Introduction to MIDI and Computer Music: Reason: Time-scaling Audio
You can time-scale an audio clip — that is, stretch or shrink its duration — without affecting the pitch of the audio. You can do this either to a clip as a whole, or to specific parts of the clip. You can also quantize parts of a clip that have a strong rhythmic character to a note grid, just as you would for MIDI notes.
While holding down the option (or alt) key, drag one of the resize handles to time-scale the clip. You will see the time-scale clock cursor, and the audio waveform will scale while you drag the handle.
This works well if you want to bring in the audio file for a vocal and then find a tempo for the sequence that matches the tempo of the vocal. Import the audio file, disable stretch for its clip, and play the sequence with the metronome turned on. Change the tempo and drag the clip left and right until you find that the beats in the clip match the metronome clicks. Do not edit the left edge of the clip before you have found the best sequence tempo. Do not ever change the tempo after this point if you later edit the left edge of the audio clip, for reasons explained below.
WARNING: If you disable stretch for a clip that does not begin at the start of its underlying sound file, and then you change the tempo, you will find that the contents of the clip have shifted to reveal a different part of the underlying file. This will be frustrating if you have already made a lot of precise edits that are now ruined. It is possible to get back on track by using the Comp Editor, but this can be painful.
An alternative is to follow this procedure, if you have a lot of edits and want to change the tempo without time-scaling the audio.
Otherwise, you’ll want to scale the start times and durations of the audio clips so that your edits will sound the same at the new tempo as they did at the old tempo.
Compute a scale tempo percentage with this formula:
scale tempo percentage = (old tempo / new tempo) * 100
So if you change from 120 BPM to 60 BPM, the result is 200%.
Choose Window > Show Tool Window, and find the Scale Tempo section in that window. Be sure all the clips are selected, specify the percentage that you computed above, and press Apply.
The clips should snap into place, and they should sound the same as they did at the old tempo.
Well, okay, that was painful also.
Reason automatically segments audio files according to its estimation of transient peaks, or places where the audio suddenly gets louder. So in a drum loop, you would see transients for each drum hit. These segments are called slices in Reason. A slice is defined by two adjacent slice markers. You can move or delete existing markers, as well as create your own.
To see slice markers for an audio clip, double-click the clip to place it in edit mode.
The slice markers are the triangles with descending white vertical lines, which slice through the audio waveform at transients.
There are several things you can do with slices.
You can select multiple slices by dragging across the clip. Then drag the gray bar at the bottom of the clip to move the slices together.
If you want to return to the original slice marker positions, choose the Edit > Revert Slices menu command.