Standardization
CGMS-A is standardized through the IEC, CEA, EIA-J and ETSI as follows:
| Video Format | Standard | Line Number(s) | Bit number(s) | Type |
|---|---|---|---|---|
| 480i60 (525i / "NTSC") | IEC-61880 | 20, 283 | 7, 8 | A |
| 480i60 (525i / "NTSC") | CEA-608 | 284 | 4, 3 | - |
| 480p60 (525p) | EIA-J CPR1204-1 | 41 | - | A |
| 480p60 (525p) | IEC-61880-2 | 41 | 7, 8 | A |
| 480p60 (525p) | CEA-805 | 41 | 7, 8 | A |
| 480p60 (525p) | CEA-805 | 40 | 38 (G0), 39 (G1) | B |
| 720p60 | CEA-805 | 24 | 7, 8 | A |
| 720p60 | CEA-805 | 23 | 38 (G0), 39 (G1) | B |
| 1080i60 | CEA-805 | 19, 582 | 7, 8 | A |
| 1080i60 | CEA-805 | 18, 581 | 38 (G0), 39 (G1) | B |
| 576i50 (625i / "PAL") | EN-300-294 | 23 | 12, 13 | - |
| 576p50 (625p) | IEC-62375 | 43 | 12, 13 | - |
In all these standards, the CGMS-A information is only two out of many bits of information that are defined.
On 60 Hz systems (commonly known as "NTSC"), the system is highly extensible, though beyond the CGMS-A bits, only the aspect ratio of the video signal and the analogue protection system (APS) bits are commonly used. The signalling is typically present on every video frame, but CEA-805-D states that "the transmission rate for any given packet type defined in CEA-805-D shall be at least once every three frames", meaning that in theory for two out of three frames, different header values can be used to send data not defined in the standard. Type A signalling (20 bits in total; the only type defined for 480i) offers some extensibility by re-using the 14 data bits via one of the 14 undefined values for the four header bits. Type B signalling (134 bits in total) already defines bits to carry an Active Format Descriptor, Colorimetry, Redistribution Control, and a pixel-accurate definition of the location of any letterbox or pillarbox bars in the image, plus two bytes reserved for future use. Different header bit values may also be used for further extensibility.
On 50 Hz systems (commonly, though incorrectly known as "PAL"), the bits that are widely used and interpreted as CGMS-A are not named as such, and are added at the end of an existing signalling standard originally created for the PALplus video format (but still in common use in Europe in standard PAL video) called Widescreen signaling.
Read more about this topic: CGMS