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There are some quick-and-dirty ways used by among others some VCRs, where the frame rate is NOT converted. I have for instance a (European = PAL) Thomson VCR which can play back NTSC tapes, and produces something called "PAL60" - where the coding is PAL and the frame rate is 60 Hz (like NTSC).
Many TVs seem to manage that -- either as is or after minor adjustment of vertical lock.
There may be chips capable of that kind of conversion, and definitely the task is MUCH simpler. I have not researched the details, but it might be worth looking at.