Serial Copy Management System

The Serial Copy Management System (SCMS) is a copy protection scheme that was created in response to the digital audio tape (DAT) invention, in order to prevent DAT recorders from making second-generation or serial copies. SCMS sets a "copy" bit in all copies, which prevents anyone from making further copies of those first copies. It does not, however, limit the number of first-generation copies made from a master.

History
SCMS was created as a compromise between electronics manufacturers, which wanted to make DAT machines available in the United States, and the RIAA, which previously hampered the availability of DAT machines in the United States by threatening lawsuits. The RIAA did not want low-cost digital recorders readily available, since it felt that such technology would result in widespread piracy. These lawsuit threats prevented DAT decks from becoming readily available.

In 1992, the RIAA and the electronics companies reached a compromise by passing the Audio Home Recording Act (AHRA). Under this law, blank digital media (including DAT tapes) would be taxed, with the money going to the RIAA, and a new copy protection scheme, SCMS, would be incorporated into DAT recorders.

Technical details
The copy protection looks for some bits written on the subcode data. There are three states of these bits: copy allowed (00), copy once (11) and copy prohibited (10). If the source has the copy bits 00, and you make a copy of this, the copy will have the bit set as 00 too, allowing copies of the copies. If the source has the copy bits set as 11, every copy of this material will have the bits set to 10 and making a copy from the copy will be prohibited. These bits are transferred over digital links, not over analog links.