DVD GOP User Data Insertion
The user data structure that follows a H.262 GOP header is as follows (the same would a apply after a ISO/IEC 14496-2 GOP header):
| Length | Name | Type | Default |
|---|---|---|---|
| 32 bits | user_data_start_code | patterned bslbf | 0x000001B2 |
| 16 bits | user_identifier | ASCII bslbf | CC |
| 8 bits | user_data_type_code | uimsbf | 1 |
| 8 bits | caption_block_size | inverted uimsbf | 0xf8 |
| 8 bits | caption_block_count | odd parity uimsbf | 0x9e |
| X*24 bits | caption_block | binary | free form |
bslbf: bit string, left bit first ; uimsbf: unsigned integer, most significant bit first
Caption blocks are inserted after the sequence and GOP headers, so each block is for one second of video which would end up being one or two long lines or three to four short lines of text. Also that means if the caption_block_count is greater than 30 then the block contains both interleaved caption fields and one could devise the framing rate from the caption_block_count. However since the data is grouped together the framing rate will almost always be 30/1.001, unlike the ATSC method that inserts one byte pair for each field after the picture header making framing rates of 24/1.001 possible for HD content. Since when a decoder does a 3:2 pull-down for NTSC output the captions will remain in sync.
DVD Caption Block
| Length | Name | Type | Default |
|---|---|---|---|
| 8 bits | caption_field | inverted uimsbf | 0xff |
| 8 bits | caption_first_byte | odd parity uimsbf | 0x80 |
| 8 bits | caption_second_byte | odd parity uimsbf | 0x80 |
Read more about this topic: EIA-608
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