Audio watermarking
The term "watermark" is highly associated with information hiding science. The so-called information hiding can be understood as hiding another kind of information in one kind of information. Therefore, the audio watermarking technique refers to a technique of embedding additional information in an original signal by modifying an audio signal. A large number of related studies have produced quite a number of techniques for embedding and extracting watermark data in audio signals. Most watermarking systems operate on data in a linear domain (ie, PCM watermarks). A small portion of the data that has been compressed can be watermarked (referred to as a bitstream watermark).
The degree of modification of the audio data by the watermarking technique is slight, and the average person's ear cannot distinguish between the original signal and the watermarked audio signal. A dedicated watermark detector can read the embedded information from the signal to which the watermark information has been added.
Fraunhofer IIS has developed a robust audio watermarking technology. The feature of this technology is to ensure that the embedded information remains readable and can be read even after processing such as MP3 compression/decompression or analog transmission. More information is available on the Music Trace website, which currently owns the technology's license from Fraunhofer IIS and is responsible for maintaining the technology.
General idea
The basic idea of ​​watermarking technology can be expressed by the following figure. The key is the additional signal that can not be heard but can be parsed by a special watermark parser. The embedding process of watermark data is performed by relying on a keyword that must be known in the decoding process.
Necessary conditions for watermarking
A well-designed watermark embedding system should have some attributes that are necessary to improve security and effectiveness. The watermark should have:
· Hidden, should not be heard;
· should be transparent to statistics or analysis;
· Resistant to digital signal processing operations;
·Anti-tampering;
· Directly related to the music as a whole, not just at the beginning of the music (head of the file) or somewhere in the music;
· The meaning can be determined by keywords;
The above requirements can be achieved by combining two basic technologies. The two technologies are "PsychoacousTIcs" and "Spread Spectrum" modulation.
The human psychoacoustic properties obtained by the first technique by modeling and analyzing the human auditory system ensure that additional data is not heard. Spread spectrum modulation spreads the embedded information across the entire time domain plane, providing a high degree of robustness against a wide range of signal processing attacks.
application
Data (watermark) compatible transmission
Basically, the watermarking algorithm adds a new data transmission path to the existing music content distribution path. The transmission of data (watermarks) can be backward compatible to some extent, and it can be considered that each existing music content distribution path can transmit music with watermarks. Therefore, watermarking technology can be used in a wide range of applications.
Digital Rights Management (DRM)
Digital rights management is often considered the primary application of watermarking technology. Watermarking technology meets the requirements of digital rights management, such as ownership authentication, digital media access control, tracking illegal copies, and more.
Metadata META data (note: descriptive content used to express the essence of things)
There are many achievable applications for watermark based technology in addition to IPMP usage. One of the important ideas is to transfer metadata along with the music. Metadata can include information such as composers, singers, music types, and the like.
broadcast
Another application of audio watermarking technology is in the field of broadcasting. Includes program type identification, ad search, broadcast coverage research, and more.
Audio quality
Although the watermark information can be concealed from the listener by a well-designed method, strictly speaking, any additional information in the audio signal is a modification of the original signal and thus potentially perceived by the listener. In fact, the degree of audio quality degradation caused by the use of Fraunhofer IIS watermarking technology is very low, and the audio signal quality is lower than that of various popular encoding technologies such as MPEG-2 AAC.
Bit stream watermark
Currently, compressed music distribution based on the Internet or other channels has become more popular and popular. However, controlling the use and redistribution of published content is currently not possible. Watermarking techniques based on association with bitstreams can provide a useful mechanism to track illegal copying or attach copyright information to multimedia content to prevent possible illegal distribution.
Compressed domain watermark embedding
The bitstream watermarking system can embed a watermark directly in the already compressed audio signal. This technique can be applied to the case where the bit stream is pre-compressed to save storage space, such as an Internet music provider.
Linear domain watermark analysis
Although the embedding process takes place in the compressed domain, the resolution of the watermark is always done in the linear domain (ie the audio data that has been decompressed). This is because the decompression of the bitstream must not destroy the watermark that has been embedded. This allows compatibility between PCM (Pulse Code Modulation, ie digitized audio signal) watermark and bitstream watermarking, allowing the same watermark parser to be used for watermark data embedded in the compressed or linear domain. Figure 2 is a composite process.
income
The main application of bitstream watermarking is to personalize media content. This means that during the publishing process (such as downloading or online listening), information about the publishing process, such as the user ID, can be embedded in the content. This enables tracking of content. The advancement of the system includes:
Fast processing (immediate watermark embedding by the server)
Pre-compressed audio can be stored.
· Personalized watermark embedding
·Compatible with PCM watermark technology
Availability
Currently, a watermark embedding system suitable for MPEG-2 AAC pre-compressed bitstream is being developed and tested. It relies on an ad hoc AAC audio encoder that can simultaneously transmit information along with the bitstream, allowing joint optimization of the audio encoding and watermark embedding process in order to improve audio quality, enhance watermark decoding performance, and increase processing speed.
PCM watermark
The PCM watermarking system embeds a watermark in a linear audio signal relative to a bitstream watermarking system. The basic idea is to hide a wideband data signal at the audio signal mask threshold. This makes the watermark signal not heard. Figure 3 is a flow chart.
Watermark analysis
The watermark parser, simply, is a software-implemented spread spectrum receiver that is based on the correlation of pseudo-random signals. The watermark itself can be viewed as a sequence of information bits whose meaning is determined by the user of the system. This means that these bits can express any information, or any information can be transmitted via this "watermark channel".
Inaudible
The inaudibility of the watermark is guaranteed by our masking model. The auditory test proves that the embedding of the watermark does not produce an appreciable distortion even for the signal used for authentication.
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