CGMS-A

From Wikipedia, the free encyclopedia

CGMS-A (Copy Generation Management System Analogue) is a copy protection mechanism for analog television signals. It is not related to the Broadcast flag, which is designed for use in digital television signals.

CGMS-A has been in existence since 1995, and has been standardized by various organizations including the IEC and IEA/CEA. It is used in devices such as PVRs/DVRs, DVD players and recorders, D-VHS, and Blu-ray recorders, as well certain television broadcasts. More recent TiVo firmwares comply with CGMS-A signals.

Implementation of CGMS-A is required for certain applications by DVD CCA license. D-VHS and some DVD recorders comply with CGMS-A signal on analog inputs. The technology requires minimal signal processing.

CGMS-A is standardized in IEC 61880 to appear in the 525i and 480i video signal as vertical blanking interval (VBI) data on line 20. IEC 61880-2 and in EIA-J (without specifying the meanings of the bits) places it to line 41 of 520p signal. EIA/CEA-608-B puts it to line 21 for 525i and 480i. EIA/CEA-805 for component video interfaces standardizes it to line 41 for 480p signal, line 24 for 720p signal, and line 19 for 1080i signal.

EIA-608-B specifies meaning of the 7-bit field placed on the data lines. The bits 4 and 3 contain the CGMS-A values, the bits 2 and 1 contain the Analog Protection System (APS) value, the bit 0 is the Analog Source Bit (ASB) specifying if the signal originates from a pre-recorded material, bits 5 and 6 are reserved.

CGMS-A is signalled by 2 bits in the vertical blanking interval (VBI) signal of analog television broadcasts according to the following matrix:

0 0 CopyFreely Unlimited copies may be made of the content.
0 1 CopyNoMore* One generation of copies has already been made; no further copying is allowed.
1 0 CopyOnce One generation of copies may be made
1 1 CopyNever No copies may be made of the content.
  • - CopyNoMore was not a part of the original standard. The 0,1 value originally was "Reserved".

The signal itself can be easily stripped by normalizing the VBI, eg. using some of the video stabilizers in use to counter the side effects of Macrovision butchering of the VBI. The scheme can be made more robust by adding the Rights Assertion Mark (RAM); when the RAM is present but CGMS-A is not, the copying is denied, turning the scheme into a permission-based one. The RAM can be encoded by using the VEIL technology.

See also:

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