Video Time Stretch Calculator
Use our free Video time stretch Calculator to learn and practice. Get step-by-step solutions with explanations and examples.
Formula
Speed Factor = Original Duration / Target Duration
Where speed factor greater than 1 means faster playback and less than 1 means slower. Target frames = target duration times target FPS. Pitch shift (semitones) = 12 x log2(speed factor). Duration change = target - original. New frames needed = max(0, target frames - original frames).
Worked Examples
Example 1: Creating Slow Motion from Standard Footage
Problem: A 10-second clip shot at 24 fps needs to be slowed to 40% speed for a dramatic slow-motion effect, output at 24 fps.
Solution: Speed factor = 0.40 (40%)\nNew duration = 10 / 0.40 = 25 seconds\nOriginal frames = 10 x 24 = 240\nTarget frames = 25 x 24 = 600\nNew frames needed = 600 - 240 = 360 interpolated frames\nPitch shift (if no correction) = 12 x log2(0.40) = -15.86 semitones
Result: 25 sec output | 360 new frames via interpolation | Quality: Significant impact
Example 2: Speed Ramp for Action Sequence
Problem: A 30-second fight scene needs to be compressed to 20 seconds while maintaining 30 fps output from 30 fps source.
Solution: Speed factor = 30 / 20 = 1.50 (150% speed)\nOriginal frames = 30 x 30 = 900\nTarget frames = 20 x 30 = 600\nFrames to remove = 900 - 600 = 300 (every 3rd frame dropped)\nPitch shift = 12 x log2(1.50) = +7.02 semitones\nWith pitch lock: audio maintains original pitch
Result: 150% speed | 300 frames dropped | Pitch shift: +7.02 semitones (correctable)
Frequently Asked Questions
What is the difference between time stretching and speed change?
A pure speed change alters both the duration and the effective frame rate of playback. Playing a 24 fps clip at double speed shows it at effectively 48 fps worth of content in the time of 24 fps, halving the duration but using all original frames. Time stretching changes the duration while maintaining the original frame rate output, requiring frame interpolation or removal. Additionally, a simple speed change without pitch correction alters audio pitch proportionally, while time stretching with pitch lock maintains the original audio pitch using algorithms like phase vocoding or granular synthesis. The distinction matters in professional post-production where maintaining consistent frame rate and audio quality is essential.
How does time stretching affect video quality?
Quality degradation depends on the stretch amount and algorithm used. Small changes (under 10%) typically produce nearly invisible artifacts. Moderate changes (10-30%) may show minor motion interpolation artifacts like ghosting around fast-moving objects or warping at object edges. Large changes (over 50%) often produce noticeable artifacts including swimming textures, object splitting, and morphing distortions. Optical flow-based interpolation (used in tools like Twixtor, DaVinci Resolve, and Adobe Premiere) produces the best results but can still struggle with complex motion, occlusion, and fine details like hair or particle effects. Recording at higher frame rates (120+ fps) and then slowing down produces the cleanest slow motion.
What happens to audio when video is time-stretched?
Without pitch correction, changing playback speed proportionally changes audio pitch. Speeding up raises pitch, slowing down lowers it. A 2x speed increase raises pitch by one octave (12 semitones). Pitch-locked time stretching uses algorithms to maintain the original pitch while changing duration. Phase vocoder algorithms divide audio into overlapping windows and adjust phase relationships, but can introduce metallic artifacts. Granular synthesis chops audio into tiny grains and rearranges them, which can cause subtle rhythmic artifacts. Formant-preserving algorithms additionally maintain vocal characteristics. For small speed changes (under 20%), modern algorithms produce nearly transparent results. Larger changes may require manual audio sweetening.
How do frame rate conversions interact with time stretching?
When the source and target frame rates differ, time stretching becomes more complex. Converting 24 fps to 30 fps without speed change requires creating 25% more frames (6 new frames per second). Converting with speed change compounds the requirements. For example, slowing 24 fps footage by 50% to play at 30 fps requires generating 36 new frames for every 24 original frames (150% new frame creation). The classic 24-to-29.97 fps conversion for NTSC broadcast uses 3:2 pulldown, which is not true time stretching but rather field duplication. Modern workflows increasingly avoid pulldown in favor of native frame rate delivery, but frame rate conversion remains necessary for certain broadcast and cinema distribution requirements.
What tools are used for professional video time stretching?
Professional time stretching tools include RE:Vision Effects Twixtor (the industry standard plugin, using optical flow with per-pixel motion estimation), DaVinci Resolve Speed Warp (built-in optical flow retiming), Adobe Premiere Pro Optical Flow (frame interpolation retiming), After Effects Time Warp and Pixel Motion (two levels of quality), Final Cut Pro Optical Flow (integrated retiming), and Nuke Kronos (high-end compositing retiming tool). Hardware solutions like Cintel Film Scanners and Blackmagic Design products also offer real-time retiming. Each tool has different strengths. Twixtor excels at extreme slow motion, DaVinci Resolve offers excellent quality in an integrated color grading workflow, and Nuke provides the most control for VFX shots.
How does time stretching work in digital audio workstations for video scoring?
In DAWs used for film and video scoring, time stretching is essential for fitting music cues to specific scene timings. Composers often write to a temporary timing and then adjust to match the final edit. DAWs like Pro Tools, Logic Pro, and Cubase offer real-time time stretching of audio regions with pitch lock. The Elastique algorithm (used in Cubase and many other DAWs) and Complex Pro (in Ableton Live) produce high-quality results for small to moderate changes. For film scoring, musicians may also adjust tempo maps instead of stretching audio, having the DAW follow a tempo track that accelerates or decelerates to hit specific cue points. This avoids audio artifacts entirely since the performance adapts naturally.