[sdiy] vocoder filters

[Mattias Rickardsson]

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!
:-)

/mr


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
> required.
>
>
>
> 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.  Cheers.
>
>
> ------------------------------
>
> *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.
>
>
>
> Cheers,
>
> Dave Dixon
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
>
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20190901/69f578f6/attachment.htm>