3-34
Algorithms and Parameters
Lexicon
Much of the art of pitch shifting, therefore, is in determining the
appropriate intervals for sampling and the optimal points for
splicing. As the PCM 81 Pitch algorithms gives you significant
control over these operations, you may find it helpful to keep in
mind some of the other factors which can affect pitch shift quality:
• Amount of pitch shifting
Large shift intervals require more splicing. As each splice can be
the source of an artifact, increasing their number increases the
likelihood of audible artifacts.
• Pitch-complexity of the input material
Pure tones have simple waveforms which can be correlated
easily for smooth splicing. Pitch shifting of simple tones such as
those generated by a solo voice or a monophonic instrument
generally produces fewer artifacts than pitch shifting of wide
bandwidth material such as piano chords, full orchestra, etc.
• Low frequency content of the input material
Pitch shifting low frequency material requires the addition or
removal of larger segments of audio than high frequency
material. The increased size of these segments can introduce
delays which are heard as artifacts.
• Transient content of the input material
When large amounts of pitch shifting are applied to complex
input material with dense percussive transients (drums,
strummed mandolin, complex speech, etc.) some of the
transients may lose definition or be shifted slightly in time.
ABOUT THE PITCH ALGORITHMS (continued)
Configuration and routing control is provided by a dedicated
Submixer incorporated into each of these five algorithms. Any
combination of reverb and effects blocks can be arranged in any
configuration by changing the Submixer’s controls. As the
Submixer has continuously variable controls, you can alter the
configurations on the fly — changing, for example, from reverb-
into-effects to effects-into-reverb. (Use of the Submixer, which
appears as a separate row on the PCM 81 parameter matrix, is
described fully after the algorithm and parameter descriptions.)
Parameters which are common to the 4-Voice and 6-Voice
algorithms are not repeated here. The Pitch Correct algorithm,
designed for use with monophonic vocal sources, is described at
the end of this section.
ABOUT PITCH SHIFTING
Digital pitch shifting requires temporal compression or expansion
of audio data and the addition or removal of small audio segments
to compensate.
Raising pitch is accomplished by compressing the audio data —
making it run faster. As this shortens the audio segment, a copy of
a section of the audio being processed is added to fill out the
remaining time. To lower the pitch, the signal is expanded,
requiring the removal of a section of the audio. These operations
are performed repeatedly at high speed, at intervals which vary
according to the type of audio signal being processed.
At each point where audio is added or removed, the audio signal
must be rejoined, or spliced. If added or removed segments were
perfectly correlated with the remaining audio, splicing would be
seamless. In reality, these splice points are the source of audible
artifacts which can be minimized, but not completely eliminated.
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