A lot of cross format television conversion is done offline. There are several DVD packages that offer offline PAL <--> NTSC conversion – including cross conversion (technically MPEG <--> DTV) from the myriad of MPEG based web video formats.
Cross conversion can use any method commonly in use for TV system format conversion, but typically (in order to reduce complexity and memory use) it is left up to the codec to do the conversion.
Most modern DVDs are converted from 525 <--> 625 lines in this way, as it is very economical for most programming that originates at EDTV resolution.
Read more about this topic: Television Standards Conversion
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