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Audio Watermarking

General

With the aid of audio watermarking technology it is possible to embed additional information in an audio track. To achieve this, the audio signal of a music recording, an audio book or a commercial is slightly modified in a defined manner. This modification is so slight that the human ear cannot perceive an acoustic difference. Audio watermarking technology thus affords an opportunity to generate copies of a recording which are perceived by listeners as identical to the original but which may differ from one another on the basis of the embedded information.

Only software which embodies an understanding of the type of embedding and embedding parameters is capable of extracting such additional data that were embedded previously. Without such software or if incorrect embedding parameters were selected it is not possible to access these additional data. This prevents unauthorized extraction of embedded information and makes the technique very reliable.

This characteristic is utilized by MusicTrace in a targeted manner. Every MusicTrace customer receives a unique set of embedding parameters. Consequently, each customer is only capable of extracting that information which he embedded himself. Accessing embedded information of other customers, by contrast, is not possible.

In addition to the inaudibility of the watermark and process security, two other factors play an important role. The first of these is the data rate of the watermark, i.e., an indication of the volume of data which can be transmitted in a given period of time. The other is the robustness of the watermark. Robustness is an indication how reliably a watermark can be extracted after an intentional attack or after transmission and the inherent signal modifications. The watermarking process implemented by MusicTrace was investigated by the European Broadcasting Union (EBU) in terms of robustness. Forms of attack investigated included analog conversion of the signal, digital audio coding or repeated filtering of the signal. This revealed that the watermark can no longer be extracted only when the quality of the audio signal has been substantially degraded as a result of the attack.

The factors data rate and robustness are mutually dependent. If more inaudible information are to be transmitted in a certain time the robustness of the watermark declines as a consequence. For embedding additional data, MusicTrace utilizes data containers that permit an acceptably high data rate and robustness. The two most commonly used data containers permit transmission of 48-bit additional data in 5 seconds with a very high robustness or 48-bit additional data in 2.7 seconds with slightly lower robustness.

Fields of Application

Because audio watermark technology in principle opens the possibility for transmitting embedded additional data in the audio track but imposes no restrictions in terms of the type of additional data, this technology has a broad range of potential applications. A few typical scenarios for audio watermarking are briefly sketched in the following.

  • Embedding of Copyright Information: During production, copyright information in the form of a watermark can be anchored directly in the recording. This makes it possible to check at a later time whether a competitor, for example, has taken samples of music played on a valuable instrument and used them in his product without permission. With the aid of the watermark, it is also possible to provide copyright verification in the event that a competitor claims he produced a given title.

  • Embedding of Recipient Information: It can also be expedient to utilize audio watermarking of promotional recordings provided to radio stations or the press or when music tracks or audio books are sold by an Internet shop. Here the idea is to personalize every recording distributed. In such cases information is embedded as a watermark that can be used at a later time to monitor recipients. This can be the recipient's customer number, for example. If these recordings are found later on the Internet, the embedded data can be used to identify the person to whom the recorded material was originally distributed.

    The advantage of the watermarking technique over the Digital Rights Management (DRM) technique is that the original multimedia format is not changed by the watermark. To illustrate this, if a watermark is embedded in an MP3 file, the result is an MP3 file that can be played on any commercially-available MP3 player. It is therefore not necessary for customers to purchase special playback devices. Furthermore, the watermark remains in the recording even in the event of format conversion, even if the material undergoes analog conversion.

  • Embedding of Title Information: The audio watermarking technique can in principle also be implemented for monitoring radio broadcasts. To do this, each title that is to be monitored must be provided with a unique identifier in the form of a watermark. A computer subsequently scans radio programs and investigates the signal for watermarking. Even though the audio fingerprinting technique is usually implemented to perform this task there are cases in which the audio watermarking technique offers certain advantages. This is the case, for example, when specific television commercials are to be monitored that have various video tracks but an identical audio track.

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