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.
Phase-Continued STFT TSM