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 fields. (It should be noted that caption industry people typically refer to the odd fields as "Field 1" and the even fields as "Field 2".) To assure adequate caption performance wherever a usable picture can be obtained, a low instantaneous data rate of 503 kilobits/second (32H) was 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".
Two bytes of data, using seven bits, odd parity format, are possible on a given line. The rise time is controlled (2T) and the amplitude of data and clock run-in is 50IRE units. Using only Line 21, Field 1, of the VBI, a delivery rate of about 3600 characters or 500 words per minute is theoretically possible (depending upon the particular caption style, discussed later).
Tests conducted by PBS determined that the typical reading rate for captioning is about 125 words per minute. (Incidentally, spoken dialogue may exceed 200 words per minute.)Particularly with the availability of Field 2, the data delivery capacity (or "data bandwidth") far exceeds the requirements of simple program related captioning in a single language. Therefore, the closed captioning system allows for additional "channels" of program related information to be included in the Line 21 data stream. In addition, multiple channels of non-program related information is possible.
Currently I have something in mind right now about making a better use of this technology... bad thing television is one-way.