iMovie -- 24fps in, 23.98fps out -- WHY?

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Hello everyone. Here’s an esoteric problem that maybe one of you might have encountered and fixed.

I am using iMovie 10.2.2 to add a watermark to a movie. The original file is 24fps, but upon export iMovie always changes it to 23.98fps. Apple Support was stymied, basically saying that “iMovie does what iMovie does.” There are no frame rate settings in iMovie, and all output formats had the same results.

Any suggestions? I am not ready (yet) to buy more software to add watermarks, and can’t use online services due to the commercial nature of the film.

Thanks for your thoughts if you have any ideas.

Jeff

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Believe it or not, 23.98 fps is the standard frame rate for “24p” video.

The reason goes back to the earliest days of color television. Black-and-white television had a frame rate of 30 fps (that is, 60 fields per second, with interlacing). When color was introduced, they had to slightly reduce the frame rate in order to add the colorburst signal without exceeding the radio bandwidth mandated by law (and expected by existing television tuners).

The frame rate is reduced by 1/1000 producing a frame rate of approximately 29.97 frames per second (30/1.001) or 59.94 fields per second (60/1.001). The slowdown is low enough that it is within the error margin permitted by TV tuners, so B&W sets have no problem syncing to color TV signals.

In the digital age, there is really no need for this, but despite that, digital TV broadcasts all use frame rates of 29.96 and 59.94 fps (which is presented to users as 30 and 60). This is probably for backward compatibility with NTSC and existing NTSC-based equipment, but you would think that they could have chosen 30 and 60 for HD broadcasts. Nevertheless, they did not, I’m guessing so they could reuse their existing hardware/software that is all designed for these frame rates. This means, unfortunately, that video editing tools that export to TV-compatible video (e.g. DVD and Blu-Ray, as well as for broadcast) all work at 29.96 and 59.94 fps. Video sources that are actually 30 and 60 fps must be converted.

Movies on film have always used 24 fps. In order to convert a movie for a B&W broadcast (at 30 fps), it is necessary to increase the frame rate to 30 fps. In its simplest form, 6 of each second’s 24 frames (that is 1/4 of them) are duplicated. In actual practice, it’s a bit more complicated, where image data from adjacent frames are blended using adjacent fields. The conversion process is known as the Telecine 2:3 pull-down.

Of course, in reality it is uglier than this because unless you plan on using an antiquated B&W broadcast signal, you really need to convert those 24 fps to 29.97 fps, not 30. The Telecine process takes care of this by slowing down the source frame rate by 1/1000 (producing a 23.98 fps source). The Wikipedia page provides a good summary of the process.

With a 23.98 fps source, you can use a 2:3 pulldown, increasing the number of frames by exactly 25% to produce the required 29.97 and 59.94 fps.

You may occasionally run across video files (especially older ones produced with cheap non-professional software) that captured a video signal (from broadcast, VHS, etc.) and didn’t take the 1/1000 ratio into effect, treating the signal as if it was 30 fps instead of 29.97. The resulting video ends up running slightly too fast. You won’t notice the speedup, but you will notice the audio drifting out of sync for all but the shortest clips.

See also:

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Great reply, David, very informative—thank you!

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David — Thank you! What a remarkably useful and fulsome reply. I will keep it to hand for the future. I’ve learnt bits and pieces of this situation of the years but you really offered a complete explanation that is clear and easy to follow. Wonderful. Much, much appreciated.

But interestingly (to me, anyway), I still have my problem. I am one of those few folks who actually needs the video to stay exactly at 24 fps. I work in the animation production business, and my immediate task to hand is to prepare to create the English audio version of a Japanese animated film. We sync to the frame, so while most folks never care about 23.976 fps vs 24 fps, it makes a world of difference to me! So I’m still stuck with this problem. Of course, I should not be using iMovie for this professional level task. But it’s all I’ve got right now, and I prefer not to invest in something grander just for this one task.

A quick side story – a couple years back I produced 14 seconds of animation here in Tokyo for an Amazon TV show being made in England. We delivered the required 336 frames (24 fps x 14 seconds), on time. Then I got some serious grief from someone at the London company – he accused me of short-changing them! Turns out that he (or someone at his company) had converted it at some point after delivery (maybe accidentally – maybe an iMovie problem) to their 25 fps PAL broadcast standard. So yes, 336 frames was only 13.44 seconds long! In the end it all worked out (I guess Amazon uses 23.976 fps as its standard, being US-based), but it was not a pleasant few days there and I was the one thrown under the bus for a while.

Thank you for the excellent explanation, David. It really does help and fill in a lot of important bits of information that I was missing.

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So what have you done to sort the situation? (interested to know.)

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This is apparently not all that unusual:

I’m not sure about the accuracy of that statement, since the article doesn’t include references, but if it’s true, it would explain what happened.

You would hope that anyone converting film-rate (24 fps) video to PAL would know about this, but apparently one important person who should know, didn’t.

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Thanks for asking, @jimthing. Sadly, I had to simply give up and ask someone else to do what I needed done. He was a good sport (and gently chided me for trying to use an amateur tool for a professional task).