30fps to 23.98fps Frame-Rate Conversion

There are several important choices when considering the editing of material which originates on film or 1080-24p. The final result is the first and most important consideration because it dictates the choices to be made in the production work-flow leading to it.

Choice - What Accuracy Is Required?

"Perfect"

If a perfectly frame-accurate 24fps result is required, such as for mastering feature films from 24p or when a film cut-list is needed for negative cutting, the requirements at all stages are most rigorous.

In particular, a 24-frame accurate offline edit system is required. A 30-frame editing system cannot produce frame accurate results. This means the video and audio must be digitized accurately and any associated film data (Flex file, Aton) edge numbers, film-framing) must be retained. This, in turn, leads to the requirement that the telecine transfer of film or 1080-24p to standard NTSC video (480-60i) be managed correctly. This transfer must also treat the audio sampling correctly. In addition, perfect results can only be achieved by verifying and/or correcting any data inaccuracies resulting from transfers or digitization within the offline system itself. A knowledgeable operator must perform this data checking before editing proceeds.
 

"Really Good"

Several economies can be realized if perfect accuracy is not required. In particular, significant economical savings can be realized by using a normal 30-frame editing system for offline. Since the framing accuracy will not be transported through the offline process, additional economies may be achieved because the quality of film-framing data in the telecine, video transfers, and digitizing is less demanding. It also means the data verification process is not required.

Of course this process will lead to inaccuracies in the final result. It is important to understand the ramifications these inaccuracies may have on workflow and the final result to determine if this is acceptable to clients and operators.

Offline Edit Systems for 24p

Most projects will go through an offline-editing phase. The choice of the offline system employed is a critical decision in the planned workflow because it determines the frame-accuracies that can be achieved.

To achieve perfect frame and sample accuracy it is necessary to employ an offline edit system that is capable of maintaining the 24-frame accuracy throughout the project. A normal 30-frame editing system is simply not capable of it, and there is no way around that fact.

The reason is that NTSC video is a 30 frames-per-second format, while film is 24 frames-per-second. 30 just doesn't fit into 24, or, the other way round, there are several ways 24 can be fit into 30.

If you've got 30 balls and 24 cups to put them in you'll always have 6 balls left over no matter what choice of which ball to put into which cup. Or, again, the other way round, if you've got 24 balls and 30 cups to put them in you'll always have 6 empty cups.

This fact cannot be overcome by any clever method - there will always be an ambiguity in any conversion from 30 to 24.

Mathematically (you can skip this part if you want), you need to divide 30 into 24, or 30/24. You can simplify this to 5/4 (six is the lowest common divisor of 30 and 24, so 30/6 = 5 and 24/6 = 4). Dividing 5 by 4 does not result in an integer: 5/4 = 1.25. That ".25" represents a part*of one of the left-over balls.

Film and 1080-24p Transfer to NTSC Video - 2:3 "Film" Framing in NTSC Interlaced Fields

NTSC video is actually a 60 "fields-per-second" format which runs at "59.94". Two "fields" make a "frame", and the frames run at "29.97" frames-per-second, slightly slower than true 30 frames-per-second. Film, on the other hand, is a "true" 24 frame-per-second format.

(Nitpickers will point out that "59.94" and "29.97" are short-hand for the actual rates of 60000/1001, or 59.940059940059900 (to 15 points of precision) for NTSC field rate and 30000/1001, or 29.97002997002990 for NTSC frame rate. This is a subtle but important fact, especially for any engineering task that involves the actual running speed of video or film.)

 

To make up the differences between NTSC and film during the telecine process two things must happen. First, the film must be played back slightly slower than its original speed of true 24fps speed so that 24 film frames occur in exactly the same time as 30 video frames occur. This speed is "23.98"fps*. Second, the film frames must be "embedded" into the NTSC interlaced fields in a 2:3 pattern, or "cadence". Fortunately, 24 film frames fit exactly into NTSC's 60 fields when this pattern is applied.

*(Nitpickers will note that the "23.98" rate is actually 24000/1001, or 23.976023976023977 (to 15 points of precision).)

 

1080-24p has a natural frame rate of "23.98" (not true 24!), so its speed need not be changed when transferring to NTSC 480-60i video, but its 24 frames must be embedded into the NTSC fields in exactly the same manner used with film. This process is typically provided by internal processing in the 24p VTR, and available as a special output.

Once film or 24p are transferred to NTSC video in this manner they may be considered to be identical with respect to the relationship between the video fields and the embedded 24 frames they contain.

 In fact a frame "resolution" of 60 is required. That is exactly what a 24-frame capable edit system is doing, behind the scenes.

You can see this Mathematically - lowest common divisor of 60 and 24 is 4, so 60/24 becomes 15/6. 15/6 = 2.5. Thus, any edit system that has only a 30-frame resolution cannot resolve the difference.