CGMS-A
 | This article needs additional citations for verification. (December 2007) |
Copy Generation Management System - Analog (CGMS-A) is a copy protection mechanism for analog television signals. It is not the same as the Broadcast flag, which is designed for use in digital television signals. The concept is the same as the Broadcast flag[1]. There is a digital form of CGMS specified as CGMS-D which is required by the DTCP ("5C") protection standard.
History
CGMS-A has been in existence since 1995, and has been standardized by various organizations including the IEC and EIA/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.
Applications
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.
Standardization
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. According to ETSI EN 300 294 for 625 line SD output resolutions it is signaled via widescreen signaling (WSS) in line 23 (bits number 12 and 13, group 4). 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.
Signalling
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".
Removal
The signal itself can be easily stripped by normalizing the VBI, e.g. using a video stabilizer to counter the side effects from Macrovision's manipulation of the VBI. CGMS-A VBI data is commingled or generally near captioning signals, so removal of CGMS-A will likely remove captioning as well.
The scheme can be made more robust by adding the Rights Assertion Mark (RAM); when the RAM is present but CGMS-A is not, copying is denied, turning the scheme into a permission-based one. The RAM can be encoded by using the VEIL technology.
See also
- Copy Generation Management System - Analog (PowerPoint)
- Microsoft VIDEOPARAMETERS structure for Windows GDI video connections
- Techdirt: Microsoft: It's Not The Broadcast Flag, It's A Different Flag