Television standards conversion

Television standards conversion is the process of changing a television transmission or recording from one video system to another. Converting video between different numbers of lines, frame rates, and color models in video pictures is a complex technical problem. However, the international exchange of television programming makes standards conversion necessary so that video may be viewed in another nation with a differing standard. Typically video is fed into video standards converter which produces a copy according to a different video standard. One of the most common conversions is between the NTSC and PAL standards.

The first known case of television systems conversion was in Europe a few years after World War II, mainly with the RTF (France) and the BBC (UK) trying to exchange their black and white 441 line and 405 line programming. The problem got worse with the introduction of color standards PAL, SECAM (both 625 lines), and the French black and white 819 line service. Until the 1980s, standards conversion was so difficult that 24 frame/s 16 mm or 35mm film was the preferred medium of programming interchange.^{[citation needed]}

Perhaps the most technically challenging conversion to make is the PAL and SÉCAM to NTSC conversion.

The NTSC standard is temporally and spatially incompatible with both PAL and SÉCAM. Aside from the line count being different, converting to a format that requires 60 fields every second from a format that has only 50 fields poses difficulty. Every second, an additional 10 fields must be generated—the converter has to create new frames (from the existing input) in real time.

Conversion between PAL and SÉCAM does not require similar timing changes, but still requires color encoding and sound conversion.

TV contains many hidden signals. One signal type that is not transferred, except on some very expensive converters, is the closed captioning signal. Teletext signals do not need to be transferred, but the captioning data stream should be if it is technologically possible to do so.

With HDTV broadcasting, this is less of an issue, for the most part meaning only passing the captioning datastream on to the new source material. However, DVB and ATSC have significantly different captioning datastream types.

Information theory and the Nyquist–Shannon sampling theorem imply that conversion from one television standard to another will be easier if the conversion