International Captioning Project
Video Standards: A Brief
Explanation
A simple explanation of worldwide video systems is difficult to achieve, but such knowledge is essential when trying to understand why video and data produced in one country may not be readily used in another.
When television was first introduced, all transmissions were in black and white. When color television systems were proposed, it was decided that absolute compatibility was a must: monochrome receivers must be able to produce high- quality black-and-white images from a color broadcast, and color receivers must be able to produce high-quality images from a monochrome broadcast. In 1953, the United States Federal Communications Commission (FCC) approved a system which met these standards, which was the work of the National Television System Committee (NTSC). Thus, the official video standard of the U.S. (and Japan, Mexico, Canada, Central America, parts of South America, the Caribbean and other countries) is known as the NTSC system (see Appendix A for a list of NTSC countries).
Concurrently, European countries were struggling with ways to adopt color TV systems which would still support viewers with black-and-white sets. By the mid- 1960s, two more video standards were in use around the world: Phase Alternation Line (PAL) and Sequential Coleur Avec Memoire (SECAM). PAL is used throughout most of Europe and the South Pacific; SECAM is used by France, some Eastern European countries, parts of Africa and much of the former Soviet Union. To make matters even more complicated, some countries which use PAL and SECAM do so with minor deviations, resulting in similar but not 100% compatible formats (PAL-M, PAL-N, PAL-I or MESECAM; see Appendix A for a list of PAL and SECAM countries).
At the risk of oversimplification, suffice it to say that there are many incompatibilities between the three systems. However, the most basic are in the processing of the video signal itself.
All televisions display video by painting a series of horizontal lines (each line consisting two fields) across their screens. In NTSC countries, the standard TV picture is said to consist of 262.5 lines per field, operating at a speed of 60 fields per second (or 60Hz). PAL and SECAM countries use a TV picture which consists of 312.5 lines per field, operating at 50 fields per second (or 50Hz). Regardless of the number of lines, the first 21 or so are not used for the picture. Known as the vertical blanking interval, these 21 lines are mostly dark; they appear as the black bar between rolling pictures when the TV set's vertical-hold control is misadjusted.
In North America, closed captioning is one of the earliest uses of the VBI for transmitting data. With a little effort, you can actually see the captioning data on line 21. On a monitor whose picture has been shrunk (underscanned), or with careful manipulation of the vertical-hold control, study the top of the picture where it meets the black bar of the VBI. Line 21 is at the bottom of the VBI; nearby VBI lines may contain engineering test signals, color synchronization, teletext, time and source data, network messaging or other material. The picture usually starts on line 22, although some broadcasters use line 22 for other data. (PAL and SECAM countries carry subtitling data on lines 17-20.)
