CGMS-A is standardized through the IEC, CEA, EIA-J and ETSI as follows: 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
analog 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 Description,
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.
Contradicting Type A standards Some references quote EIA-J CPR1204-1 as the authoritative reference for CGMS-A on 480p60 (525p) systems, since this was the first published standard to mention CGMS-A on 480p. This EIA-J document does not define the meaning of the bits, only their timing on the analogue video signal. The 480p signalling is based on the existing 480i standard but with a double speed clock, and IEC-61880-2 formalises this by defining bit meanings which are the same as for 480i. However, CEA-805 re-defines the aspect ratio signalling bits. Hence 480p Type A line 40 CGMMS-A data generated in accordance with CEA-805 cannot signal the aspect ratio of the video image, and in this way is incompatible with the same data generated in accordance with IEC-61880-2, and is no longer a straight "double speed clock" version of the 480i standard.
CEA-805 CGMS-A Type B confusion CEA-805 is now on its fourth major version (CEA-805-D), and there have been errata issued to at least one version. CEA-805-D recognises that, in respect of Type B signalling, earlier versions of the standard were unclear regarding the
order of bits as represented in the analogue video signal vs as used for the CRC calculation, and also
which bits were to be used for the CRC calculation. Issue D requires sink devices to perform multiple CRC calculations for Type B signalling, taking account of various possible implementations in source devices. There is no such confusion surrounding Type A signalling. == Signalling ==