Why 23.976/29.97 FPS but not 24/30 FPS?

[Jedi Master]

Why do some cameras have 23.976 and 29.97 FPS settings, but not 24 and 30 FPS settings?

I know the history behind how 23.976 FPS* came about, but what I’m asking here is why some cameras don’t support 24 and 30 FPS when it’s just a simple software change.

*For anyone who doesn’t already know, and is interested, it came about when color TV was introduced in the U.S. in 1953. B&W sets used 30 FPS (actually two interlaced frames at 60 per second), and when color was added, in a way that had to be compatible with existing B&W sets, it was found that the color subcarrier and sound carriers interacted and created a stationary dot pattern in the picture. This was fixed by reducing the refresh frequency by 0.1% to 59.94 Hz. When a film shot at 24 FPS is shown on TV, it’ll run four seconds longer per hour, so the frame rate was changed to 23.976 FPS and we’ve been stuck with it ever since, even though NTSC video has been dead for years.

[kye]

It's an interesting question, but I think a clue is likely evident from the question.  If we rephrase the question to be "Why don't they support 24/30p when it's a trivial technical change?" then it becomes obvious it's not a technical decision, and in a capitalist consumerist marketplace, if something isn't technical then it's probably economic.....

Also, if you just record in 23.976p instead of 24p, put the clips onto a 24p timeline, and have your NLE configured to re-time by selecting the nearest frame, then it will be frame perfect for any clips under ~20s long.  

If being frame/subframe perfect worries you, then I would suggest you do not ever consider the fact that almost all computer displays are 60p or some other refresh rate that isn't a multiple of 24, so when your video is viewed by almost all viewers, it'll be going through something like a 3:2 pull-down, so most frames will be off by a significant percentage and motion will be sputtering all over the place.

I've been thinking about this lately, and there are some very interesting things going on.  For example, if you record 30p and put it on a 24p timeline, and then display that 24p timeline on a 60p display, almost every frames from the original 30p will be time-perfect with zero time-shift, but there is a repeating pattern of the odd repeated frame, so the feel will still be that of 30p rather than 24p.  If you put 24p on a 24p timeline and view that on a 60p display, almost all the frames will be off by some significant percentage, but it will still feel like 24p.

I've come to really dislike the feel of 30p - it feels like 60p but only about half as 'slippery', so this stuff matters to me, but might not be visible to others.