[PD] understanding 09.pvoc.pd

[Ben Saylor]

I'm doing a research project involving phase vocoders and looking at 
09.pvoc.pd in the example patches as a reference implementation.  I was 
wondering if anyone could help me to understand it.  Specifically, how 
is the "instantaneous frequency", obtained by comparing phases in 
successive frames, used?  What is the relationship between the two 
segments of the source array that are read at the same time and the 
phase-real, phase-imag arrays?

I basically understand the procedure outlined in Computer Music Tutorial 
(Roads) on pages 1105-6:

1.  read M samples of input signal
2.  window
3.  zero-pad the windowed frame (this seems to be absent from the patch)
4.  FFT
5.  convert from rectangular to polar, differentiate the phase to obtain 
instantaneous frequency
6.  apply any modifications (time-stretch, pitch-shift, etc)
7.  IFFT
8.  window again if phase spectrum was altered
9.  overlap-add the output frames

but step 6 is sketchy, and I don't understand how the instantaneous 
frequency is actually *used* in resynthesis.  Any help would be 
appreciated.

Thanks,
Ben