[HELP] Convert 29.97 to 23.976

[Selur]

• if your source is NTSC intelaced, going to 23.976 fps is really not recommended since you would throw away motion data and might end up with jerky motions.
  
• if your source is NTSC telecine (no matter if it's soft telecine or not, since Hybrid tries to handle them the same way: decode as 29.97 and ivtc), you should use an IVTC filter, yes using QTGMC for deinterlacing as part of IVTC might help to preserve some more quality.
  
• if your source is NTSC with a non-standard but constant telecine pattern you can use a IVTC method and adjust the settings to match the pattern.
  
• if your source is an NTSC source with a non-standard non-constant telecine pattern, using QTGMC in Bob-mode and using sRestore to get rid of the duplicates is recommended.
  
• if your souce is NTSC and hybrid (= wild mix of everything) you could bob the content with QTGMC and use sRestore to remove duplicates and ghosting.
  
• if your source is PAL and non-standard interlaced your best chance, aside from doing some serious analysis of the source - and adjust filtering manually, is to QTGMC Bob and using sRestore in case you want/need to remove duplicates. Depending on the content you could also use QTGMC bob deinterle, use sRestore to a frame rate where you get rid of all duplicates and then use an interpolation method (Interframe/SVP, Rife, FrameRateConverter,..) to get to a specific higher frame rate, but depending on the interpolation method you might need another sRestore round after the interpolation.
  

Sometime NTSC 29.97 source also recorded as simply interlaced (same as PAL usually),...

'usally' *gig* that totally depends on the content you handle. Personally, I as got as never have to deal with NTSC content that isn't telecined or PAL content that isn't progressive. If I get interlaced PAL content it's usually 10+years old or from a DVD. The real horror starts if you get content that was converted multiple times between NTSC and PAL and may be even consists of a wild mix of interlaced, field shifted, telecined content and some parts that use some really uncommon handling.

-> If your content doesn't fit in the normal categories: learn to properly analyse such content and write Vapoursynth/Avisynth scripts on your own.

What's the best way of de-interlacing 29.97 video using QTGMC to 23.976 and keeping the same running time?

QTGMC doesn't do 29.97 -> 23.976 pulldown, that is either IVTC oder some other pulldown.

Cu Selur