Finding Sample Loop Points
A loop point belongs on a steady, whole-cycle stretch, and must survive transposition and a stereo check.
Related
A two-second organ recording has to hold a twenty-second chord. The sample is not stretched; the waveform circles through a steady stretch, and the loop points mark where that circle starts and ends.
Two seconds holding twenty
The attack stays outside the loop, the decay stays outside it, and only the calm middle repeats. Put the loop inside a fading section and the level rises on every pass, so the sustain breathes. Put it on one side of a vibrato cycle and the vibrato freezes. Make the loop short enough and its repetition becomes a pitch sitting on top of the note.
Four loop settings
| Shape | Where it is used | Audible risk |
|---|---|---|
| Forward | The usual sustained note | Pulses when the seam misses a whole cycle |
| Alternating | Material with a symmetrical waveform | A new transient where direction reverses |
| Release loop | The tail after note-off | The tail never ends and sounds stuck |
| One-shot | Percussion, plucked strings, piano | Looping destroys the natural decay |
Sustains usually run forward. Alternating loops suit synthetic material whose waveform is already symmetrical and are rare in acoustic recordings. A release loop needs its own points; do not share them with the sustain.
The order to work in
1. Find a middle stretch with steady amplitude and stable overtones, away from attack and decay. 2. Align start and end to zero crossings, then nudge them onto whole cycles. 3. Listen without a fade first; add a 20 to 80 ms crossfade only if a click remains. 4. Check the spectrum for added low end or a scooped midrange from comb filtering. 5. Transpose an octave up and down and listen again.
Watch the waveform and the spectrum. Beating can pass for tone colour when it is really two out-of-phase waveforms summed. A crossfade hides a click but not a phase difference, and a longer fade makes the comb filtering clearer.
Transposition is the real test
Raise the sample an octave and the loop lasts half as long in time, so its repetition rate doubles. A whole-cycle loop stays whole; a loop that only looked aligned reveals a new pulse. In stereo, check both channels: a few samples of difference between left and right makes the image drift on every pass. Audition a sustain for a minute, then try it at the top and bottom of the range.
Samples that should not loop
Piano, plucked strings and percussion are already decaying. A loop turns that decay into a drone that sounds like an organ. A voice or string line with slow vibrato locks one phase and loses its direction. Breath and bow noise can turn into a periodic gasp. For these, record a longer sample or stitch several takes instead of forcing a pair of points.
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
- SFZ Format 规范，loop_start、loop_end 与 loop_mode 的定义（公开规范）。
- Native Instruments Kontakt 手册，Wave Editor 的循环与交叉淡化（官方文档）。
- Ken C. Pohlmann, Principles of Digital Audio，采样回放与接缝。