Making a DCP: Frame Rate

Aaron Owen

Author
May 2
    -    
4 min read

A long time ago, the cinema world standardized at 24 frames per second as a compromise to achieve the right amount of smoothness of motion, using the least amount of film.

However in the video world, true 24fps is rare. For legacy compatibility reasons, most of the time if you’re shooting video at 24fps, it is really being captured at 23.976. If you’re in the United States, odds are that unless you’ve shot your film specifically for cinema, you’ve probably chosen to shoot your film at 23.976 (commonly referred to as 24p or 23.98) or 29.97. This presents a challenge when preparing for Digital Cinema Package (DCP) creation as we’ll see in a moment.

There are two flavors of DCP in common use today: the older InterOp (IOP) and the newer SMPTE. The legacy IOP format could only use 24 or 48 fps. The current SMPTE format allows for a wider range of frame rates including 24, 25, 30, 48, 50, and 60. Support for SMPTE DCPs has grown worldwide and as a result, Hollywood Studios have switched over to the newer format as of April 9th 2019. But the thing that both formats have in common when it comes to frame rate is that only non-fractional / integer frame rates are allowed.

What this means is that during the DCP creation process, fractional frame rates must be converted through a process called conforming. This process tells each frame to be on the screen for a slightly shorter amount of time. This doesn’t change the amount of frames in your film, just the overall duration. Because of this timing change, the audio will also need to be adjusted to stay in sync over the duration of your film. But if the audio is adjusted, how do you ensure that the film is perfectly in sync after the conversion? This is where two-pops and tail-pops come in. If you’ve included a two pop for each channel at the beginning of the film, then you have confidence that the beginning of your film is in sync. But to be 100% sure that the entire movie is frame accurate after the conversion, you’ll need to include a tail-pop. If the audible pop’s waveform lines up with the visul tail-pop frame, then you’re guranteed to be in sync.

If you’re using a professional DCP creation service like Cinematiq, this frame-rate conversion step is handled seamlessly as part of the process. But in order for the technician to be absolutely sure that your film is in sync, the film must include both two and tail-pops.

Takeaway: At picture lock, when you’re preparing to send your film to an audio engineer for mixing, make sure to create a two-pop and tail-pop. Here’s a short instruction video about how to do so:

What About 29.97 / 30 FPS?

If you’ve shot your film at 29.97 frames per second, you’ve probably made a mistake. In this day and age, there is not really a good reason to shoot a new film at 29.97 fps. Even if you’re shooting for a television release, it is pretty easy to convert a film from 23.98/24fps to 29.97/30fps through a process called pull-down, but it is much harder to go the other way. Of all the frame-rate conversions, converting from 29.97 to 24 is the hardest to make look good, and the results are often completely dependent on the type of motion in the shot. The smoother the motion the worse it will look. Examples include slow moving aerial shots, smooth camera pans or subjects like cars moving through frame at a constant rate of speed. If you’re converting an entire feature film, some shots will look fine after conversion while others will fall apart entirely.

Many documentary filmmakers in the United States who are creating work primarily for television or web distribution channels choose to finish their film at 29.97fps. It represents a common consolidation frame rate for interviews shot at 23.976 fps, European archival at 25 fps, and drone or action cam footage at 60 fps. Finishing the film at 29.97/30fps can make life easier in post, but as you head to distribution or film festivals, you may find a 24 frame per second requirement and end up converting your entire film anyway. It will yield better results to spend the time and money adjusting individual shots in post-production, than to treat your film as a whole once it is complete.

You might have noticed that the SMPTE DCP format supports a wider ranger of frame rates including 30fps. Releasing a film at 30 fps in the SMPTE format used to be a problem, and would require contacting the theater and running tests before your screening to ensure compatibility. With the wide adoption of the SMPTE format in the US, there’s less of a requirement to adhere to the 24fps standard, but it’s still a good practice if you want to play content in the cinema.

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