23.976 vs 24 fps: What Happens When You Make a DCP

A DCP can't be 23.976 fps. The cinema standards only allow whole-number frame rates, so a film finished at 23.976 becomes a 24 fps digital cinema package. Nothing is added, dropped or blended. Every frame of your film is kept and simply plays 0.1% faster, which makes the film 3.6 seconds shorter per hour of runtime. The sound has to be sped up by the same amount to stay in sync. When picture and sound travel together in one file, DCP software does both for you and you don't need to do anything. The trouble starts when the sound arrives separately and only the picture gets converted. So export your master at 23.976, keep the audio in the same file, and let the DCP step do the conversion.

Can a DCP be 23.976 fps?

No. Every DCP standard lists whole-number frame rates only.

What this means for you: 24 is the only rate that every cinema in the world is obliged to play. A 23.976 film lands on it with the smallest possible change.

What happens to a 23.976 film when it becomes a DCP?

The frames stay the same and play a little faster. This is called a pull-up, or a conform.

23.976 is really 24,000 frames every 1,001 seconds. Playing those same frames at 24 per second speeds the film up by exactly 1001/1000, or 0.1%. A one-hour film runs 3.6 seconds shorter. A short we made in September went from 1,189.02 seconds at 23.976 to 1,187.83 seconds at 24, just over a second shorter across 20 minutes.

Nobody in the audience can see that speed change. It's the same pull-up that has moved films between video and cinema rates for decades, and it's why converting 23.976 to 24 is the easy conversion. Converting 29.97 or 25 to 24 means inventing or throwing away frames, and that's where motion starts to stutter.

Does the sound go out of sync?

Not if picture and sound are converted together. It drifts if only one of them is.

If your master is a single file with the sound inside it, DCP software speeds up both by the same 0.1%. We encode with easyDCP, which plays the picture frame for frame at 24 and stretches the audio to the same new length, so they stay locked from first frame to last. DCP-o-matic does the same job.

Separate sound files are where it goes wrong. A 5.1 mix delivered as six WAV files, timed to the 23.976 picture, is 0.1% too long for a 24 fps DCP. Left alone, it falls behind the picture by about a frame every 42 seconds. By the end of a 90-minute feature, the words land more than five seconds after the lips move. Those WAVs need the same 1001/1000 speed-up before they go into the DCP. A sound mixer can deliver them at 24 if you ask.

The way to prove a conversion worked is a two-pop at the head and a tail-pop at the end. If the pop lines up with its flash frame at both ends, the whole film is in sync. Here's how two-pops and tail-pops work.

Should I change my film to 24 fps before I export?

No. Export at the frame rate you edited in and let the DCP step do the pull-up.

Changing frame rate in your editing software is where things break. A timeline switched from 23.976 to 24 can blend frames, retime clips or leave the audio at the old length, and you may not notice until a test screening. Our Instant DCP takes 23.976 masters exactly as they are and conforms them to 24 during the encode.

What about 29.97, 25 and 60 fps?

29.97 becomes 30 by the same 0.1% pull-up, and 59.94 becomes 60. 25 needs nothing at all.

• 25 fps is in the interop profile, so it is the safest rate after 24 and 48.
• 30 and 60 fps are legal SMPTE, but no cinema is obliged to play them, and older servers refuse. Before you ship a 30 or 60 fps DCP, ask the venue to confirm their server plays it. In October we made 60 fps dance shorts for a festival into 30 fps DCPs, keeping every other frame, and the festival had the theater confirm 30 before the screening.
• 29.97 to 24 is the hard one. It can't be done by a speed change, and the result depends on the motion in every shot. We cover that in our frame rate guide.

The DCP name tells the projectionist the rate. A 24 fps DCP carries no frame-rate tag. A 30 fps one should say so in its name (SHR-30 for a 30 fps short, for example), under the ISDCF naming convention.

A checklist before you send your master

• Export at the rate you edited in. 23.976 is fine.
• Keep picture and sound in one file. ProRes 422 HQ with the mix embedded is ideal (see our delivery specs).
• If you have separate WAV stems, tell your lab or your mixer which frame rate they were timed to.
• Put a two-pop on the head and a tail-pop on the end.
• If you finished at 29.97 or 60, decide on 30 or 24 before the deadline, and ask the venue.