Make your mark

Posted on 3 Jul 2002 at 15:59

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If images are your livelihood or music your bread and butter, you may despair at how easy it is to copy and manipulate such things in our digital times but help is at hand in the form of digital watermarking. Mike Bedford explains

The advantages of digital techniques in music and photography are widely appreciated among PC users. The single most important benefit is the ease of copying and manipulating digital formats. We can copy a digital image or a track of digital music at the click of a mouse n and it will be a perfect copy, identical in every way to the original. This differs markedly from analogue methods such as copying photographs by chemical means or copying audio tracks from tape to tape, where each generation gains additional noise and degrades in quality.
Ease of manipulation is just as important. Convoluted darkroom tricks have all but disappeared, and photos are manipulated on a PC with a copy of Adobe Photoshop. Similarly, music production has changed out of all recognition over a decade or so.
Digital techniques don't just simplify processes that could be achieved using analogue methods; they bring completely new possibilities. Given enough skill, amazing effects can be produced in the darkroom but, because of the expertise required, few photographers do more than correcting for poor exposure or colour balance. With digital manipulation, the sky's the limit. You can even create total works of fabrication by taking elements or people from one photograph and importing them into another.
Unfortunately, while the ability to make a perfect copy of a DVD you've rented may be a bonus for you, someone else thinks otherwise. To the movie studios, whose revenues depend on their control of the distribution of their products, this is a serious drawback. An amateur photographer might revel in the ability to modify photographs to their heart's content. To those involved in law enforcement, though, the fact that digital photos can be manipulated easily and therefore be inadmissible as evidence is hardly a boon. Newspaper editors wary of libel cases may have to pass on a scoop unless they can verify that a digital photo has not been tampered with.
We hear a lot about copy protection and its use in preventing copyright violation of digital music and videos. A technique we hear less about, although it can be used to police both digital copying and digital manipulation, is digital watermarking.
Water good idea
We're all familiar with the concept of watermarks. If you hold a £10 note up to the light, you'll see an image of the Queen's head in the pale oval area. This is probably the most commonly encountered application of watermarking. So what makes a watermark so interesting? First, it's invisible under normal conditions n you have to do something special to see it such as holding it to the light. Second, it's integrated within the paper and so is difficult to tamper with, especially compared to markings on the surface of a paper, which n with sufficient ingenuity n can be removed or altered.
These important features are shared with digital watermarks. Clearly, a digital watermark on an audio CD would be unacceptable if it was audible under normal conditions as this would interfere with the music. Similarly, a watermark that degraded the quality of a digital photograph would be of no use. A digital watermark must be imperceptible.
The most obvious way of putting additional data n details of the copyright holder, for example n into a file would be to include it as a header in front of the audio or graphical information. This way, software or electronic equipment would skip that header and the image or audio would be unaffected. Only software that needed to access the information in the header would do so. This would not provide the second pertinent feature of a watermark: that it is integrated into the data. Unless this is achieved, it would be easy for someone to modify or remove the watermark. Therefore, a digital watermark, like a physical one, is integrated completely with the audio, video or image data into which it's embedded.