The concept of overcranking and undercranking in the world of film and video production goes back to the origins of motion picture technology. The earliest film cameras required the camera operator to manually crank the film mechanism – they didn’t have internal motors. A good camera operator was partially judged by how constant of a frame rate they could maintain while cranking the film through the camera.
Prior to the introduction of sound, the correct frame rate was 18fps. If the camera was cranked faster than 18fps (overcranking), then the playback speed during projection was in slow motion. If the camera was cranked slower than 18fps (undercranking), the motion was sped up. With sound, the default frame rate shifted from 18 to 24fps. One by-product of this shift is that the projection of old B&W films gained that fast, jerky motion we often incorrectly attribute to “old time movies” today. That characteristic motion is because they are no longer played at their intended speeds.
While manual film cranking seems anachronistic in modern times, it had the benefit of in-camera, variable-speed capture – aka speed ramps. There are modern film cameras that include controlled mechanisms to still be able to do that today – in production, not in post.
With the advent of videotape recording, the television industry was locked into constant recording speeds. Variable-speed recording wasn’t possible using tape transport mechanisms. Once color technology was established, the standard record, playback, and broadcast frame rates became 29.97fps and/or 25.0fps worldwide. Motion picture films captured at 24.0fps were transferred to video at the slightly slower rate of 23.976fps (23.98) in the US and converted to 29.97 by employing pulldown – a method to repeat certain frames according to a specific cadence. (I’ll skip the field versus frame, interlaced versus progressive scan discussion.)
Once we shifted to high definition, an additional frame rate category of 59.94fps was added to the mix. All of this was still pinned to physical videotape transports and constant frame rates. Slomo and fast speed effects required specialized videotape or disk pack recorders that could playback at variable speeds. A few disk recorders could record at different speeds, but in general, it was a post-production function.
Production shifted to in-camera, file-based recording. Post shifted to digital, computer-based, rather than electro-mechanical methods. The nexus of these two shifts is that the industry is no longer locked into a limited number of frame rates. So-called off-speed recording is now possible with nearly every professional production camera. All NLEs can handle multiple frame rates within the same timeline (albeit at a constant timeline frame rate).
Modern video displays, the web, and streaming delivery platforms enable viewers to view videos mastered at different frame rates, without being dependent on the broadcast transmission standard in their country or region. Common, possible system frame rates today include 23.98, 24.0, 25.0, 29.97, 30.0, 59.94, and 60.0fps. If you master in one of these, anyone around the world can see your video on a computer, smart phone, or tablet.
Record rate versus system/target rate
Since cameras can now record at different rates, it is imperative that the production team and the post team are on the same page. If the camera operator records everything at 29.97 (including sync sound), but the post is designed to be at 23.98, then the editor has four options. 1) Play the files as real-time (29.97 in a 23.98 sequence), which will cause frames to be dropped, resulting in some stuttering on motion. 2) Play the footage at the slowed speed, so that there is a one-to-one relationship of frames, which doesn’t work for sync sound. 3) Go through a frame rate conversion before editing starts, which will result in blended and/or dropped frames. 4) Change the sequence setting to 29.97, which may or may not be acceptable for final delivery.
Professional production cameras allow the operator to set both the system or target frame rate, in addition to the actual recording rate. These may be called different names in the menus, but the concepts are the same. The system or target rate is the base frame rate at which this file will be edited and/or played. The record rate is the frame rate at which images are exposed. When the record rate is higher than the target rate, you are effectively overcranking. That is, you are recording slow motion in-camera.
(Note: from here on I will use simplified instead of integer numbers in this post.) A record rate of 48fps and a target rate of 24fps results in an automatic 50% slow motion playback speed in post, with a one-to-one frame relationship (no duplicated or blended frames). Conversely, a record rate of 12fps with a target rate of 24fps results in playback that is fast motion at 200%. That’s the basis for hyperlapse/timelapse footage.
The good news is that professional production cameras embed the pertinent metadata into the file so that editing and player software automatically knows what to do. Import an ARRI Alexa file that was recorded at 120fps with a target rate of 24fps (23.98/23.976) into Final Cut Pro X or Premiere Pro and it will automatically playback in slow motion. The browser will identify the correct target rate and the clip’s timecode will be based on that same rate.
The bad news as that many cameras used in production today are consumer products or at best “prosumer” cameras. They are relatively “dumb” when it comes to such settings and metadata. Record 30fps on a Canon 5D or Sony A7S and you get 30fps playback. If you are cutting that into a 24fps (23.98) sequence, you will have to decide how to treat it. If the use is for non-sound-sync B-roll footage, then altering the frame rate (making it play slow motion) is fine. In many cases, like drone shots and handheld footage, that will be an intentional choice. The slower footage helps to smooth out the vibration introduced by using such a lightweight camera.
The worst recordings are those made with iPhone, iPads, or similar devices. These use variable-bit-rate codecs and variable-frame-rate recordings, making them especially difficult in post. For example, an iPhone recording at 30.0fps isn’t exactly at that speed. It wobbles around that rate – sometimes slightly slower and something faster. My recommendation for that type of footage is to always transcode to an optimized format before editing. If you must shoot with one of these devices, you really need to invest in the FiLMiC Pro application, which will give you a certain level of professional control over the iPhone/iPad camera.
Time and storage permitting, I generally recommend transcoding consumer/prosumer formats into professional, optimized editing formats, like Avid DNxHD/HR or Apple ProRes. If you are dealing with speed differences, then set your file conversion to change the frame rate. In our 30 over 24 example (29.97 record/23.98 target), the new footage will be slowed accordingly with matching timecode. Recognize that any embedded audio will also be slowed, which changes its sample rate. If this is just for B-roll and cutaways, then no problem, because you aren’t using that audio. However, one quirk of Final Cut Pro X is that even when silent, the altered sample rate of the audio on the clip can induce strange sound artifacts upon export. So in FCPX, make sure to detach and delete audio from any such clip on your timeline.
This may have a different name in any given application, but interpret footage is a function to make the application think that the file should be played at a different rate than it was recorded at. You may find this in your NLE, but also in your encoding software. Plus, there are apps that can re-write the QuickTime header information without transcoding the file. Then that file shows up at the desired rate inside of the NLE. In the case of FCPX, the same potential audio issues can arise as described above if you go this route.
In an NLE like Premiere or Resolve, it’s possible to bring in 30-frame files into a 24-frame project. Then highlight these clips in the browser and modify the frame rate. Instant fix, right? Well, not so fast. While I use this in some cases myself, it comes with some caveats. Interpreting footage often results in mismatched clip linking when you are using the internal proxy workflow. The proxy and full-res files don’t sync up to each other. Likewise, in a roundtrip with Resolve, file relinking in Resolve will be incorrect. It may result in not being able to relink these files at all, because the timecode that Resolve looks for falls outside of the boundaries of the file. So use this function with caution.
There’s a rub when work with standard speed changes (not frame rate offsets). Many editors simply apply an arbitrary speed based on what looks right to them. Unfortunately this introduces issues like skipping frames. To perfectly apply slow or fast motion to a clip, you MUST stick to simple multiples of that rate, much like traditional film post. A 200% speed increase is a proper multiple. 150% is not. The former means you are playing every other frame from a clip for smooth action. The latter results in only one fourth of the frames being eliminated in playback, leaving you with some unevenness in the movement.
Naturally there are times when you simply want the speed you picked, even if it’s something like 177%. That’s when you have to play with the interpolation options of your NLE. Typically these include frame duplication, frame blending, and optical flow. All will give you different looks. When it comes to optical flow, some NLEs handle this better than others. Optical flow “creates” new in-between frames. In the best case it can truly look like a shot was captured at that native frame rate. However, the computation is tricky and may often lead to unwanted image artifacts.
If you use Resolve for a color correction roundtrip, changes in motion interpolation in Resolve are pointless, unless the final export of the timeline is from Resolve. If clips go back to your NLE for finishing, then it will be that software which determines the quality of motion effects. Twixtor is a plug-in that many editors use when they need even more refined control over motion effects.
Doing the math
Now that I’ve discussed interpreting footage and the ways to deal with standard speed changes, let’s look at how best to handle off-speed clips. The proper workflow in most NLEs is to import the footage at its native frame rate. Then, when you cut the clip into the sequence, alter the speed to the proper rate for frames to play one-to-one (no blended, duplicate, or skipped frames). Final Cut Pro X handles this in the best manner, because it provides an automatic speed adjustment command. This not only makes the correct speed change, but also takes care of any potential audio sample rate issues. With other NLEs, like Premiere Pro, you will have to work out the math manually.
The easiest way to get a value that yields clean frames (one-to-one frame rate) is to simply divide the timeline frame rate by the clip frame rate. The answer is the percentage to apply to the clip’s speed in the timeline. Simple numbers yield the same math results as integer numbers. If you are in a 23.98 timeline and have 29.97 clips, then 24 divided by 30 equals .8 – i.e. 80% slow motion speed. A 59.94fps clip is 40%. A 25fps clip is 96%.
Going in the other direction, if you are editing in a 29.97 timeline and add a 23.98 clip, the NLE will normally add a pulldown cadence (duplicated frames). If you want this to be one-to-one, if will have to be sped up. But the calculation is the same. 30 divided by 24 results in a 125% speed adjustment. And so on.
Understanding the nuances of frame rates and following these simple guidelines will give you a better finished product. It’s the kind of polish that will make your videos stand out from those of your fellow editors.
© 2019 Oliver Peters