Resynthesis
You can analyze the frequency components of a recorded sound and then resynthesize—reconstruct—a representation of the sound using additive or spectral techniques. See Additive synthesis and Spectral synthesis.
At a basic conceptual level, additive synthesis and spectral (modeling) synthesis are similar in that both techniques can recreate complex sounds by adding together many simpler signals. In practice, however, the two methods are very different. Additive resynthesis attempts to understand the harmonic structure of an audio file and then recreates each harmonic partial with a separate sine wave. Spectral resynthesis instead analyzes the changing frequency spectrum of the signal and attempts to recreate these spectral characteristics in the resynthesized signal.
An additive resynthesis system generates a series of sine waves, with appropriate pitches and levels over time, for each harmonic. It does this by calculating the frequency and amplitude of each harmonic in the overall frequency spectrum of the analyzed sound. After the sound has been resynthesized in this fashion, you can adjust the frequency and amplitude of any harmonic (sine wave). Theoretically, you could even restructure a harmonic sound to make it inharmonic, for example.
In a spectral resynthesis system, the audible spectrum is split into a large number of “spectral bins,” and the distribution of energy across these bins is analyzed. The sound is recreated by filling each spectral bin with the required amount of signal, either using sine waves or filtered noise, and the results are then summed.
The difference in approach means the two techniques are suited to different types of sounds. Additive resynthesis excels at recreating single notes with a clear harmonic structure. Spectral resynthesis is better suited for complex inharmonic sounds such as drums, or polyphonic material containing chords rather than individual notes.
Logic Pro for Mac Alchemy and Sample Alchemy can resynthesize sounds using additive or spectral methods. It can also perform resynthesis using a combination of the two techniques, which is useful for sounds that feature both a clear pitch component and a noisy component. Examples of such sounds are the hammer strike of a piano and the string tone, or the breath noise of a flute and the flute tone.
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