Example Vocoder Waveforms

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Test Case
- Two constant-amplitude sinusoids
- Frequencies separated by 10 Hz (10 beats/sec)
Vocoder Parameters
- 2x time expansion
- Vocoder frame size = 45ms $\;\leftrightarrow\;$ 22 Hz
  (frame size $\approx$ half a beat pulse)
- Vocoder hop size = 1/4 frame
  (Hann or Hamming window)
Vocoder Types
- Classic Vocoder (Dan Ellis's Matlab Code)
- Salsman's Phase-Locked Vocoder (Puckette, Laroche, Dolson)
- SOLA-FS by Hejna and Musicus (Dan Ellis's Matlab Code)

**Figure 9.26:** Classic Vocoder Waveforms at 2X Expansion.
$\includegraphics[width=\twidth]{eps/pv-ellis-wave}$

**Figure 9.27:** Classic Vocoder Spectra at 2X Expansion.
$\includegraphics[width=\twidth]{eps/pv-ellis-spec}$

**Figure 9.28:** Phase-Locked Vocoder Waveforms at 2X Time Expansion.
$\includegraphics[width=\twidth]{eps/pv-salsman-wave}$

Amplitude envelope better preserved

**Figure 9.29:** Phase-Locked Vocoder Spectra at 2X Time Expansion.
$\includegraphics[width=\twidth]{eps/pv-salsman-spec}$

**Figure 9.30:** SOLA-FS Waveforms at 2X Expansion.
$\includegraphics[width=\twidth]{eps/pv-solafs-wave}$

Time domain method
Phase preserved within each frame
Phase not continuous across frames, but frames cross-fade at ``best locations'' in time, based on maximum cross-correlation

**Figure 9.31:** SOLA-FS Spectra at 2X Expansion.
$\includegraphics[width=\twidth]{eps/pv-solafs-spec}$

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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.

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