The process of generating a watermark signal and incorporating it into the content is called embedding. From a very high level, the watermark embedding reference model is depicted in Figure 2:
The protected content passes through the DRM where usage rights are verified and the content decrypted. The content plain-text then passes through a decoder to produce the baseband signal. The DRM provides some information to a Forensic Message Generation process. The Forensic Message is encoded for error control, to form a Forensic Message codeword. Finally, the codeword is transformed into a watermark signal suitable for modulating the content.
This model is called blind embedding because the watermark generator is insensitive to the nature of the content. Blind embedding is of relatively low complexity, but does not exploit local characteristics of the content to make the watermark less perceptible. The watermark must be of low enough energy so as not to damage sensitive areas of the content (low textured objects), but strong enough to be recovered from a degraded copy. It is thus challenging to strike a satisfactory balance between perceptibility and robustness in the blind embedding model.
A more sophisticated approach, termed informed embedding, analyzes the content and modulates the watermark signal so as to take advantage of the masking properties of the content. In the video domain, masking is a function of spatial and temporal frequency, brightness, contrast, and edge orientation. By choosing propitious locations, and by tailoring watermark energy and other characteristics to the local content, informed techniques can embed a much more robust watermark than can a blind embedding technique, without exceeding perceptibility thresholds. The detailed model for informed embedding is shown in Figure 4:
The preceding models are termed single ended as their implementation is entirely within the rendering device. Single ended watermarking architecture presents the implementer with a few challenges. The content analysis block must perform complex calculations on the baseband video signal. Data rates are typically in the tens of megabits per second, and greater for HD content. The need to examine entire frames, as well as intensive computation to generate the watermark signal, are likely to drive a requirement for significant buffering where the watermark and content are merged. The processing required to perform the content analysis step is also likely to add significant expense to the device.
Secondly, security issues arise with single ended architecture, as shown in Figure 5. Typical DRM implementations protect only the DRM, exposing unmarked plain text content to eavesdropping. A much larger security envelope is required to ensure that only watermarked content is accessible, and thereby protect the investment in watermarking. In many consumer devices, the security envelope is defined by a smart card, ASIC, or similar component. Integrating complex processes, such as video decoding and content analysis, into such devices is impractical.
A third aspect to consider is renewal. Renewal can be accomplished by downloading new software or firmware to the device, within the limitations imposed by its capabilities. The renewal process can become burdensome if it is necessary to employ ASICs to handle high video baseband data rates, particularly those associated with high-definition content. Security of the renewal process must be considered as well, so as to deprive the pirate of less secure, outdated devices.
