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Filed under: Editing •Motion Graphics •Post Production •

Urban Legends of Video

Legend #3: Drop Frame footage runs at a different speed than Non-Drop

Black and white video originally ran at 30 frames per second in the US. When color was introduced, there was concern with potential signal interference issues, so the frame rate was slowed down by 0.1% to approximately 29.97 fps. This speed change adds up to about two fewer frames of video being played per minute. Over the course of one hour, video time is now 3.6 seconds behind real time – just enough to chop off your production company’s logo at the end of that one hour drama you produced for television…

This slowdown also threw off the timecode used to tell how far you were into a tape or a program – the effect was like a clock running slightly slow. The longer the tape or program, the more noticeable this difference becomes. To compensate for this, a timecode counting method called drop frame was invented. Drop frame counting skips certain frame numbers – namely, two every minute, except for the “tens” of minutes (00, 10, etc.) – to get the video clock close again. An example of this is shown in the table below:

Unfortunately, many people incorrectly assume from the name “Drop Frame” that actual video frames get dropped (they don’t), or that a non-drop counting method implies the original speed of 30 fps (it doesn’t). These misconceptions are most common in audio software, some of which even used to skip audio samples when drop frame timecode was in use, potentially adding clicks to the soundtrack at this points.

The reality is that NTSC color video runs at 29.97 fps, regardless of the method you choose to use to number those frames. The only thing that should be dropped are the numbers used to label those frames, not any actual content itself.

Since drop frame math can be confusing, it is common to use non-drop timecode for 29.97 fps work under a half hour in length. Drop frame timecode is usually displayed slightly different inside software or on the timecode readout on a tape deck: The normal colons (:) between numbers are usually replaced with semicolons or simple periods. Some video software (such as After Effects) defaults to drop frame counting when you initialize their preferences; keep an eye out for it and set it to non-drop if you tend to work on shorter pieces. Note that most DV tapes (and therefore, editing timelines) also default to drop frame. And be particularly aware of using audio software in drop frame mode, as some applications actually dropped “frames” (and therefore, skip bits of audio) – try to work in non-drop, and adjust for the numbering difference later.

Reality

It is not reasonable to expect every artist who creates video content to be born knowing the intricacies of the technical side of video. But unfortunately, you’ll have to learn them sooner or later: Especially when other users – or even the manuals to your software or hardware – may be giving you bad advice. Hopefully this article has helped to demystify and set the record straight on some of the more common pieces of bad advice and “urban legends” floating around out there. The result will be less time wasted re-working a video output that’s wrong.

Special thanks to Don Nelson of Avid’s Advanced Technology unit for additional information on the field dominance issue.

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