Cactus Data Shield

Cactus Data Shield (CDS) is a form of CD/DVD copy protection for audio compact discs developed by Midbar Tech now owned by Macrovision. It has been used extensively by EMI and BMG and their subsidiaries, see Copy Control. CDS relies on two components: multiple sessions, and data corruption.

As of September 2006, all of Macrovision's CD copy protection products, including CDS, have quietly disappeared from their website. [http://www.macrovision.com/] It is also unclear if EMI is still using the technology.

Multiple Sessions

CDS discs contain an initial audio session like that of an unprotected disc. The disc also contains a second (data) session in addition to the first audio session with a corrupt "Table of Contents", a software player configured for auto-play, and a lower-quality, compressed version of the same audio held in the first audio session for the software player to play. Typically those drives that don't recognise the audio session launch the player and the user can only listen to the low quality compressed audio using this player. Reportedly few discs include Macintosh versions of the software, and generally these computers have no problems, being able to see an audio and data session is present on the disc.

The second session on the disc causes some CD players to hang, typically some car players (allegedly using CD-ROM drive mechanisms) and some MP3 capable players that can see but not understand the second data session.

The second session has been circumvented by another method, which is to either place masking tape around the disc near the edge, or mark a strip next to the edge with permanent marker. Because the bad Table of Contents for the second (data) session of the disk is near the edge, this method can hide the second session, leaving only the first audio session visible.

On a Windows computer disabling Auto-play either once when loading the disc, or permanently, can stop the software player launching and may be all that is required to access the audio session for drives that recognise both sessions.

A side effect of the second session containing the music in compressed form is that the maximum length of music on a CDS disc is reduced, being approximately 70 minutes. The remaining space is use for the compressed audio (and the player software and other files though these are small by comparison).

Data Corruption

The second aspect of Cactus Data Shield is careful corruption of the audio data, as described in the Midbar patent "Prevention of disk piracy" US patent number 6,425,098. As usual for patents this is freely available on the US Patent Office web site.

In summary the method described detects during mastering when the waveform of the music comes close to being a straight line for at least the size of a "frame" (information on the disc is encoded in blocks called frames, each frame contains about 1/75 of a second of music for a CDDA disc) and marks the frame as a data frame (as opposed to audio) in subcode and replaces the content with loud audio hash.

Interpretation of Corrupted Data

An old-fashioned CD player reading subcode correctly sees a missing audio frame and interpolates any missing information that it cannot correct using information from neighbouring frames. Because these missing frames occur at points where the waveform was nearly a straight line anyway, this interpolation is very accurate and generally transparent to the user.

What happens with computer drives is very specific to the hardware and firmware of the drive in question, assuming they have at least seen past the second data session and can play the audio session.

Some older drives simply ignore the subcode and "play" the data frames, resulting in loud audio glitches. Some are overwhelmed by the number of errors needing correction and interpolation, and these drives may then output occasional glitches. Ripping at the drive's minimum speed can reduce or eliminate this effect.

The majority of new drives can successfully correct and interpolate all missing audio even at maximum ripping speeds.

Impact

The techniques used on Cactus Data Shield discs mean the discs do not conform to the red book Compact Disc Digital Audio standards, and they therefore do not bear the Redbook logo. For this reason they should not be referred to as CDDA (Compact Disc Digital Audio).

There are also ethical/legal issues surrounding the selling of discs that won't reliably play in all players, where added errors make the disc more easily affected by accumulated lifetime damage, and where actual audio data has been omitted.

The first ever commercially released CD to utilise copy-protection was "White Lilies Island" by Natalie Imbruglia, which used the Cactus Data Shield and was released in November 2001. With only a minor mention of the CDS in the small print of the CD case, the album was the subject of many complaints from consumers who found that they could not play the CD on non-Windows computers, games consoles and some other devices. Cases included the Xbox repeatedly playing only a small portion of track 1, whilst Playstation 2 users could play track 2 but not track 1. BMG later provided unprotected copies both commercially and to those who had the original disc. Only the initial European release was copy-protected.

ee also

* CDS - Cactus Data ShieldCDS100/CDS200/CDS300/Totalplay

References

[http://space.newscientist.com/article/dn1586 Fans get free replacement of copy-protected CD]

External links

[http://www.macrovision.com Macrovision Website]