SMPTE Non-Drop / SMPTE Drop

SMPTE Drop (29.97fps)

다소 복잡한 Time code 문제인데 NTSC의 30fps code는 실제로는 29.97fps로 일반적인 동화상 규격인 30fps와 0.03초의 차이가 납니다. 
이 차이를 없애기 위해 SMPTE는 매 10분 째를 제외한 매 분마다 처음 2 fame(00과 01 frame)을 다시 부여하게 되는데 이렇게 2frame씩을 Drop시키는 code를 SMPTE Drop 방식이라고 합니다.

SMPTE Non-Drop 29.97fps

Drop Frame을 감안하지 않고 계산하는 일반적인 Time Code 방식이며 (30fps와 비교하여) 0.3%의 오차가 발생합니다.

SMPTE 25fps(EBU)

PAL, SECOM 등 유럽과 동구권이 채택하는 Time Code입니다.

SMPTE 24fps(Film)

일반적인 영화에 채택되는 Time Code입니다.

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To Drop Or Not To Drop,
That Is The Question

by Pam Malouf-Cundy

There are many options for editing shows now that digital editing has come to dominate our craft. A show can be shot on videotape at 30 fps (frames per second) and then loaded into an editing system (as a 30 fps video project) using either drop or non-drop time code. A show shot on 24 fps (35mm film or 16mm film) can be loaded into a digital system as either a 30 fps video project or loaded into a digital system as a 24 fps film project. Whether your final product is a video online or a transfer of the cut negative to tape does makes a difference as to how you should decide to load and edit your show.

However, for the purposes of this article, I'd like to simply examine the options of drop and non-drop time codes to take them out of the equation. If two shows were cut identically, one using drop-frame timecode and one using non-drop timecode - both film cut lists (or both onlines) would be precisely the same. Many people, particularly "film" people, are under the false impression that non-drop is more accurate. They think that drop-frame timecode actually drops frames. This is absolutely false.

Drop and non-drop timecodes are both methods of counting video frames. You would be correct to think of them like two languages. Just like "non" in French equals "no" in English. Each timecode counts and keeps tracks of video frames - they do not alter or drop video frames. They only count video frames and they count differently. In fact, drop frame timecode counts video frames accurately in relationship to real time. This is why all the networks and cable companies require that the final tape masters to be delivered are on drop-frame timecode.

In the old days, before the invention of SMPTE time code, editors could not get frame accurate control of both the playback and record VTRs. Timecode provided a tremendous advantage over the older methods of editing electronically including permitting multiple machines to run in sync (dubbers, switchers, etc.), permitting frame- and field-accurate edits, and allowing editors to calculate scene durations quickly and accurately. SMPTE code (originally referred to as SMPTE/EBU time code) is indexed in hours, minutes, seconds and frames (30 fps in NTSC and 25 fps in PAL).
Everyone refers to NTSC video as 30 fps, but in actual, factual, gorey detail it is really 29.97 frames per second. No human has dared create a timecode that counts hundredths of frames (it would be too confusing,) - hence we have drop and non-drop. Clock time (electricity) operates at 60 Hz but the NTSC standard television operates at 59.94 Hz. If you double 29.97, you get 59.94 - hence - two fields per video frame. For now, that's all we'll say about "fields". Because of this speed difference between 59.94 Hz and 60Hz, there is an expanding elapsed time difference between the clock and timecode readings.

Drop-frame timecode counts each video frame but when that .03 finally adds up to a video frame - it skips ahead (or drops) a number. It does not drop a film or video frame, it merely skips a number and continues counting. This allows it to keep accurate time. So if you're cutting a scene using drop frame time code, and the duration reads as, say, 30 minutes and 0 frames, then you can be assured the duration is really 30 minutes.

Non-drop timecode counts (labels) every single video frame and does not make any allowances for the fact that it is really 29.97 fps. Therefore, if you cut a scene using non-drop time code and the time reads as, say 30 minutes and 0 frames, this is not the actual running time of the scene. The total time must be converted in order to know the true running time. This is why non-drop lengths are always shorter than the real time. For example - it's counting 3000 frames for 100 seconds when it's really 2997 frames that equals 100 seconds. In other words, non-drop time code will count/label 3000 frames as 100 seconds, when it is actually 100 seconds and 3 frames. This is why the networks and cable companies require that their masters be delivered on drop frame time code.
The actual difference amounts to 108 frames per hour, or 86 seconds for each 24 hour period. This equals 54 video frames per 30 minutes. So if you are cutting a show using non-drop timecode, at the end of a 30 minute program, you would need to add 1 second, 24 video frames to get the accurate time. It can be roughly calculated that a program using non-drop timecode is two seconds shorter per half hour than the non-drop timecode displayed.

Remember, both timecodes are merely labeling methods and do not alter the visual picture in any way, shape or form. Therefore, choosing whether to edit in drop or non-drop normally is the editors' choice or may be determined by the preference of your post supervisor, tape house or editing system limitations. Drop and non-drop have one thing in common - hour 1:00:00:00. (The hour actually doesn't matter as long as it's 00:00:00.) This is why you can take an output (tape assembly) of a non-drop timecode show and insert edit it onto a drop-frame video master at 1:00:00:00 and it will lay across perfectly. It's the same reason you can take a non-drop show, and create a drop-frame online list using 1:00:00:00 as the record in.

A tape image which is going to be transferred to film (such as a CGI - computer generated image) must be compressed (no 3/2 pull down) and put onto a non-drop timecode master in order to most easily transfer to film, but it still doesn't matter if you're cutting your show drop or non-drop. The film (from the tape) is then transferred to tape dailies (drop or non-drop) just like your regular dailies. To fully explain this, it is necessary to explain video fields in detail but I feel explaining "drop and non-drop time code" is enough technical information for one sitting. For more about the options mentioned in paragraph one, see my upcoming article "How Do 24 Film Frames Equal 30 Video Frames?" (to be published in the Sept/Oct Newsletter).

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Pam Malouf started as an assistant editor, working with film, in 1976.
She has edited on the Montage, Laser Edit,
Ediflex tape, Ediflex Digital and the Avid.
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