EIA-608

EIA-608, also known as line 21 captions or CEA-608, is a standard used for displaying closed captioning (CC) on analog NTSC television broadcasts in the United States, Canada, and Mexico. Developed by the now defunct Electronic Industries Alliance (EIA), it allows text such as dialogue and sound effects to be shown on screen to aid deaf or hard of hearing viewers in following television programs. EIA-608 was tightly connected to the NTSC broadcasting standard. As such, the transition to the digital ATSC standard (or other competing digital standards) in North America has rendered 608 obsolete in active broadcasting. Its digital successor, EIA-708 or CTA-708, is intended to take over in areas where ATSC is used. As a subtitle format, EIA-608 captioning is classified as a closed, analog, in-band (transmitted inside the video stream), and text-based protocol (the latter contrasts with bitmap images of the caption characters, as seen on DVDs).

The system works by sending the caption data on a part of the TV signal that viewers aren't intended to see under normal operating conditions, called the vertical blanking interval (VBI). When broadcasting NTSC signals, the size of the "image" transmitted is larger than the actual display, creating an area (the VBI) that is intentionally invisible to the viewer. The VBI exists as an "imaginary" extended region above the screen (there also exists another section below the screen, but it is not relevant to EIA-608). There are many horizontal lines within the invisible regions that can be used for the transmission of non-video data. Line 21 was selected for "transmission, reception, and display of caption data", in addition to generic text information and metadata.

Initially launched in 1980, the standard received US government endorsement after the Television Decoder Circuitry Act (1990) mandated the availability of closed captioning decoding hardware, specifically for the EIA-608 format, by July 1993 on all consumer TVs with screen sizes of at least 13 inches. On the broadcasting side, in 1997, the US Federal Communications Committee rolled out new guidelines for a ramp-up to full enforcement of CC availability on all programing, to be achieved by 2006. Both Canada and the US used the ability of EIA-608 to send generic metadata in order to digitise parental controls. Line 21 could contain optional age guidelines and content descriptors in differing formats (e.g. "DSLV"), enabling the interpretation, display, and potential automated age restriction (e.g. the US V-chip system) of content.

Although originally developed for captioning (and short plain text messages and metadata, such as the parental control data), the standard was also to be extensible and an all-purpose metadata carrier, similar to and inspired by Teletext, which had been invented almost a decade earlier in the UK. Teletext used the same manipulation of the VBI and rapidly spread throughout Europe, where it became a cultural institution. EIA-608 grew to support some limited extra services, known as "eXtended Data Services" (XDS, rendered "eXtended" in official documentation), which included details about program titles or instructions for recording shows (i.e. durations and start or stop signals for overruns). EIA-608 is a basic analog emulation of some of the features found in later DVR systems, set-top boxes, and other equipment offered by digital TV providers, cable (and satellite) providers, and later still by "smart TVs". For example, the addition of program titles and durations was a precursor to modern electronic program guides. Teletext, however, was able to provide program listings as early as 1974 with the full rollout of the BBC's Ceefax.

The specification has been subsumed by the American National Standards Institute and the latest version is entitled Line 21 Data Services (ANSI/CTA-608-E S-2019), which began work in 2008 and was finalized in 2019. It is unclear whether there will be another version given that, as of 2022, all three countries have completely finished their transition to all-digital TV and thus EIA-608 is obsolete as a broadcast protocol. However, there is still a lot of legacy media with CCs in various versions of the 608 format encoded into their video streams.