Serial Copy Management System
The Serial Copy Management System ( SCMS ) is a copy protection mechanism for digital recording devices (e.g. DAT recorder, MiniDisc , CD recorder) that only allows one generation of copies of a distribution item . The information in subcode bit 6 of the digital data stream is used for the identification. 00 means free copying, 11 a digital copy allowed, 10 no more digital copy allowed. Digital transfers of material with the code 11 become 10, whereas 00 remains permanently marked as 00. The system only works with digital direct copies, for example via an S / PDIF interface between CD player and DAT recorder.
SCMS was required by law for digital audio recording devices by the US AHRA . The same process is used for DVDs under the name CGMS . A similar technology is being planned for digital television, the “broadcast flag”.
The process was originally developed so that consumers could not transfer CDs digitally to DAT in bulk. However, the option of private backup should be retained. However, since DAT has not achieved any notable importance in the end-user market (the devices are much more complex and expensive to manufacture than analog tape drives due to the skewed tape) and the ability to copy CDs directly has made copying to DAT completely superfluous, this process has been implemented little importance today.
The digital CD recorders, which were also intended for digital recording and came onto the market much later, recognize a control bit “10” as “no digital copy allowed”, but they either lack the ability to write the SCMS bits onto the CD ( the CD has been defined before SCMS) or the CD players with digital output ignore the SCMS information stored on the CD, so that when the CD is played in a CD player, the bit sequence "11" for "a digital copy allowed" appears. The corresponding bits are also normally ignored on a PC, since sound processing on the PC usually involves a large number of loading and saving processes, which would not be possible with a single generation of copies.
Professional DAT devices do not have this copy protection mechanism anyway, but they cost several times what consumer devices cost. Since, however, more and more inexpensive consumer devices are being used instead of professional equipment, especially in small recording studios with a limited budget, and there the copy protection is a major obstacle to the reproduction of self-produced works, there are various legal and illegal additional devices on the market that can be digitally converted Signal are looped in and filter out the SCMS bits or set to "00".
By defining the data format for PCM audio (which also contains these SCMS bits) and z. B. packed therein MPEG audio or AC-3 are the SCMS bits z. B. also in the audio data stream of a digital radio or television station sent via DVB . In the case of direct output via a digital S / PDIF interface, digital recording of the sound can (at least theoretically) be permitted or prohibited under the control of the transmitter. Usually these control bits are not used and have the value "00".
Modern DVD players usually output the value "00" when playing audio CDs whose SCMS bits have the value "11" or "10". The reason for this is that, unlike older, "pure" CD players, the PCM stream on the CD is no longer routed directly via S / PDIF, but rather ATAPI DVD-ROM drives and the firmware of the DVD are used -Player reads the CD audio data via the IDE interface. These are then converted back into a serial PCM data stream by the firmware itself, whereby the SCMS bits (especially with cheap devices) are not taken into account.