[sdiy] vocoder filters

David G Dixon dixon at mail.ubc.ca
Mon Sep 2 01:11:46 CEST 2019

What do I hope to accomplish?  Not sure.  Mostly to have fun getting stuck
into another stupidly large electronics project.  I don't do digital -
coding seems too much like work (I'm an engineering professor, and I
specialized in mathematical models in my academic work, before I saw the
light and started developing new processes instead - for extraction of
copper from ores and concentrates, in case you're curious).



From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of
Bernard Arthur Hutchins, Jr
Sent: Sunday, September 01, 2019 3:45 PM
To: synth-diy at synth-diy.org
Cc: Bernard Arthur Hutchins, Jr
Subject: Re: [sdiy] vocoder filters


The general set of vocoder problems mentioned here yield to a specific
digital approach.


First, the need for MANY (topographically similar) filters with unknown or
uncertain parameters is an obvious job for programming.


Second, obviously, the equivalent of filtering can be done by taking an FFT
(analysis), custom weighting the frequency domain description (the dynamics)
and inverting the thusly modified FFT (synthesis).


Thirdly, since the FFT frequencies are equally spaced in Hz (requiring huge
FFT sizes) the FFT is NOT well-suited to the audio/speech problem. Instead,
look at the well-studied approach of MULTI-RATE DSP where lower rates at
lower frequencies are used.  (It's magic).  Things like
"perfect-reconstruction filers".  See: P.P. Vaidyanathan "Multirate Systems
and Filter Banks" (Prentice-Hall, 1993) and N.J. Fliege, "Multirate Digital
Signal Processing: Multirate Systems, Filter Banks, Wavelets" (Wiley 1994).


A simple introductfion:  <http://electronotes.netfirms.com/AN358.pdf>


But first of all - What are you hoping to accomplish?   



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