I believe some of you are confusing a 120hz refresh rate with 120 FPS frame interpolation, which isn’t surprising since TV manufactures have been advertising this as simply “120hz” for years.
The motion-smoothing feature you see on TVs generates artificial intermediate frames between the real recorded frames of the video source (primarily using what they call block matching). This brings the video frame rate up to 120 FPS, but 4/5 of the frames are these interpolated constructs. The algorithms aren’t great, so you can get artifacts when a lot of things are moving at once. Even in the best cases it significantly changes what you’re seeing vs the creator’s intent, but that’s more of a personal preference matter.
Once you have 120 frames, you do need a 120hz panel to display them, which is why this term is used in marketing.
This interpolation can be done in software – if you’re interested, there’s a program called SVP which can perform interpolation to any frame rate on a PC. I don’t know if anything like this for the iPad, but it’s technically possible to make and the iPP is fast enough to do the processing on the fly.
A 120hz display is simply capable of refreshing 120 times per second; this does not mean there will be 120 frames of data to display in all situations. If the source is artificial (scrolling animations) then you will as long as the system can render fast enough. If you actually have 120fps video, that would work too.
The advantage of this display for normal (24fps) video content is that it can actually refresh at 24hz, making the timing between frames uniform.
When you play 24fps content on a 60hz display, there’s no way to perfectly match the frame rate to the refresh rate, so you end up displaying one frame for 2 refreshes and the next for 3 refreshes (and repeat). This creates a sense that the movement is not steady, almost like it is constantly slowing down and speeding up. This is called telecine jitter, or 3:2 pull down. Many people have ONLY seen video on a 60hz screen so they may not even be aware of it, but the motion is much more stable on a 24hz (or any integer multiple) display.
Wow, I have been spent days googling trying to find an article that talks about ProMotion and all that it can be used with. The example I gave was watching fast action videos such as sports on ESPN app or old football games on YouTube. I was wondering if the ProMotion would smooth out the video like TVs do with the motion-smooth feature. But I guess the new iPad Pros only have a 120hz panel and therefore wouldn't accomplish this (I doubt ESPN or YouTube streams 120 fps since that is now what cable TV does).
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TLDR: Don’t confuse TV “120hz” interpolation smoothing with a 120hz panel refresh rate.