ESI ESI2000 Operation Manual page 126

Background:
Auto Correlation
Background:
Zero Crossing
118
ESI Operation Manual
Typical Applications
Typical cut/copy/paste applications would be to splice the beginning of
one sample to the end of another, or to mix two samples together to
conserve memory. (Do this by copying an entire sample, then pasting it
at the beginning of the second sample using the mix option.) You can
splice an attack transient on to a synthesizer waveform loop to produce
realistic sounds that take up virtually no memory, or take a pop or click
out of a sample. Another application is flanging and chorusing. Paste
(mix) a sample to itself, offset from the beginning by a few hundred
samples or so, to thicken up the sound.
We suggest you practice cut, copy, and paste techniques on a spoken
phrase. Samples of political speeches can be particularly amusing when
subjected to cut and paste operations.
Auto Correlation
First of all, just what does auto correlation mean? Auto correlation
simply means to automatically compare and tweak into a complemen-
tary or parallel relationship. The computer analyzes the signal around
the loop points you have specified and then moves the end point of the
loop until it finds a section of the wave that closely matches the section
around the start point. Auto correlation may be used again and again
with the computer moving the analysis window slightly each time to try
to zero in on the optimum loop.
Zero Crossing
The term zero crossing refers to the point at which the positive slope of a
waveform passes through zero. In many digital processing applications,
such as splicing and looping, it is useful to locate zero crossings in order
to make glitch-free joins and loops. On some signals, however, a simple
zero crossing may not be effective because the signal contains excessive
noise or low-level, high-frequency harmonics. In these cases, every few
samples may cross through zero. By setting a zero crossing threshold, we
can ignore low-level zero crossings and wait for the signal to reach a
certain level before choosing the next zero-crossing. Zero crossing
threshold sets a level that a signal must exceed before the next zero
crossing with a positive slope is selected. The selected zero crossing
threshold is used in the Auto-Truncate function in the Sample Manage-
ment module, Sample Setup (5), or whenever you manually select a zero
crossing using the cursor keys.
The diagram on the following page illustrates zero crossing in action.
The small x marks the initial position. To move forward through the
sound to the next zero crossing, press the right cursor button, as shown
in the upper diagram. The ESI will find the first zero crossing on the
positive slope after the signal has crossed the designated threshold.
To move backward through the sound to the next zero crossing, as
shown in the lower diagram, press the left cursor button. Again, the ESI
finds the first zero crossing on the positive slope after the signal has
crossed the designated threshold.