23.976 is not a frame rate. It is a speed change.

23.976 is 24 divided by 1.001, and everything difficult about frame rates follows from that one line. Going between 24 and 23.976 is not a conversion: no frame is added, dropped or repeated, the picture is untouched and perfect, and the clip simply plays 1.001 times longer. Which means the sound has to come with it — and if it does not, the programme drifts 3.6 seconds an hour, from a change that left every frame exactly where it was.

The engine is free and needs no account. Paste the timeline and every conversion under it comes back as the exact ratio it is, with the repeat pattern written out — because judder is not a quality or an encoder setting, it is a pattern falling out of a ratio. 24 into 30 is 4:5 and gives 2 1 1 1. 25 into 23.976 is 1001:960 and repeats every forty seconds, which is why it looks so much worse despite moving fewer frames. And it computes the drift on the clips that are speed changes rather than conversions, which is the fault that survives review: there is nothing to see in the picture, and every clip is in sync at its head. Every lane ships a worked example that costs nothing. Writing the report needs credits.