EEG HD490 - Grand Alliance Protocol

HD490 Smart Encoder
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Appendix C: Grand Alliance Interface Protocol
The following table describes the Data Packet Structure used by EEG
equipment to send caption data to Grand Alliance protocol ATSC
encoders. This protocol has been proven compatible with encoders from
all major manufacturers supporting GA protocol.
Byte
Name
Value
Meaning
0
SOH
0x01
ASCII SOH, start of packet
1
Type
0x41
ASCII “A”, ATVCC data
0x31
ASCII “1”, NTSC field 1 data
0x32
ASCII “2”, NTSC field 2 data
2
Count
5+n
Packet size, in bytes, including header
and trailer bytes.
3
Data 1
CEA-708 data bytes.
4
Data 2
2+n
Data n
3+n
Checksum
<varies>
1 byte checksum. The sum of all bytes
in the packet must be zero, modulo
256.
4+n
EOT
0x04
ASCII EOT, end of packet
1. The maximum packet size is 128 (0x80).
Notes:
2. Because the packet size (Count) includes the header and trailer bytes,
the minimum valid count is 5. This corresponds to a packet with zero
data bytes.
3. This packet structure is applied only to the data for the closed caption
serial stream input to the ATSC encoder. Outgoing bytes in the
ATSC stream follow the CEA-708B standard.