Audio Watermarking

With the perfect transfer of audio as data, the widespread copying of CDs and the apparently ubiquitous belief that downloading mp3s of copyright material is acceptable, the idea that audio signals could be indelibly marked to express ownership and establish authenticity has gained ground in recent years. The analogy could be made with bank notes where the paper is marked with an “invisible” logo (watermark) which makes forgery more difficult, hence the adoption of the term “audio watermark” to describe this technique. Several techniques have been proposed.

Least Significant Bit (LSB) Modification

In this technique (which has many variants) the least-significant bit (the smallest bit of the sixteen or twenty-four) is retired from use in the audio signal and devoted instead to signalling. Provided that efforts are many to pseudo-randomise the activity of the bit, so that its contribution in audio terms is solely to add a small amount of noise, this technique can be regarded as being effectively “inaudible”. The technique suffers from being fragile because virtually any form of signal processing (gain change or sample-rate conversion) can destroy the extra data signal and it is certain to be lost in any digital to analogue conversion and re-recording.

Low Level Tones

Other systems have been proposed based on extra data signals embedded as low-level tones carefully selected to be masked by the audio signal. Sometimes the systems suggest inaudible tones towards the edges of the top-frequency (Nyquist) limit where they will be inaudible to the majority. The problem with the latter technique is that the watermarking is easily removed by low-pass filtering.

Echo hiding

Another way that data can be “hidden” within the audio signal is by the addition of low-level echoes added to the signal itself. By modulating the delay of the echo, extra data can be conveyed. In the decoder these echoes can be detected by auto-correlating the cepstrum of the signal.

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