What are the technical aspects of closed caption?

Closed caption information is added to Line 21 of the Vertical Blanking Interval (VBI).

It may be added to either or both the odd and even fields of the television signal. However, the primary language program related information appears in the odd field known as field 1.

To assure adequate caption performance, a low data rate of 503 kilobits/second (32H) is chosen. Data is preceded by a seven-cycle sine wave similar to color burst (called the "Clock Run-In") and three start bits that are always 0, 0, and 1.

Using only the odd field of Line 21, a delivery rate of about 500 words per minute is theoretically possible. However, according to tests conducted by PBS (Public Broadcasting Service) it has been determined that the highest reading rate for captioning is about 200 words per minute, which is technically possible. (...)