<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns="http://www.w3.org/TR/REC-html40">
<head>
<meta http-equiv=Content-Type content="text/html; charset=us-ascii">
<meta name=Generator content="Microsoft Word 11 (filtered medium)">
<!--[if !mso]>
<style style="display:none;">
v\:* {behavior:url(#default#VML);}
o\:* {behavior:url(#default#VML);}
w\:* {behavior:url(#default#VML);}
.shape {behavior:url(#default#VML);}
</style>
<![endif]-->
<style>
<!--
/* Font Definitions */
@font-face
{font-family:Tahoma;
panose-1:2 11 6 4 3 5 4 4 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman";}
a:link, span.MsoHyperlink
{color:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{color:blue;
text-decoration:underline;}
p
{margin:0in;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman";}
span.EmailStyle18
{mso-style-type:personal-reply;
font-family:Arial;
color:navy;}
@page Section1
{size:8.5in 11.0in;
margin:1.0in 1.25in 1.0in 1.25in;}
div.Section1
{page:Section1;}
-->
</style>
</head>
<body lang=EN-US link=blue vlink=blue>
<div class=Section1>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>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).<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<div>
<div class=MsoNormal align=center style='text-align:center'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>
<hr size=2 width="100%" align=center tabindex=-1>
</span></font></div>
<p class=MsoNormal><b><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma;font-weight:bold'>From:</span></font></b><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'> Synth-diy
[mailto:synth-diy-bounces@synth-diy.org] <b><span style='font-weight:bold'>On
Behalf Of </span></b>Bernard Arthur Hutchins, Jr<br>
<b><span style='font-weight:bold'>Sent:</span></b> Sunday, September 01, 2019
3:45 PM<br>
<b><span style='font-weight:bold'>To:</span></b> synth-diy@synth-diy.org<br>
<b><span style='font-weight:bold'>Cc:</span></b> Bernard Arthur Hutchins, Jr<br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: [sdiy] vocoder
filters</span></font><o:p></o:p></p>
</div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<div id=divtagdefaultwrapper>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 color=black face="Times New Roman"><span style='font-size:12.0pt;
color:black'>The general set of vocoder problems mentioned here yield to a
specific digital approach.<o:p></o:p></span></font></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 color=black face="Times New Roman"><span style='font-size:12.0pt;
color:black'><o:p> </o:p></span></font></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 color=black face="Times New Roman"><span style='font-size:12.0pt;
color:black'>First, the need for MANY (topographically similar) filters with
unknown or uncertain parameters is an obvious job for programming.<o:p></o:p></span></font></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 color=black face="Times New Roman"><span style='font-size:12.0pt;
color:black'><o:p> </o:p></span></font></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 color=black face="Times New Roman"><span style='font-size:12.0pt;
color:black'>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).<o:p></o:p></span></font></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 color=black face="Times New Roman"><span style='font-size:12.0pt;
color:black'><o:p> </o:p></span></font></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 color=black face="Times New Roman"><span style='font-size:12.0pt;
color:black'>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).<o:p></o:p></span></font></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 color=black face="Times New Roman"><span style='font-size:12.0pt;
color:black'><o:p> </o:p></span></font></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=2 color=black face="Times New Roman"><span style='font-size:10.0pt;
color:black'>A simple introductfion: </span></font><font color=black><span
style='color:black'><a href="http://electronotes.netfirms.com/AN358.pdf"
id=LPlnk604580 previewremoved=true><font size=2><span style='font-size:10.0pt'>http://electronotes.netfirms.com/AN358.pdf</span></font></a><o:p></o:p></span></font></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 color=black face="Times New Roman"><span style='font-size:12.0pt;
color:black'><o:p> </o:p></span></font></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 color=black face="Times New Roman"><span style='font-size:12.0pt;
color:black'>But first of all – What are you hoping to
accomplish? <o:p></o:p></span></font></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 color=black face="Times New Roman"><span style='font-size:12.0pt;
color:black'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=black face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:black'><o:p> </o:p></span></font></p>
</div>
</div>
</body>
</html>