10.6

722Logic Pro Instruments
Depending on the sophistication of the additive synthesizer you are using, you will either
have individual envelope control over the level and pitch of each sine wave, or you will be
limited to envelope control over groups of sine waves—one envelope per sound and its
harmonics, or all odd or all even harmonics, for example. In practical terms, working with
groups of related harmonics is the best approach due to the mathematical relationships
between them and the impact this has on the overall tone when adjusting them en masse,
rather than individually.
Alchemy can be used as a true additive synthesizer, where you create sounds from scratch
with sine waves, with full control of the level, pitch, and pan position of each harmonic.
Alchemy also allows you to resynthesize imported samples with additive (and spectral)
synthesis techniques. See Resynthesis.
Some aspects of the additive synthesis approach are also used in Vintage B3 and other
drawbar organs. In Vintage B3 you start with a basic tone and add harmonics to it, to
build up a richer sound. The level relationships between the fundamental tone and each
harmonic are determined by how far you pull each drawbar out. Because Vintage B3
doesn’t provide envelope control over each harmonic, it is limited to organ emulations.
Spectral synthesis
Spectral (modeling) synthesis lets you build a sound by combining multiple (sine wave)
harmonics and filtered noise signals. This synthesis method shares many underlying
principles with vocoders, but tracks peaks in the overall spectrum, rather than individual
amplitudes and frequencies in the signal.
Alchemy provides a flexible spectral synthesis implementation, known technically as
multiresolution sinusoidal modeling. In other words, a custom filter bank is used to
analyze peaks (and other elements) in the frequency spectrum of the signal. Harmonic
components, based on the spectral analysis, are modeled as a combination of sine waves
and white noise passed through a filter that changes over time. The noise components are
typically used to model “percussive” elements such as a piano strike or a speech fricative
in a vocal sample, for example. The (sine wave) harmonic components are used to model
the piano note or remainder of the vocal sound. The output of the modeled sound is a
combination of the frequencies and levels of the detected harmonic components and the
noise signal passed through a time-variable filter.
The spectral synthesis engine in Alchemy can be used to create sounds from scratch, by
drawing or painting in the spectral edit window. You can also import and convert an image
file into a spectrogram (an image of the frequency spectrum) in the spectral edit window.
You can then edit this converted image with the drawing and painting tools. Alchemy
analyzes the spectrogram and replaces peaks and percussive components with sine
harmonics and filtered noise elements to create a sound.
Alchemy can also break imported samples down into “spectral bins,” with each bin storing
the amplitude and phase values in the given frequency band. These bins are used to
resynthesize (or reconstruct an approximation of) the original sound. See Resynthesis. In
noise mode, the amplitude values are used to generate filtered noise for each bin. In pitch
mode, the amplitude and phase values are used to synthesize a sine wave for each bin. The
signals associated with each bin are then summed and sent to other parts of the Alchemy
synthesis engine.