Serial Copy Management System
The Serial Copy Management System or SCMS is a copy protection
Copy protection
Copy protection, also known as content protection, copy obstruction, copy prevention and copy restriction, refer to techniques used for preventing the reproduction of software, films, music, and other media, usually for copyright reasons.- Terminology :Media corporations have always used the term...

 scheme that was created in response to the digital audio tape
Digital Audio Tape
Digital Audio Tape is a signal recording and playback medium developed by Sony and introduced in 1987. In appearance it is similar to a compact audio cassette, using 4 mm magnetic tape enclosed in a protective shell, but is roughly half the size at 73 mm × 54 mm × 10.5 mm. As...

 (DAT) invention, in order to prevent DAT recorders from making second-generation or serial copies. SCMS sets a "copy" bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...

 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
Master recording
A multitrack recording master tape, disk or computer files on which productions are developed for later mixing, is known as the multi-track master, while the tape, disk or computer files holding a mix is called a mixed master.It is standard practice to make a copy of a master recording, known as...


SCMS was also included in consumer MiniDisc
The disc is permanently housed in a cartridge with a sliding door, similar to the casing of a 3.5" floppy disk. This shutter is opened automatically by a mechanism upon insertion. The audio discs can either be recordable or premastered. Recordable MiniDiscs use a magneto-optical system to record...

 and DCC
Digital Compact Cassette
Digital Compact Cassette was a magnetic tape sound recording format introduced by Philips and Matsushita in late 1992 and pitched as a successor to the standard analog cassette. It was also a direct competitor to Sony's MiniDisc but neither format toppled the then ubiquitous analog cassette...

 players and recorders.


SCMS was created as a compromise between electronics manufacturers, mainly Sony and Phillips
-Places:In the United States:*Phillips, California, unincorporated community in El Dorado County*Phillips, Maine, town in Franklin County*Phillips, Nebraska, village in Hamilton County*Phillips, Minneapolis, Minnesota, community in the city of Minneapolis...

, who wanted to make DAT machines available in the United States, and the RIAA, which had previously hampered the availability of DAT machines in the US with the threat of 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 resulted in a chilling effect, which prevented DAT decks from becoming readily affordable.

In 1987, a member of the RIAA proposed a system where DAT recorders would have copy protection in them. The copy protection would look for the presence of frequencies in a particular high-frequency band; if there was no audio present in this band, the recorder would assume that the music in question was copy protected, and would not allow recording of the music. The record companies would then release all music with this particular frequency band filtered out. It would be illegal to manufacture a DAT machine with the presence of audio in this frequency band; the RIAA was lobbying Congress to make this the law of the land.

The reaction to this proposed scheme was very negative. The Home Recording Rights Coalition
Home Recording Rights Coalition
The Home Recording Rights Coalition is a non-profit advocacy organization in the U.S., whose mission is to protect the rights of consumers to view, listen to, and record radio and television broadcasts....

 orchestrated a letter writing campaign opposing this scheme. Editorials in musician's and home stereo magazines attacked this scheme. The proposed law never made it out of committee.

Even after this law was shot down, the RIAA still threatened to sue anyone who released an affordable consumer DAT recorder in the US. No one made such a recorder available.

In 1992, Congress passed the Audio Home Recording Act
Audio Home Recording Act
The Audio Home Recording Act of 1992 amended the United States copyright law by adding Chapter 10, "Digital Audio Recording Devices and Media"...

. In 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 enforced. Blank analog media, such as cassette tapes, were not subject to the tax. SCMS was compulsory in digital media because there is zero deterioration of quality from copy to copy. SCMS was universally disliked by home musicians who used DAT decks to record their own music; the acronym was pronounced as a derogatory term, "scums".

Technical details

SCMS copy protection looks for bits written in the subcode data in a digital link. There are three states of these bits: copy allowed (00), copy once (10) and copy prohibited (11). If the source has the copy bits 00, and a copy is made of the source, the copy itself will have the bit set as 00 as well, allowing copies to be made from copies. If the source has the copy bits set as 10, every copy of this material will have the bits set to 11, and making a copy from the copy would be prohibited.


Software and design defects in certain models of consumer Minidisc player allow SCMS to be defeated. Professional-grade Minidisc systems come with SCMS disabled, but these run at several thousand US dollars.

European electronic hobby magazine Elektor
Elektor is a monthly magazine about all aspects of electronics, first published as "Elektuur" in the Netherlands in 1960, and now published worldwide in many languages including English, German, Dutch, French, Greek, Spanish, Swedish, Portuguese and Italian with distribution in over 50 countries...

published a construction project in the 1990s. The device, once completed, was designed to be inserted in the digital link between SCMS enabled devices (the article was designed around the optical TOSLINK
TOSLINK is a standardized optical fiber connection system. Also known generically as an "optical audio cable," its most common use is in consumer audio equipment , where it carries a digital audio stream from components such as MiniDisc, CD and DVD players, DAT recorders, computers, and modern...

 interface, but it would have been easy to adapt it to the S/PDIF
S/PDIF is a digital audio interconnect used in consumer audio equipment over relatively short distances. The signal is transmitted over either a coaxial cable with RCA connectors or a fiber optic cable with TOSLINK connectors. S/PDIF interconnects components in home theaters and other digital high...

 coaxial link). The circuit intercepted the SCMS control bits, (10 and 11) and substituted the 'Copy Allowed' (00) code. Similar functionality is often also included in commercially available bitrate-converters, like the Behringer
Behringer is an audio equipment company founded by Uli Behringer in 1989, in Willich, Germany. Behringer was listed as the 14th largest manufacturer of music products in 2007. Behringer is a multi-national group of companies, with direct marketing presence in 10 countries or territories and a...


There is another way that SCMS can be defeated, but it requires copying the Table of Contents from a blank disc that already allows copying, to a recorded 'copy disallowed' disc. The method is laborious, and suffers the disadvantage that the track marks and titles are lost in the process.
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