EIA-608, also known as line 21 captions and CEA-608, used to be the standard for closed captioning for NTSC TV broadcasts in the United States, Canada and Mexico. It also specifies a Extended Data Service, which is a means for including a VCR control service with a now/next EPG 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 a 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 960 bit/s. 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 a 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 the EIA-608 caption pairs. 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 Taiwanese 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.
Read more about EIA-608: Channels, DVD GOP User Data Insertion, Extended Data Service, Characters, Control Commands