Pirates on the Airwaves - Technology Information

Emedia Professional, Sept, 1999 by Linden deCarmo

Since digital audio streams are normally filtered to remove quantization errors, an audio watermark theoretically can be inserted during filtering without affecting sound quality. For example, a filter may monitor the audio stream to prevent clipping. When it detects clipping, the filter smoothes out the wave to prevent an irritating pop. While it is smoothing, the filter can add a digital signature (or watermark) to the stream. Watermarks can also be stashed in the amplitude phase (or volume component) of the waveform. This solution is the least desirable because the ear is sensitive to volume manipulations and because hackers can detect unusual patterns in the amplitude of the wave.

Another technique for hiding a watermark is to place it in an undetectable frequency. Because the brain will discard apparently redundant frequencies, a watermark can be placed on such a frequency without affecting fidelity. You must be careful when using this approach, however, as a perceptual encoder such as MP3 may detect this frequency as unnecessary and strip out the watermark when the stream is recompressed. Frequency-based watermarks also tend to avoid frequencies above 22KHz since they aren't detectable by most humans and are discarded by most samplers.

Since pirates can write algorithms to detect the presence of filter and frequency watermarks, time-based watermarks are usually interspersed within the content. These watermarks are located at random (or pseudo-random locations) within the stream to eliminate predictability. Although frequency, amplitude, and time-based watermarks can be used independently, most watermarking solutions combine two or more watermark alternatives to create a more robust solution. Furthermore, most companies enhance these alternatives with proprietary techniques to differentiate themselves from competitors.

THE BATTLE FOR DOMINANCE

The audio industry is about to make a transition from the uncompressed 44KHz, 16-bits per sample audio format established by the Red Book CD format into the 96KHz, 24-bit, multichannel audio specification defined for DVD-Audio. Greater sample resolution and higher sampling rates result in digital audio streams that more closely approximate the analog original and, consequently, sound better. Unfortunately, this transition could be painful for some audio watermarking vendors since it may expose weaknesses in their algorithms.

For example, if a company designed a hardware filter that assumed a 16-bit sample size, this filter would break if it tried to insert a watermark into a stream with 24-bit samples. Other poorly designed solutions will blow up when they have to deal with hiding watermarks in frequencies higher than 22KHz. In addition, some audio purists are concerned that the noise introduced by sub-standard watermarking algorithms will degrade sound quality to CD levels and thereby negate the benefits of increased sample resolution and sampling rates.

As we discovered with DVD-Video, watermarking is only one aspect of a complete audio copy protection solution. Content holders that deliver on physical media are likely to complement it with a CSS-like algorithm to prevent unauthorized access to the content.