[sdiy] vocoder filters

David G Dixon dixon at mail.ubc.ca
Sun Sep 1 20:28:24 CEST 2019

Thing 1: I had a look at the MFOS vocoder schematic, and lo and behold, Ray
used single opamp filters!  Giving this some thought, I can design a single
opamp filter which will give more or less the same frequency response near
the corner as a double (or quadruple) opamp filter.


Thing 2: Looking at Jurgen Haible’s overall frequency response for all the
filters together, it looks like a series of peaks with a “floor” at about
-3dB, and the peaks at about -1dB – yes, that’s right, the whole action of
the filters altogether is only within about a 2-dB range of amplitude.
Hence, what is happening further down with these individual responses is of
absolutely no consequence.  I would probably opt for slightly pointier
responses near the tops to give more of a comb effect and better separation
of the channels.


Conclusions: I can achieve more or less exactly the same results as the JH
Living Vocoder with a quarter of the filter circuitry.  With the double
feedback filter design, one has complete control over the frequency, gain,
and bandwidth.  Hence, I would strive to position the peak at 0 dB, and to
make the slope of the response near the peak similar to the 8-pole filters
with low Q employed by JH.  With the filter design I’m using, the frequency
will be tuned by a 500-ohm trimpot – I’m using single turn trimpots to make
tuning less suicide-inducing (I bought a bag of 100 from Electronic Goldmine
for 20 cents each).  I will simply tinker with the design until I have a
frequency response which looks very similar to JH’s filters – it’s all on a
spreadsheet – all I have to do is specify f_0, Q and the amplitude at f_0,
and pick a standard capacitor value, and the design is calculated
automatically.  Depending upon the Q value chosen, the filters can all take
a combination of the same two standard resistor values once the capacitor is
chosen, with the third resistor value represented by the trimpot (which is
500 ohms based on selecting a Q value of 10).


This is all an experiment, so I don’t expect to succeed completely on the
first attempt.  I also must confess that I haven’t really given much thought
to things such as compression, pre-amplifying the vocals, sibilance, etc,
etc, so I still have a lot of work to do.  However, I’m going to start with
the basic vocoder and expand on it as needed, rather than try to design the
whole thing from the ground up right away.  I make all my PCBs in a modular
fashion anyway, so no big deal.


My original idea was to make a vocoder with 88 channels – one for each
semitone of the piano range – with resonating filters that would ring out
with a sine wave when excited.  In this way, the device would “sing” some
ethereal version of whatever music was fed to it, in tune.  Maybe, with
single opamp filters, this is doable.




From: Mattias Rickardsson [mailto:mr at analogue.org] 
Sent: Sunday, September 01, 2019 8:55 AM
To: David G Dixon
Cc: David Moylan; Synth DIY
Subject: Re: [sdiy] vocoder filters


The next level is to ponder on the unwanted higher-frequency AM effects from
controlling a vocoder band VCA with the "best" (fastest) envelope follower,
and whether a slower response could be more optimal. So much fun! :-)





Den lör 31 aug. 2019 23:23David G Dixon <dixon at mail.ubc.ca> skrev:

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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