Employees of record companies, radio stations and magazines receive a fair amount of promos for review with a serial number embedded in the music for security purposes. This is done so in the unlikely event that a leak may occur, it can be traced to the recipient of the CD. Besides the personal watermark within the music, they all come with a different recipient name and personal catalog number printed on the disc.
The watermark is very dynamic as it can not be changed or destroyed by extracting clips of the music, or by using any compression technology like mp3 and it even survives if someone records the content with an analog tape recorder and then back to digital. This is done by adding an audio track that computers would pick up, but which music fans could not hear, an art and science known as steganography. Due to the vast number of hiding techniques, detecting them all is infeasible and indeed detecting the presence of any could be time consuming.
Steganography can be split into two types: Fragile and Robust. While in Fragile steganography the information is easily destroyed if the file is modified, in Robust steganography the information is hidden in a part of the file where its removal would be easily perceived — the amount of changes required to remove it would render the file useless.
The two main types of Robust marking are Fingerprinting, which involves hiding a unique identifier for the customer who originally acquired the file and therefore is allowed to use it, and Watermarks, which identify the copyright owner of the file, not the customer. Whereas fingerprints are used to identify people who violate the license agreement, watermarks help with prosecuting those who have an illegal copy. Ideally fingerprinting should be used, but for mass production of CDs and DVDs, it is not feasible to give each disc a separate fingerprint.
It is possible to embed data within an audio files by taking advantage of human perception to mask a frequency. When two tones with similar frequencies are played at the same time, the listener only hears the louder tone while the quieter one is masked. Similarly, temporal masking occurs when a low-level signal occurs immediately before or after a stronger one as it takes us time to adjust to hearing the new frequency.
Since many of the formats used for digital media take advantage of compression standards such as MPEG to reduce file sizes by removing the parts which are not perceived by the users, the mark should be embedded in the perceptually most significant parts of the file to ensure it survives the compression process, but this would result in a loss of quality because some of the information would be lost. A simple technique involves embedding the mark in the least significant bits which will minimise the distortion. However, it also makes it relatively easy to locate and remove the mark. An improvement is to embed the mark only in the least significant bits of randomly chosen data within the file.
In the past, to protect the music, a record company would come as far as inserting dropouts at various points in songs on an advance CD with different locations for each person receiving the disc to help narrow down where downloaded copies came from. Recently, big companies are not only putting notification on the CD that it is burned specifically for a certain person, but they sometimes also send a separate note telling that the CD can be traced and should not be duplicated, left with or heard by any other party; and sometimes the artist's name on the actual CD is changed to prevent theft. In the case of Linkin Park, the acronym of the band's initials is kept (LP), but without the instructions the CD recipient is given, there's no way to know who the artist is before release. Names used in the past by the band include "Lion Pride", "Larry Potter" and "Little Ponies".
- LPCatalog - News, February 02, 2017
- BBC News | NEW MEDIA | Digital music's security flaws exposed, August 16, 2001
- Introduction, March 04, 2004
- Promo CD watermarking : Is this for real? | Steve Hoffman Music Forums, July 19, 2005