EIA-608
EIA-608, also known as "line 21 captions" and "CEA-608", was once the standard for closed captioning for NTSC TV broadcasts in the United States, Canada and Mexico. It also specifies an "Extended Data Service", which is a means for including a VCR control service with an electronic program guide for NTSC transmissions that operates on the even line 21 field, similar to the TeleText based VPS that operates on line 16 which is used in PAL countries.
It was developed by the Electronic Industries Alliance and required by law to be implemented in most television receivers made in the United States.
EIA-608 captions are transmitted on either the odd or even fields of line 21 with an odd parity bit in the non-visible active video data area in NTSC broadcasts, and are also sometimes present in the picture user data in ATSC transmissions. It uses a fixed bandwidth of 480 bit/s per line 21 field for a maximum of 32 characters per line per caption for a 30 frame broadcast. The odd field captions relate to the primary audio track and the even field captions related to the SAP or secondary audio track which is generally a second language translation of the primary audio, such as a French or Spanish translation of an English-speaking TV show.
Raw EIA-608 caption byte pairs are becoming less prevalent as digital television replaces analog. ATSC broadcasts instead use the EIA-708 caption protocol to encapsulate both the EIA-608 caption pairs as well as add a native EIA-708 stream. EIA-608 has had revisions with the addition of extended character sets to fully support the representation of the Spanish, French, German languages, and cross section of other Western European languages. EIA-608 was also extended to support two byte characters for the Korean and Japanese markets. The full version of EIA-708 has support for more character sets and better caption positioning options; however, because of existing EIA-608 hardware and revisions to the format, there has been little or no real world use of the format besides simple 608 to 708 inline conversions.
Channels
EIA-608 defines four channels of caption information, so that a program could, for example, have captions in four different languages. There are two channels, called 1 and 2 by the standard, in each of the two fields of a frame. However, the channels are often presented to users numbered simply as CC1-2 for the odd field and CC3-4 for the even field. However due to bandwidth limitations on either field, CC1 and CC3 are the only ones used, meaning that there has been little use for the second channel. Early Spanish SAP captioned broadcasts first used the second channel CC2 because the original caption decoders only read the first odd field, but later switched to using CC3 for bandwidth reasons. Due to the same bandwidth reasons XDS was never used by Spanish-speaking stations.Within each channel, there are two streams of information which might be considered sub-channels: one carries "captions" and the other "text." The latter is not in common use due to the lack of hardware support and bandwidth available. Text is signaled by the use of text commands and can be used for a formatted URL string with a 16-bit checksum that designates a web site that the captions relate to or a local station communication channel.
This layering is based on the OSI Protocol Reference Model:
DVD GOP User Data Insertion
The user data structure that follows a H.262 GOP header is as follows :| 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 |
| 1 bit | caption_odd_field_first | flag | 1 |
| 1 bit | caption_filler | alignment | 0 |
| 5 bits | caption_block_count | uimsbf | 15 |
| 1 bit | caption_extra_field_added | flag | 0 |
| 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.
| Length | Name | Type | Default |
| 7 bits | caption_filler | alignment | 0x7f |
| 1 bit | caption_odd_field | uimsbf | 1 or 0 |
| 8 bits | caption_first_byte | odd parity uimsbf | 0x80 |
| 8 bits | caption_second_byte | odd parity uimsbf | 0x80 |
DVB Transport Insertion
The packet-ed structure that is inserted before the H.222 video packet is as follows for a frame of associated video:| Length | Name | Type | Default |
| 32 bits | private_stream_1_start_code | patterned bslbf | 0x000001BD |
| 16 bits | PES_packet_length | uimsbf | 176 |
| 2 bits | PES_version | uimsbf | 2 |
| 1 bit | PES_priority | flag | 0 |
| 2 bits | PES_scrambling_control | uimsbf | 0 |
| 1 bit | data_alignment_indicator | flag | 1 |
| 2 bits | copyright original | uimsbf | 0 |
| 2 bits | PTS_DTS_flag | uimsbf | 2 |
| 6 bits | various_PES_flags | uimsbf | 0 |
| 8 bits | PES_header_data_length | uimsbf | 36 |
| 40 bits | PTS | uimsbf | varies |
| 248 bits | stuffing_bytes | uimsbf | 255 |
| 8 bits | data_identifier | uimsbf | 153 |
| 8 bits | data_unit_id | uimsbf | 197 |
| 8 bits | data_unit_length | uimsbf | 3 |
| 2 bits | reserved_future_use | uimsbf | 3 |
| 1 bit | field_parity | flag | 0 |
| 5 bits | line_offset | uimsbf | 21 |
| 16 bits | closed_captioning_data_block | uimsbf | 608 caption |
| 8 bits | data_unit_id | uimsbf | 197 |
| 8 bits | data_unit_length | uimsbf | 3 |
| 2 bits | reserved_future_use | uimsbf | 3 |
| 1 bit | field_parity | flag | 1 |
| 5 bits | line_offset | uimsbf | 21 |
| 16 bits | closed_captioning_data_block | uimsbf | 608 caption |
| 8 bits | data_unit_id | uimsbf | 255 |
| 8 bits | data_unit_length | uimsbf | 124 |
| 124*8 bits | stuffing_bytes | uimsbf | 255 |
bslbf: bit string, left bit first ; uimsbf: unsigned integer, most significant bit first
This structure was designed for any digital VBI data and was optimized to carry three or more 43-byte Teletext packets. e.g. a page header and two associated lines. For Teletext subtitles, the data_unit_id is set to 3. In this form, captions have to be separated into byte pairs spread over frames in one second of video rather than grouped into one block as with the DVD structure. The same is true for Teletext subtitles with more than one line of text.
SDI/MXF [SMPTE 291M] Insertion
The packet-ed structure that is inserted before the SMPTE 259M active video frame or MXF essence video packet is coded as follows for a frame of associated video:| Length | Name | Type | Default |
| 16 or 128 bits | ancillary_flag or ancillary_header | patterned bslbf or 7 uimsbf | 0xFFFF or varies |
| 8 bits | data_id | uimsbf | 97 |
| 8 bits | secondary_data_id | uimsbf | 2 |
| 8 bits | data_count | uimsbf | varies |
| X*24 bits | caption_data_block | binary | free form |
bslbf: bit string, left bit first ; uimsbf: unsigned integer, most significant bit first
This structure was designed for any digital audio or metadata that is to be synchronized with a video frame. SDI transports every eight bits in a 10 bit aligned packet, unlike MXF which is byte aligned and the ancillary flag bytes are replaced by 128 bit header.
| Length | Name | Type | Default |
| 1 bit | caption_odd_field | flag | 1 |
| 2 bits | caption_reserved | uimsbf | 0 |
| 5 bits | caption_line_offset | uimsbf | 15 |
| 8 bits | caption_first_byte | odd parity uimsbf | 0x80 |
| 8 bits | caption_second_byte | odd parity uimsbf | 0x80 |
Extended Data Service
The EIA-608 data stream format includes Extended Data Service, a variety of information about the transmission. It is all optional,:- program name
- offensiveness rating
- program category
Characters
Extended characters are a later addition to the standard and their decoding is optional.
EIA-608 provides controls for the color of the foreground and background of the text, underlining, blinking, and italics. The default color scheme is white characters on a black background, all opaque.
The Transparent Space special character implies a transparent background even in the absence of any background control commands. As the foreground of this character is a blank space, it really means a gap in the close caption text.
Non-Caption Data
This is used to either pad out the field line when no captions are sent or for the eXtended Data Service.
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
null pad |P|0|0|0|0|0|0|0| |P|0|0|0|0|0|0|0| XDS metadata |P|0|0|0| CLASS | |P|0|0|0| TYPE |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
15 8 7 0 15 8 7 0
Basic North American character set
A command with bits 13 or 14 on directs the receiver to display two basic characters at the current cursor position for the current mode. Each character is a code point, as follows.
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
modified 7-bit ASCII |P| CHARACTER1 | |P| CHARACTER2 |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
15 8 7 0
The code is almost identical to ASCII; the exceptions are shown in red.
In the table above, SB represents a solid block. The apostrophe, which may originally have been intended to be a neutral apostrophe as in ASCII, is now recommended to be rendered as a right single quotation mark. For a neutral single quote/apostrophe, the plain single quote from the extended character set should be used. Special North American character setThe only real use in North America of this set is the use of the Eighth note character to denote changes from spoken dialogue to singing or musical only scenes.It is an acceptable broadcast engineering practice when translating EIA-608 to Teletext for PAL compatible countries to substitute this character for a number sign because of its similarity to a sharp. A command to display a special character has a first byte of 0x11 or 0x19. The second byte is a code point in the range 0x30-0x3F as follows.
|