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.
Logic Pro for Mac Alchemy and Sample Alchemy provide 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.
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