TechTip - Closed Captioning History

Closed Captioning History

Captions were introduced in the 1970s when the first open-captioned shows were run on American public television stations. The U.S. government funded captioning technology research, and in 1976 the U.S. Federal Communications Commission (FCC) reserved Line 21 of the broadcasting signal for closed caption transmission. Open captions are always displayed; closed captions can be enabled and disabled by the viewer. Line 21 data can have many uses and is governed by multiple overlapping specifications in support of government and industry regulations.  A little history may aid in the understanding of the features.  

In the 1970’s, extra bandwidth was exploited in the television broadcast signal that occurs in a normally dead period while the electron gun is being repositioned to start the painting of each new field.  A US federal mandate required that most broadcasts include closed captioning. It was determined that two 8-bit ASCII characters along with a synchronization signal could reliably be included in the vertical retrace interval (line 21) right before the first visible horizontal line.   Simple synchronization and decode circuitry was mandated to be included in all TVs that would provide the extraction and storage of the line 21 data and allow the TV user to select the closed caption characters as an overlay to the video.  Other uses of the line 21 bandwidth include the Copy Generation Management System (CGMS) that provides a three-state flag that can prevent new industry and federally mandated TV decoders from copying video content. The flags are “Copy Freely”, “Copy Once”, and “Copy Never”.  The old Macrovision technology of interfering with the TV broadcast synchronization signals to prevent video copying was also embedded in line 21 data with Analog Protection System (APS) flags that instruct the video decoder to interfere with selected synchronization signals. Also video content rating, program type information, and program schedule data is broadcast via the line 21

The advent of MPEG compressed video brought with it more possibilities for higher bandwidth closed captioning channels and other supplementary data channels.  One of the objectives was to make the new system backwards compatible with older TV technologies IE allow for just two bytes of data per field. This evolution has resulted in a large array of specifications that support the new and old capabilities. Line 21 data can be added to user_data of each frame of elementary MPEG video of the SEI headers of AVC.  Line 21 data can also be added as a separate channel within a MPEG transport stream.

Digital Television offers a variety of display sizes and aspect ratios. To accommodate these different dimensions, the CEA-708 specification includes enhanced captioning display features as compared with the older CEA-608 specification.  CEA-608 decoders were required to place the captioning overlay in a defined section of the screen and with a specified font size.  CEA-708 decoders can draw the captioning text in a user defined window anywhere on the display. The TV users themselves can interactively move and re-size this captioning window. The captioning commands define anchor points that will not move when this window is resized.  As such, CEA-708 requires quite a bit more control overhead to utilize the advanced features.  This makes CEA-608 streams not easily converted into full featured CEA-708 streams.  CEA-708 packets do include four optional bytes per frame which are designated as NTSC captioning bytes for backward compatibility with CEA-608 decoders.  It is possible to add both the DTV CEA-708 captioning in additional to the two NTSC bytes in the CEA-708 data.

Multiple organizations publish different standard for the carriage of closed caption and auxiliary data in streams broadcast over different means. The Consumer Electronics Association publishes CEA-608-C Line 21 Data Service that details data formats for closed captioning services and extended data (including CGMS and APS) for NTSC broadcast.  The CEA publishes CEA-708-B Digital Television Closed Captioning that details usage of a 9600 bps closed channel (ten times the bandwidth of the original channel).  The Advanced Television Systems Committee Inc (ATSC) publishes the Digital Television Standard A/53 specification that details a format for adding CEA-708/608 data to user_data of an MPEG-2 stream.  The International Organization for Standardization (ISO) publishes the ISO/IEC 13818-2 MPEG video standard that details the inclusion of user_data in compressed video streams.  The Society of Cable and Telecommunications Engineers (SCTE) publishes the ANSI/SCTE 20 2004 Methods for Carriage of Closed Captions and Non real-time Sampled Video and ANSI/SCTE 21 2001 Standard for Carriage of NTSC VBI Data in Cable Digital Transport Streams specification that defines additional VBI services.


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