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In an ideal world, a DCP would be created using content at the same video frame and audio sampling rates as the DCP. This is not, however, always possible.
There are some limitations to video and audio frame rates in DCPs. This is complicated by the fact that not all projectors will play DCPs at the same frame rates. It is possible to create a DCP which one projector will play fine, but another (of a different type) will refuse to play.
The only rates that are guaranteed to work on all DCI projectors are 24 frames per second (fps) for video and 48kHz for audio. If you are sending DCPs to unknown places it is wise to consider using these rates if at all possible.
Many projectors now in the wild support additional video frame rates: 25, 30, 48, 50 and 60 fps.
DCP-o-matic has a few tricks to allow you to use content that is not in one of the ‘approved’ rates.
Audio is easy: DCP-o-matic can resample to 48kHz from any source rate with minimal loss in quality.
Video rate conversion is harder. DCP-o-matic's basic strategy to deal with a non-supported content rate is to run it at the wrong speed, and to adjust the audio to keep it in sync.
Let us consider the example of a 25fps source for which you want to create a 24fps DCP. DCP-o-matic will put the frames from the source directly into the DCP without modification, but will tell the projector to play them back at 24fps. This means that the DCP's video will run slightly slower than the original.
If DCP-o-matic did nothing else, the result of this would be that the audio would be running at the original speed with the video running slowly. Hence the audio would drift slowly out of sync. To avoid this, DCP-o-matic also resamples the audio such that the projector will play it too slow by the same amount. Hence it will sound slightly different but will remain in sync with the video.
For very low or high frame rates, DCP-o-matic can also skip or duplicate frames.