Digital watermarks are and will continue to be used to protect music from piracy and to ensure copyrights. As I have discussed in previous chapters, audio watermarking is done by introducing subtle changes into a music file in a particular way. The watermark could be something similar to a particular tone at a particular frequency repeated periodically or something more sophisticated such as simulating the acoustics at the location where the music was recorded, and then altering those acoustics. Some examples of this include BlueSpike's technology that removes a few select tones in a very narrow band. Verance adds signals that are just out of the range of human perception. Others adjust the sound by changing the frequency slightly.
Watermarks in digital music could also carry a variety of information. Some may simply encode a message indicating the file is copyrighted. Another, more sophisticated version could include artist or copyright information (or both). The watermark could even be customized to track down the original owner of a pirated file.
The big enemy of digital watermarks with respect to music is compression. Because compression excludes unwanted information, compression software could be used as a tool for stripping away digital watermarks. Because the watermark typically resides in areas beyond the perceptible auditory range in humans, a compression algorithm would detect and remove it during compression.
The Secure Digital Music Initiative (SDMI), whose charter is to set a common standard for computer, electronic, and entertainment companies, is working with a watermark that simply tries to identify copyrighted music to computers and MP3 players.