producing a basis signal from the watermarked signal independently from any anticipated watermark data. The basis signal is divided into the same segments and the segmented basis signal is scaled by the same cube-root sine function to smooth the basis signal at segment boundaries. Repetitive iterations of the watermark data are encoded in the watermarked signal. Accordingly, a number of segments which correspond to a particular bit of the watermark data are collected. The segments corresponding to th
Brownell Jason S.
Levine Earl
Hayes Gail O.
Ivey James D.
Leaning Jeffrey S.
Liquid Audio Inc.
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