120 fps proxies no longer work in Premiere Pro

Well ... though it seems like it should be, "interpret footage" isn't meant for changing clip speed. That's meant for use such as going from 29.97 to 23.976, where the 'cadence pulldown' has to be done correctly. And that's why using Interpret Footage for slowing clips before making proxies doesn't work. It ain't the right tool.

Speed/Duration is the correct tool. Apply that, then make proxies, it works perfectly.

Now ... using Speed/Duration is a bit of a pain because you can't simply give a framerate, you have to specify the change by either percentage or time unit. Such as, use at 40% speed or play over 00:01:23 of time on the timeline. Not nearly as easy. You need to compute the percentage change, really. Make a postit by your computer for those you regularly use.

Not that any of us are happy with that but this way it simply works. Change your clip speed with Speed/Duration, then make proxies, the world is happy. Right.

And 4k 10-bit long-GOP files like that are a hella pain for the computer ... it's got to create and store to RAM what, up to 100 frames at times before it can show one?

So it's understandable why both ... 

• the camera maker likes it ... gives you SMALL files comparatively, and a specialized chip makes writing them a quickie in-cam ... but ...
• it's nasty as all get out during editing, as the computer doesn't have the exact replicate reverse chip ... it may have some internal generic chip that helps the decode process.

And yea, you will get far better playback with an intraframe proxy, either ProRes, Cineform, or a DNx variant, proxies that are quite a bit larger on disc than a long-GOP file would be. But will playback/edit beautifully because there's no need to compute many frames to show each frame successively.

Neil