Caption Stream Encoding (block_data)
The 63 caption service sub-streams contain a mixed command and text stream, much like Telnet. There are four logical code sub-groups: CL, GL, CR, and GR. These each have single and multi-character code sets.
| CL Group: C0 | 0x00-0x1F | Subset of ASCII Control Codes |
| CR Group: C1 | 0x80-0x9F | Caption Control Codes |
| CL Group: C2 | 0x1000-0x101F | Extended Miscellaneous Control Codes |
| CR Group: C3 | 0x1080-0x109F | Extended Control Code Set 2 |
| GL Group: G0 | 0x20-0x7F | Modified version of ANSI X3.4 Printable Character Set (ASCII) |
| GR Group: G1 | 0xA0-0xFF | ISO 8859-1 Latin 1 Characters |
| GL Group: G2 | 0x1020-0x107F | Extended Control Code Set 1 |
| GR Group: G3 | 0x10A0-0x10FF | Future characters and icons |
Whenever a command character is seen any text accumulated in the parser should be flushed. Since text might need to be flushed when there is no command pending, there is a null command known as the ETX command in the C0 command set. There are also two special commands, the Reset and DelayCancel. These must be parsed with lookahead. A Delay command issued previously can be canceled at any time with a DelayCancel command, so once a Delay is seen a decoder must look ahead for a DelayCancel, and only look for a DelayCancel. A Reset command on the other hand is sent to break out from an unknown decoder state and all data before it must be ignored.
Read more about this topic: CEA-708
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