[Octal-dev] Pitch Shifter / Extracting Fourier Description

[n_nelson]

  Steve Mosher wrote:

  [ I have a question,  does  anyone  know how  to write a  reasonable
  [ pitch-shifter?   I know  that  pitch  shifting  audio  data  (i.e.
  [ -withoutchanging its speed/duration) is gruesome work,  but it can
  [ be a good correctional component for many effects.

  Pitch shifting without changing the sampling rate would seem  to re-
  quire an  identification  of the pitch as  frequency  and  amplitude
  (Fourier description)  and then changing the frequency in the sampl-
  ing generate phase (reverse Fourier transform).

  I have been experimenting with code to identify the complete Fourier
  description  of a sample sequence.   Does anyone on the list know of
  code to perform this process?  Some of my observations are:

  (1) The Fast Fourier Transform  only  detects  frequencies  in whole
      numbered values  of which useful frequencies in general are not.
      What is required is a more precise frequency detection sequence.
      The full Fourier transform can be used to more precisely analyze
      fractional ranges after the FFT identifies nearby high-amplitude
      areas.

  (2) The precision of frequency identification correlates to the num-
      ber of samples used.  I.e., though one may address any frequency
      below half the sampling  rate,  the degree  of distinction  from
      nearby  (fractional) frequencies depends  on how long  (how many
      samples in which) the frequency is present,  and would appear to
      be an aspect fundamental to all frequency detection systems such
      as human hearing.

  Neil Nelson address@hidden