The frame rate is always constant for NTSC but it can be constant in different ways. Interlaced, hard telecine (3:2 pulldown) or soft telecine (pulldown flags). I'm pretty sure an NTSC video can be a combination of all three but the output frame rate is constant.
Some encoding programs (ie Handbrake) can output variable frame rate video, so the interlaced parts would be de-interlaced to 29.970 progressive and the film parts would be 23.976fps progressive, all within the same video. Many encoding programs need to convert such video to a constant frame rate. Each method has it's pros and cons.
To be honest I don't know how to reliably detect the frame rate of DVDs with MediaInfo, but then again I'm not sure I've ever used it for that. If it reports 23.976 then it's possibly safe to assume it's film with pulldown flags. When it reports 29.970 the video could be interlaced or have pulldown applied, but it can all change and I don't know what MediaInfo checks.
When re-encoding, I'd check it visually or by letting the encoding program analyse it before deciding how to re-encode it. PAL is usually much easier. It either needs de-interlacing or it doesn't. I live in PAL land so my NTSC experience is a little limited.
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There are plenty of programs that reliably detect exhibited/resultant framerate (via framerate and/or timebase headers + pulldown flags), but few can detect INTENDED framerate. This is because once editing & encoding has been done, the only way to judge is by visual similarity (which requires smarts). Example: NTSC (aka 29.97) video with a mix of 24fps (23.976 actually) and 29.97 sources. You normally wouldn't be able to use AUTHORED pulldown flags that turn on+off, because by the time it has been edited, it's either 23.976 or 29.97. Editors rarely, if ever, pass along that kind of information.
There are apps on this site that do a decent job of "visual similarity" framerate judging of DVDs, especially DGIndex/DGMPEGDec, but even it gets it wrong sometimes. You could, of course, always just ASSUME that it is the stated, exhibited framerate and encode thusly. You would only be sacrificing some bitrate efficiency (minor quality dip) in doing so. It should play fine, regardless.
I won't re-encode anything, was just curious about this strange (to me) value.
In my experience DVDs are either 25fps (PAL) or 29.970fps (NTSC), this is the first time I see a DVD (PAL or NTSC) with 23.976fps.
I would call it a non-compliant DVD.
These two series are from the same era, same TV Studio (probably filmed with the same cameras), same manufacturer so I don't get the difference.
Because the video can be a mixed bag I'm not sure there's a program that can reliably tell you what it is without analysing it. Sometimes an entire movie might be film with pulldown flags but the studio promo stuff at the beginning is different.... that sort of thing. Some TV shows (I think Star Trek is notorious for it) are mostly "film" but the CGI parts are interlaced video. Documentaries are often a mixed bag of 29.970fps interlaced video and "film" with pulldown.
I guess if MediaInfo is reporting 23.976fps it must know it's "film" with pulldown flags, so it's reporting the original frame rate rather than the frame rate after the player applies pulldown. What part of a DVD it looks at though, I have no idea.
Normally when re-encoding I'd open the ripped video (vob files) with MeGUI. It uses DGIndex to index the video, then displays it using a preview. I'd literally step through frames one at a time. If every frame shows signs of being interlaced (you'll only see it where objects have moved between frames) like this, then it's purely interlaced (29.970fps for NTSC). If, when you step through the frames there's an obvious 3:2 pattern of interlacing artefacts and no interlacing artefacts, then it's film with 3:2 pulldown etc etc.... but of course it can change throughout.