Chapter 1

22 Everything You Ever Wanted to Know About DVD
videotapes, some DVDs are Macrovision-protected and some aren’t. For a
few Macrovision details, see STMicroelectronics’ NTSC/PAL video encoder
datasheets at www.st.com/stonline/books/.
Inexpensive devices can defeat Macrovision, although only a few work
against the new Colorstripe feature. These devices include products such as
Video Clarifier, Image Stabilizer, Color Corrector, and CopyMaster (www.
videoguys.com/sima.htm). You can also build your own (http://66.40.78.100/
Services/TECH_Notes/nineteen.html). Some DVD players can be modified
to turn off Macrovision output. Professional time-base correctors (TBCs) that
regenerate line 21 also remove Macrovision. APS affects only video, not
audio.
Copy Generation Management System (CGMS)
Each disc contains information specifying if the contents can be copied.
This is a serial copy generation management system (SCMS) designed to
prevent initial copies or generational copies (copies of copies). The CGMS
information is embedded in the outgoing video signal. For CGMS to work,
the equipment making the copy must recognize and respect the CGMS
information. The analog standard (CGMS-A) encodes the data on NTSC line
21 (in the extended data service [XDS]) or line 20. CGMS-A is recognized by
most digital camcorders and by some computer video capture cards (they
will flash a message such as “recording inhibited”). Professional time-base
correctors (TBCs) that regenerate lines 20 and 21 will remove CGMS-A
information from an analog signal. The digital standard (CGMS-D) is
included in DTCP and HDMI for digital connections, such as IEEE
1394/FireWire (www.1394ta.org). See the “Digital Copy Protection System”
and “High-Bandwidth Digital Content Protection” subsections.
Content Scrambling System (CSS)
Because of the potential for perfect digital copies, paranoid movie studios
added a tougher copy protection requirement to the DVD standard. CSS is
a data encryption and authentication scheme intended to prevent copying
video files directly from DVD-Video discs. It was developed primarily by
Matsushita and Toshiba. Each CSS licensee is given a key from a master
set of 400 keys stored on every CSS-encrypted disc, and a license can be
revoked by removing its key from future discs. The CSS decryption algorithm
exchanges keys with the drive unit to generate an encryption key that
is used to obfuscate the exchange of disc keys and title keys needed to
decrypt data from the disc. DVD players have CSS circuitry that decrypts
the data before it’s decoded and displayed, and computer DVD decoder