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Ask, June 2011

June 1, 2011
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I''ve recently discovered the benefits of audio files that can stretch to match a project''s tempo changes, but I''m trying to sort out the different stretching technologies—REX, Acid, Apple Loops, DSP-based, and so on. Which gives the best stretch quality over the widest range?
Ken Henderson, Jupiter, Florida
via e-mail

It depends on the source material. REX files “slice” percussive parts into individual hits, so they work well with drums (which have short, discrete hits), and can speed up over a wide range. However, slowing down creates gaps between slices. With short percussive sounds this isn''t a problem, but if something like a cymbal sustains over multiple slices, there will be gaps in the cymbal sustain. Making your own REX files requires Propellerhead Software''s program ReCycle.

Acidization usually speeds up well, but slowing down more than 10 to 20% can give a grainy, echo-ey sound quality. Editing the Acidization markers applied to files may improve stretching for specific tempos, but this process can be done only in Sony Acid or Cakewalk Sonar (both of which can also create Acidized files). Apple Loops use a process similar to Acidization; Logic Pro includes a utility that simplifies creating your own Apple Loops. (The June 2008 “Power App Alley” in EQ magazine shows how to do this.) Note, however, that creating stretchable files of any type is an art that takes practice to master.

Doing “offline” stretching with DSP can give surprisingly good sound quality with modern algorithms like those from iZotope and zplane, but unlike REX, Apple Loops, and Acidized files, they won''t adapt to real-time tempo changes in your host.

Finally, some programs have proprietary ways to stretch files (Pro Tools'' Elastic Audio, Cubase''s Hit Points, Ableton Live''s various internal stretching algorithms, etc.). However, most sample library files provide REX, Acidized, and/or Apple Loops formats; use whichever your host can import.

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