EIA-608, also known as "line 21 captions" and "CEA-608",^[1] was once the standard for closed captioning for NTSC TV broadcasts in the United States, Canada and Mexico. It was developed by the Electronic Industries Alliance and required by law to be implemented in most television receivers made in the United States.

This article has multiple issues. Please help improve it or discuss these issues on the talk page. (Learn how and when to remove these template messages) This article may be too technical for most readers to understand. (January 2013) This article needs additional citations for verification. (May 2018)

It 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.

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 (maximum four lines) for a 30 frame broadcast.^[2] 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.

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. The channels are often presented to users numbered simply as CC1-2 for the odd field and CC3-4 for the even field. 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:

The user data structure that follows a H.262 GOP header is as follows (the same would apply after an ISO/IEC 14496-2 GOP header):

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.

The packet-ed structure that is inserted before the H.222 video packet is as follows for a frame of associated video:

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.

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:

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.

The EIA-608 data stream format includes Extended Data Service (XDS), a variety of information about the transmission. It is all optional,:

• program name
• offensiveness rating (violence, sex, etc.)
• program category (drama, game show, etc.)

There are three sets of characters that the EIA-608 stream can direct the receiver to display: basic characters, special characters, and extended characters. A single two-byte EIA-608 command (represented by a single VBI line) can specify two basic characters, one special character, or one extended character.

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.

In these examples

  P = odd parity bit

          +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+              +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
 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

                       +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
  modified 7-bit ASCII |P|  CHARACTER1 | |P|  CHARACTER2 |
                       +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
                       15             8   7             0

 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
 |P|0|0|1|C|0|0|1| |P|0|1|1|  CHAR |
 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
 15             8   7             0
 P = odd parity ; C = second channel toggle

 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
 |P|0|0|1|C|0|1|S| |P|0|1|CHARACTER|
 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
 15             8   7             0
 P = odd parity ; C = second channel toggle ; S = char set toggle

When used all standard and extended character sets are unused in favor of the following predefined sets, care must be taken to not emulate any control commands. This is an extension submitted to the CEC by Norpak who made a similar extension to the Teletext format for the Chinese market. The main use has been to provide double byte code point captioning to the Japanese, Taiwanese and South Korean markets. A command to switch character sets has a first byte of 0x17 or 0x1F (depending upon channel). The second byte is a character set reference in the range 0x24-0x2A as follows

 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
 |P|0|0|1|C|1|1|1| |P|0|1|0|CHARSET|
 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
 15             8   7             0

Bits 15 and 7 are always odd parity bits. Bit 11 is always the channel bit.

Preamble address code with masking bit 15,11 and 7 as already defined above can be interpreted from following table

Row Preamble Standard Address and Style
(Default Row 11 = 0,top rows 1-4 = 1-2,bottom rows 12-13 = 3)
                +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+                  +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
 preamble style |P|0|0|1|C|0|ROW| |P|1|N|0|STYLE|U| preamble address |P|0|0|1|C|0|ROW| |P|1|N|1|CURSR|U|
                +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+                  +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
                15             8   7             0                   15             8   7             0
Row Preamble Extended Address and Style
(Bottom Rows 14-15 = 0,middle rows = 5-10 = 1-3)
                +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+                  +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
 preamble style |P|0|0|1|C|1|ROW| |P|1|N|0|STYLE|U| preamble address |P|0|0|1|C|1|ROW| |P|1|N|1|CURSR|U|
                +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+                  +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
                15             8   7             0                   15             8   7             0
 P = odd parity ; C = second channel toggle
 U = underline toggle ; N = next row down toggle
 (if style or cursor position not set,
  defaults are regular white text with black background
  at cursor = 0, cursor - multiple of 4)
 text style enumerations:
  {white=0,green,blue,cyan,red,yellow,magenta,italic white}

The row bits specify which of the fifteen screen rows should contain the caption text: row 11 (0000), 1 (0010), 2 (0011), 3, 4, 12, 13, 14, 15, 5, 6, 7, 8, 9, or 10 (1111).

The attributes bits allow 16 possibilities, which are: white (0000), green, blue, cyan, red, yellow, magenta, italics, indent 0, indent 4, indent 8, indent 12, indent 16, indent 20, indent 24, indent 28 (1111).

For a midrow code these are as follows: Bits 14, 13, 10, 9, 6 and 4 are always 0, bits 12, 8 and 5 are always 1. Bits 3, 2 and 1 form the color attribute 0001X10X(see the listing of attributes). Bit 0 indicates underline.

Mid Row Style Change
(style remains in effect until either next change or end of row signaled by a control or preamble)
           +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+              +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
  bg color |P|0|0|1|C|0|0|0| |P|0|1|0|COLOR|T| midrow style |P|0|0|1|C|0|0|1| |P|0|1|0|STYLE|U|
           +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+              +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
           15             8   7             0               15             8   7             0
       +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+            +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
 no bg |P|0|0|1|C|1|1|1| |P|0|1|0|1|1|0|1| black text |P|0|0|1|C|1|1|1| |P|0|1|0|1|1|1|U|
       +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+            +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
       15             8   7             0             15             8   7             0
 P = odd parity ; C = second channel toggle
 T = partially transparent ; U = underline toggle
 bg color enumerations:
 {white=0,green,blue,cyan,red,yellow,magenta,black}

For other control codes these are as follows: Bits 14, 13, 9, 6 and 4 are always 0, bits 12, 10 and 5 are always 1. Bit 8 chooses between line 21 and 284. Bits 3, 2, 1 and 0 identify the particular action.

The command bits allow 16 possibilities, which are: resume caption loading (0000), backspace (0001), delete to end of row (0100), roll-up captions 2-rows, roll-up captions 3 rows, roll-up captions 4-rows, flash on (0.25 seconds once per second), resume direct captioning, text restart, resume text display, erase displayed memory, carriage return, erase nondisplayed memory, end of caption (1111).

For tabs these are as follows: Bits 14, 13, 6, 4, 3, 2 are always 0, bits 12, 10, 9, 8, 5 are always 1. Bits 1 and 0 determine the number of tab offsets.

Considering parity bit already ignored hex value have of 2 byte data is following command: