Computer Editing: Applying Speed Effects in Post

We have all seen them: Those snazzy slow-motion instant replays on NFL games that give us (and the officials) a second look at a quick play. Or the reality-based show Survivor using speeded-up video to compress the time it takes to fly from one side of a remote location to another or show the sun going down in seconds instead of minutes. These spectacular speed effects that were once the exclusive domain of the networks or post-production facilities with hundreds of thousands of dollars worth of cumbersome switchers with frame-store devices and time-based correctors (TBCs) – to counteract the jiggle of analog studio VTRs can now be created easily (and far more inexpensively) using today’s editing programs. In the following examples, we’ll use Adobe Premiere 6.5 to create speed effects in the editing environment, but most editing programs offer some type of facility for altering the speed of your video.

Step 1: Import

Once you’ve imported your raw clips and saved your project, it’s time to start experimenting. Drag and drop the clip you wish to apply speed adjustment to from the gallery to the timeline. Once the clip is on the timeline, it’s ready to have speed adjustment applied. First, however, you’ll need to prepare the video that you want to tweak.

Step 2: Deinterlace

Video in our current NTSC system is interlaced, meaning that each 30th-of-a-second frame is comprised of two 60th-of-a-second fields, with each field containing half of the horizontal lines of resolution that a video frame is capable of displaying. When the video is played at the standard 30 frames (60 fields) per second, the two fields containing adjacent sets of lines form a frame.

If you apply slow motion (or fast motion) to a clip without first deinterlacing the image the video jiggles on playback, often severely. This is why you first need to deinterlace the video (which is called "flicker removal" in Premiere), so that both 60th-of-a-second fields are combined together, allowing smooth slow- or fast-motion playback.

In Premiere, right-click on the video portion of your clip and select "Video Options" from the context menu and then click the "Field Options" item. Select "Flicker Removal" and then click "OK." The clip is now deinterlaced, or will be when rendered. You’ll want to repeat this process for all clips to which you will apply speed effects. In Premiere 6.5, you can automatically deinterlace any slow-motion effects by making sure "Deinterlace When Speed is below 100% is selected .


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Step 3: Select Speed

Right-click on the video portion of the clip again and select "Speed." In the "Clip Speed" dialog, enter a new speed and then click "OK". A percentage lower than 100% will slow the video down and a percentage higher than 100% will speed the video up . You can go as slow as 1% and as fast as 10,000%. For most applications (such as a time-lapse sunset), 1,000% (10x normal speed) is more than adequate for fast-motion effects. 1% (or 1/100th normal speed) will make your video appear as a freeze-frame for a few seconds before advancing to the next frame, so you’ll most likely want slow down your video to no less than 30% of the original speed. Artifacts and jitters are also more pronounced at slower speeds.

Step 4: Determine Duration

If your speed-altered clip needs to fit a particular duration amongst other clips on either side of it in the timeline, try setting a new duration instead of a new speed. Again in the "Clip Speed" dialog (right-click the clip and click "Speed"), type in the numerical value (hours; minutes; seconds; frames [h;mm;ss;ff]) next to the "New Duration" item and click "OK." Your clip has now been speeded-up or slowed-down to the exact time duration you entered. In Premiere 6.5, you may have noticed a "Duration" menu item below "Speed" on the context menu: this opens the Clip Duration dialog and is another (easier?) way to do the same thing . Be careful: In some editing modes (such as a ripple edit mode), if you change the speed or duration such that the new slower video runs over subsequent clips, the other clips may be moved, altering earlier edits.

Subtlety and Timing

Duration changes don’t need to be special effects and can be used to subtly make an otherwise too short shot longer. It is up to you whether you want the speed change to be noticeable or not. Experiment and have fun, but don’t over-do it. As with any other post-production effect, use speed effects according to how the story or content of your project motivates them. Time and practice will yield some super slow-mos and spectacular speed effects.

[Sidebar: The Chipmunks]
Don’t forget the audio. When you alter the speed of your video clip, the speed of the audio portion of your video clip may be altered as well. Unless this is the desired effect, you should unlink the audio from the video before adjusting speed. In Premiere, click the Toggle Sync Mode button at the bottom of the timeline to link and unlink the audio and video.

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