Vocoder-Based Additive-Synthesis Limitations

Using the phase-vocoder to compute amplitude and frequency envelopes for additive synthesis works best for quasi-periodic signals. For inharmonic signals, the vocoder analysis method can be unwieldy: The restriction of one sinusoid per subband leads to many ``empty'' bands (since radix-2 FFT filter banks are always uniformly spaced). As a result, we have to compute many more filter bands than are actually needed, and the empty bands need to be ``pruned'' in some way (e.g., based on an energy detector within each band). The unwieldiness of a uniform filter bank for tracking inharmonic partial overtones through time led to the development of sinusoidal modeling based on the STFT, as described in §H.12.2.

Another limitation of the phase vocoder analysis was that it did not capture the attack transient very well in the amplitude and frequency envelopes computed. This is because an attack transient typically only partialy filled an STFT analysis window.

Moreover, filter-bank amplitude and frequency envelopes provide an inefficient model for signals that are noise like, such as a flute with a breathy attack.

The above limitations are addressed by sinusoidal modeling, sines+noise modeling, and sines+noise+transients modeling, as discussed starting in §9.4 below.

The phase vocoder was not typically implemented as an identity system due mainly to the large data reduction of the envelopes (piecewise linear approximation). However, it could be used as an identity system by keeping the envelopes at the full signal sampling rate and retaining the initial phase information for each channel. Instantaneous phase is then reconstructed as the initial phase plus the time-integral of the instantaneous frequency (given by the frequency envelope).

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About the Author: Julius Orion Smith III
Julius Smith's background is in electrical engineering (BS Rice 1975, PhD Stanford 1983). He is presently Professor of Music and Associate Professor (by courtesy) of Electrical Engineering at Stanford's Center for Computer Research in Music and Acoustics (CCRMA), teaching courses and pursuing research related to signal processing applied to music and audio systems. See http://ccrma.stanford.edu/~jos/ for details.