Television standards conversion is the process of changing one type of television system to another. The most common is from NTSC to PAL or the other way around. This is done so television programs in one nation may be viewed in a nation with a different standard. The video is fed through a video standards converter that changes the video to a different video system.
Converting between different numbers of lines and different frame rates in video pictures is a complex technical problem. However, the international exchange of television programming makes standards conversion necessary and in many cases mandatory.
Perhaps the most technically challenging conversion to make is the PAL to NTSC.
PAL is 625 lines at 50 fields/s
NTSC is 525 lines at 59.94 fields/s (60,000/1,001 fields/s)
The two TV standards are for all practical purposes, temporally and spatially incompatible with each other. 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 a difficulty. Every second, an additional 10 fields must be generated—the converter has to create new frames (from the existing input) in real-time.
Information theory and the Nyquist–Shannon sampling theorem imply that conversion from one television standard to another will be easier providing: