Audio Watermark
Jiamian Yang
Abstract
The recording industry is losing money. Piration should be blamed. But
are there any way to prevent the trend? Yes, the answer lies in the
watermark technology. In my paper, first I will talk about the original and
definition of audio watermark. Then the main idea, that is to say, the
mechanism behind watermark will be discussed. Thirdly, properties
would be elaborated. Then different applications are discussed. The last
but not least, a vivid case in real life is presented.
Introduction
According to figures published by the IFPI, the recording industry lost
4.2 billion dollars almost every year. The illegal manufacturer burn CDs
and sell them to the customer, who can buy the same CD the tenth price
as in some music store. What is more, some website provide download or
online listening service to music fans. However, an audio watermark
breakthrough the recording industry needs. Actually the audio watermark
is like a signature, different to every copy and piece of music. It could be
used to track the owner of illegal distribution and could be also used as a
copyright for the owner. Naturely, it has to make sure that it could not be
heard and interfere with the original sound quality and it could survive
the audio signal distortion. In my paper, first I give the origin and
definition to audio watermark, so to say, what is watermark, audio
watermark, where does it come from. Secondly I will discuss the main
idea behind the technology and then one of them-bitstream watermark.
Third, properties of audio watermark would be elaborated. That is what
requirement of an audio watermark should be met. The forth, I will talk
about the applications of audio water, how it could be used? The last but
not least, a special case of music trace company is presented. I will
concentrate on several business module based on the audio watermark
technology.
Audio watermark-Origin and definition
Actually, the word watermark comes from paper industry first. A
watermark is a design embossed into a piece of paper during its
production and used for identification of the paper and papermaker. The
watermark can be seen when the paper is held up to light. The design
could be some pattern or images which indicate quality of trademark of
the paper. Then the word watermark in introduced to digital field. It is an
identification code carrying information about the copyright owner, the
creator of the work, authorized consumers and so on(1). The watermark is
invisible and permanently embedded into digital data for copyright
protection and for checking if the data has been corrupted. The electronic
documents which is watermarked can be digital image, audio or even
video. But I will only concentrate on the audio watermark, which means a
watermark embedded in an audio stream to identify its origin.
The main idea
All watermarking system consist of two blocks-watermark embedding
system and a watermark detector system as we can see in picture one
Picture one:
In the watermark embedding block, a copyright owner uses a private key
to create an inaudible watermark on the audio file. So the private key is
used to encode the digital watermark into the music. Later on, the owner
can examine whether a given audio file, which us suspected for being
illegally copied, contains his or her own watermark and use it as legal
prove. In the watermark detector block, a public key is used to decode
the watermark from the music. The basic scheme contains a lot of
possibilities both for compressed file and for uncompressed files. Below
is how the audio watermark is embedded into the compressed files such
as MP3.
Picture 2
The mechanism is called bitstream watermarking.
Properties of audio watermark
Some basic requirements should be met by an audio watermark. These
are low computational cost, no distortion of original signal and change of
format, robustness and imperceptibility.
Low computational cost means it should not take long for a watermark to
be embedded and detected. It is a crucial factor in the watermark system.
The watermark imbedded in the audio file should in no way reduce the
quality of the audio file and change the format. If a watermark is
embedded in an MP3 file, the customer can also play it on some MP3
supported player, they do not need to buy a new player again. Robustness
refer to the ability to detect the watermark after common signal
processing operations and hostile attacks. Example of common operations
performed on audio files include noise, reduction, volume adjustment or
normalization, digital to analog conversion and so forth. As far as attacks
concerned, there is different kinds of attack on the audio watermark. They
are collusion attack, signal diminishment attacks and inversibility attacks.
A collusion attack is having copies of audio files with the same
watermark, the attacker estimate the value of watermark by averaging the
value of the audio signal. Only about 10 samples are enough for such
estimation. As a coutermeasurement to this kind of attack, it is better to
embed more than one watermark in an audio file or embed different
watermark for different files. The signal deminishment attack refer to
different signal operation such as lossy compression, modulation,
conversion, etc. Now it is possible to embed more than one watermark in
a audio file. If Bob steals the watermarked one from Alice and watermark
it with his private key. It is sure to find Alice’s watermark in Bob’s file,
but vice versa not possible. However, an attack to this procedure is
created. So it is hard to decide who is the real owner of the audio file(2).
The other property of audio watermark is imperceptibility. There should
be almost no difference between the watermarked one and the
unwatermarked one. Here is a sound wave graph of the same music with
and without watermark. As we can see, there is only small changes which
could not be noticed at the first sight and could also be ignored.
Picture 3
Applications
The audio watermark is used in many ways. First, it is used in proof of
ownership. Information during production, copyright information in the
form of a watermark can be anchored directly in the recording. Specific
ownership has different information imbedded. It could also used for
access control, the watermark becomes a trigger to give allowance for
playing the music. Tracing illegal copies is also an very important
application. Personalized information is embedded in the music. The
personalized information could be the customer number which moniter
the trace of the music. If such music is found in internet, the watermark
will be distracted and the customer number could be known for later law
actions.
Case study
Watermark is used in real life business. The fraunhofer institution in
Germany invented the watermark embedding technology and sold it to
the company music trace. Company music trace unfold two main
business module based on the technology. They are watermark
embedding for audio signals and internet search for illegal use. The
company use specific software to listen on the internet all day and night
and search the matched music. Of course, the customer should provide
several criteria for selecting the music first. For example, the music with
specific title should be caught. Then the watermarks would be distracted
as evidence. But only transaction numbers or customer numbers could be
extracted. The identity of original purchasers can only be determined by
the customers self.
Another business module is watermark embedding for audio signals.
Copy right information, recipient information and title information could
be imbedded. The imbedded content is different. The Product family
contentmark permits embedding a digital watermark in audio signals.
Depending on the product, this can involve either uncompressed or
compressed audio signal(3). ContentmarkPCm is a technology to embed
watermark in uncompressed files and the watermarked files are still
uncompressed files. The result is unchanged. The file could be PCM files
or Wave files. The contentmarkCD technology make it possible to create
different watermarks on the same copy files. ContentmarkMP3 is used in
compressed files.
Conclusion
As the conception watermark goes into digital area, recording industry
has some hope not to lose money. But it needs time. The watermark is
used as a special identification for every audio file. Different watermark
algorithm and scheme have been developed. Nevertheless, it should be
expanded and amended to cope with malicious attacks.
Reference
1. www.wikepedia.org
2. page 75, multimedia security, steganograghy and digital watermarking
technique for protection of intellectual property, Chun-Shien Lu
3. www.musictrace.de/products/contentmark.en.htm