Timestretch - Akai DD1500 User Manual

UTILITIES - DSP FUNCTIONS - 20

TIMESTRETCH

Timestretch allows you to alter the length of a sample, shortening it or lengthening it,
without changing its pitch. Before we look at how to operate the timestretch functions on
the DD1500, let us first look at how timestretch works.
Timestretch works by instructing the digital signal processor to analyse the signal and
insert or delete blocks of sample data at appropriate places and crossfades are used to
make the insertions and deletions as seamless as possible. This has the effect of
lengthening or shortening a recording. As you can see from the following diagram, blocks
of sample data have been inserted to create a 200% timestretch. The overall envelope of
the sound data is preserved but there is twice as much data in it causing it to play back
twice as slow.
SAMPLE DATA BLOCKS
ORIGINAL RECORDING
WITH 200% TIMESTRETCH
In the following diagram, data has been removed to make the recording play back faster.
ORIGINAL RECORDING
70% TIMESTRETCH
You can see that the length in both of these examples changes quite drastically but the
envelope remains pretty much the same, thereby ensuring the integrity of the sound. The
timestretch on the DD1500 works by making a copy of the original. It is necessary to make
a copy for two reasons - firstly, the DD1500 needs the original data to get the data to
insert. Secondly, in the event of a stretch going wrong, you still have the original to revert
to using UNDO Unfortunately, however, perfect results using timestretch are sometimes
difficult to achieve. It is not a limitation of software or hardware but due to the fact that,
although the processor is clever, it is not that clever and can sometimes make mistakes in
deciding which sample to insert or remove. The end result of this is that, on occasions,
especially with stretch factors exceeding 10% or so, you may get an echo or 'flam' effect
on some transients because the processor has inserted a transient. When shrinking a
recording, you may find a transient softened because the processor has decided to
remove it. You will find this to be the case more or less on all devices that feature some
form of time compression or expansion.
Version 2.00 - March, 1996 Page 193