Serial Copy Management System
The Serial Copy Management System ( SCMS ) is a copy protection mechanism for digital recording devices (eg DAT recorder, MiniDisc, CD recorder ) allows only one generation of copies of a sales piece. The information in the digital data stream Subcodebit 6 is used for the marking. This mean 00 free copying, 11 a digital copy allowed, 10 no longer allows digital copy. Digital re-recordings of material with the code 11 to be 10, whereas 00 is permanently marked as 00. The system is only effective for direct digital copies as an S / PDIF interface between CD players and DAT recorders.
SCMS was required by law by the U.S. AHRA for digital audio recording equipment. The same method is used under the name of CGMS for DVDs. For digital TV is a similar technique in planning, the " broadcast flag ".
The method was originally developed to ensure that consumers could not dub CDs digitally on DAT masse. The possibility of private backup copy but should remain. However, since DAT could not attain remarkable importance in the consumer market ( the devices are due to the band skew in producing much more elaborate and expensive than analog tape drives) and the ability to copy CDs directly, copying to DAT has completely obviated, this method has today only little importance.
Although also intended for digital recording, much later entered the market, digital CD recorder recognize a control bit " 10 " as " any digital copy allowed", but they lack either the opportunity to write the SCMS bits on the CD ( the CD has been defined before SCMS ) or the CD player with digital output ignore the data stored on the CD SCMS information so that in turn " allows a digital copy " when playing the CD in a CD player, the bit sequence "11" for appears. Also on a PC, the corresponding bits are usually ignored because the sound editing on the PC usually includes many loading and storage operations, which would not be realized by a single copy generation.
Professional DAT machines have this copy protection mechanism anyway not in principle, but cost a multiple of what cost consumer devices. However, especially in small studios with limited budget instead of professional equipment more frequently cheap consumer devices are used, and there is the copy protection a major obstacle in the duplication of self-produced works that exist on the market various legal and illegal accessories that looped into the digital signal and will filter out the SCMS bits or set to "00".
By the definition of the data format for PCM audio ( which also contains this SCMS bits) and, for example packaged therein MPEG audio or AC -3, the SCMS bits are found for example in the audio stream of a DVB by transmitted digital radio or TV station. For direct output via a S / PDIF digital interface allows thus ( at least theoretically) from the transmitter controlled a digital recording of sound allow or forbid. Most of these control bits are not used and have the value "00".
Modern DVD players do when playing audio CDs, the SCMS bits have the value "11" or "10", most still have the value "00" from. Reason for this is that no more as in older, "pure" CD players of using the CD PCM stream is eye grinds directly via S / PDIF, but rather ATAPI DVD -ROM drives are used and the firmware of the DVD CD player reads out the audio data on the IDE interface. These are then converted by the firmware itself again only in a serial PCM data stream, the SCMS bits are ignored (especially with cheap devices ).
- Copy protection