24 is a multiple of 120, so you can repeat frames 5x on the 24fps parts, and play the whole video at 120fps. The question is what the on/off time is of a 24fps

24 is a multiple of 120, so you can repeat frames 5x on the 24fps parts, and play the whole video at 120fps. The question is what the on/off time is of a 24fps film. My TV has black frame insertion to make 24fps content look better; I am personally a huge fan of black frame insertion so I would imagine that theater projectors do it or something equivalent. If so, that will go away with your "play the same frame 5 times" algorithm. (Old style film projectors did it as an implementation detail; the sample and hold nature of LCD screens is a new "problem" for 24fps content.) Info, and a demo, about black frame insertion: https://www.testufo.com/blackframes https://www.testufo.com/blackframes