Timestretch

Timestretch (or time stretching) is the process of changing the duration of an audio or video clip without altering its pitch or apparent speed. While simply changing playback speed affects both duration and pitch (playing audio faster raises its pitch), timestretch uses digital signal processing algorithms to redistribute the content across a new duration while preserving the frequency characteristics of the original. Applied to audio, a music track that's 3:45 can be timestretch to 4:00 without the tempo sounding sped-up or the pitch shifting — the algorithm inserts additional content (typically by repeating small segments imperceptibly) to fill the additional duration. Applied to video, timestretch produces slow motion or speed effects similar to speed ramping but often with different algorithmic approaches.

Audio timestretch has two primary applications in video post-production. The first is music editing: making a licensed music track fit a video's exact duration without awkward fade-outs or abrupt endings. If a video is 90 seconds and the selected music track ends naturally at 93 seconds, a 3-second compression via timestretch (a 3.2% change) is often imperceptible to listeners and produces a cleaner edit than fading the music early. Most modern DAWs and NLEs include timestretch algorithms that can handle modest compression and extension (up to approximately 15–20% without obvious artifacts). Beyond that range, the algorithmic artifacts become audible as a "flanging" or unnatural quality. The second application is dialogue editing: conforming interview audio recorded at a slightly different speed (due to camera inconsistencies or frame rate mismatches) to match the video's playback timing.

Video timestretch to produce slow motion requires high-quality algorithms — the simplest approach of duplicating frames at reduced frame rates produces choppy, artificial-looking slow motion. Higher-quality approaches use optical flow algorithms that analyze the movement between frames and generate interpolated intermediate frames, producing smooth slow motion from standard-rate footage. Tools like DaVinci Resolve's optical flow speed change, Adobe Premiere's Time Remapping with optical flow, and dedicated applications like Topaz Video AI can produce convincing slow motion from footage that wasn't shot at high frame rates — though the quality depends heavily on the amount of movement in the frame and the degree of slowdown being applied.