[sdiy] vocoder filters

David G Dixon dixon at mail.ubc.ca
Sat Aug 31 23:22:21 CEST 2019

Not really.  It's a full-wave rectifier followed by a standard LP filter
stage tuned to about 1/4 of the expected incoming frequency followed by a
notch filter tuned to twice the incoming frequency.  I also use a full wave
rectifier instead of the normal half wave rectifier, because I figure this
gives faster integration.  This is why the ripple is at twice the incoming
frequency, and a notch filter knocks it out nicely.  Through the judicious
choice of gain at the LP filter, the envelope follows the waveform tops more
or less exactly, and comes up to full strength within two periods of the
incoming waveform, with ripple which is inconsequential.  For a 10Vpp
waveform coming in, the envelope rides at 5V, which will turn on my favored
linearized 2164 VCA design to unity gain.



From: David Moylan [mailto:dave at expeditionelectronics.com] 
Sent: Saturday, August 31, 2019 5:26 AM
To: David G Dixon
Cc: synth-diy at synth-diy.org
Subject: Re: [sdiy] vocoder filters


Curious about this envelope follower you mention. Trade secret?


On Aug 31, 2019 03:48, David G Dixon <dixon at mail.ubc.ca> wrote:

Well, I know that the higher-Q filters have a longer delay, so that they
take longer to respond to the incoming waveform.  I'm thinking that a Q of
about 3 is probably about right, and with that, only a 4-pole filter is


I've got a nice design for an envelope follower which responds quickly and
has little or no ripple, so that's not a problem.


On a related note, does anyone here have problems getting the Bode plotter
in Multisim to work consistently?  I am finding with this simulation that
sometimes if I change the component values, the Bode plotter doesn't work at
all.  Also, for some simulations, changing the component values doesn't
change the filter response at all.  Multisim is sure glitchy.  It's very
frustrating.  I can sometimes fix it if I erase all of the passive
components and load new ones with the new values, rather than just changing
them, but that sort of thing is just complete bullshit.  Multisim is a sad
excuse for a professional program.  There must be something better out there



From: Paul Perry [mailto:paulfrancisperry at gmail.com] 
Sent: Friday, August 30, 2019 10:22 PM
To: David G Dixon
Subject: Re: [sdiy] vocoder filters


I don't think there is a "right" answer. To my mind, it depends on what one
wants to do with the unit. Think about what will happen when a single swept
tone is used to modify white noise. The low pass filter on the VCAs probably
has a significant effect as well.

paul perry Melbourne Australia


On Sat, 31 Aug 2019 at 14:50, David G Dixon <dixon at mail.ubc.ca> wrote:

Well, I think I might have answered my own question.  Looking again at the
JH Living Vocoder, since Jurgen Haible's filter responses overlap at about
the 8dB mark, it really should not matter at all what's going on around the
skirt of the response, and higher-Q filters with two 2-pole stages should
give very similar results to low-Q filters with four 4-pole stages (and be
much much cheaper to build).


I'd still appreciate if anyone has any specific insights into this problem.



From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of David
G Dixon
Sent: Friday, August 30, 2019 9:05 PM
To: synth-diy at synth-diy.org
Subject: [sdiy] vocoder filters


Hey SDIY Team!


I'm thinking about building a vocoder, and I have a general question about
the bandpass filters.


I've looked at Jurgen Haible's Living Vocoder, and he used 8-pole filters
with low Q.  These give a reasonably broad band with fairly steep slopes.
He makes the filters from two pairs of LP and HP.


I was thinking about using BP filter sections, but just 4-pole, and with
higher Q (around 10).  This gives a somewhat narrower band, and the slope is
steep near the corner, but fairly shallow around the skirt.  This idea uses
a lot fewer components (about half as many).


What I'm asking is, does anybody here have any insight into what the
"proper" approach to vocoder filters would be?  What is the design goal?  Do
you want significant overlap from one band to the next, or should they be
fairly distinct?  I guess I'm just looking for some general guidelines and
conventional wisdom.



Dave Dixon

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