Fixing Tempo Drift
Tell a bad splice from expressive timing before choosing between stretching and regenerating.
Related
Separate three kinds of misalignment
Quantise a whole track and the drums line up, yet the chorus arrives half a beat before the verse and the seam feels yanked. Tempo trouble comes in at least three forms: the whole piece runs slightly fast or slow, a splice lands off the beat, or the performance itself pushes and pulls. The first two need repair; the third usually does not.
Detect it by checking the downbeat every eight bars
- Lay a click under the whole track and note the first bar that drifts
- Mark the downbeat every eight bars and log any offset beyond ten milliseconds
- Listen two beats either side of every splice for stacked accents
- Solo bass and kick to separate overall drift from a single misplaced hit
- Read transient spacing in the waveform to see whether drift is gradual or a jump
- After the repair, listen twice from the top and confirm the relative tempo between sections is unchanged
Gradual drift usually comes from tempo estimation during generation; a jump usually comes from splicing. The fixes differ, so identify the kind first.
Order the fixes by cost
The cheapest repair is regenerating the two bars around the boundary so the model joins at one tempo. Next is slice alignment: cut on silence or a drum hit, move the whole section, and add a very short crossfade at the seam. Time-stretching comes last, because beyond about two percent transients soften and the tone dulls. Correct section by section with a tempo map rather than quantising the whole track; a blanket pass erases rubato and ritardando along with the breathing between sections.
When not to fix it
Triplets, compound metre and genuine ritardando should not be straightened. Forcing a jig in six-eight into 4/4, or snapping a nocturne's free timing to a grid, yields a performance that is accurate and dead. The test is simple: does the deviation repeat identically? The same offset every time is an error; a different one each time is expression.
Listen in the library
All tracks from midicn-lib — inline badge is the license tier (C1 commercial / C2 non-commercial / C3 study only).
Along the same line
Sources
- Ableton Live 手册与 Walter Piston, Harmony 中关于速度、拍号与复合拍的记法与听辨。
- 音频拼接与时间拉伸的取舍，依据数字音频编辑惯例。