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Relative-Phase-Preserving STFT TSM

Figure 10.20 shows the relative-phase-preserving (sometimes called ``phase-locked'') vocoder case in which relative phase is preserved across FFT bins. As a result, the amplitude envelope is preserved very well in each frame, and segues from one frame to the next look much better on the envelope level, but now the individual FFT bin frequencies are phase-modulated from frame to frame. Both plots show the same number of beats per second while the overall duration is doubled in the second plot, as desired. Figure 10.21 shows the corresponding spectrum; instead of distortion-modulation on the scale of the frame rate, the spectral distortion looks more broadband--consistent with phase-discontinuities across the entire spectrum from one frame to the next.

Figure 10.20: Phase-locked vocoder waveforms at 2X time expansion.

Figure 10.21: Phase-Locked Vocoder Spectra at 2X Time Expansion.

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``Spectral Audio Signal Processing'', by Julius O. Smith III, W3K Publishing, 2011, ISBN 978-0-9745607-3-1.
Copyright © 2022-02-28 by Julius O. Smith III
Center for Computer Research in Music and Acoustics (CCRMA),   Stanford University