EIA-608

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EIA-608, also known as "Line 21 captions" and "CEA-608",^[1] is a 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.

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.