[sdiy] VSM201 Vocorder Question

[Magnus Danielson]

Hi,

On a somewhat different note.
I really love that we can have this discussion. Some 15-20 years ago a 
few of us had some fun with scanners, bringing some schematics and 
manuals onto this new thing called the Internet and in particular World 
Wide Web. Building my own home-page was a big thing at the time.
Back then, there was a complete drought on real information, but piece 
by piece a few of us was able to share things to get a pile.
Many more people have since been able to study these schematics, they 
have inspired new designs and some went on to make clones of larger and 
larger things.

It's interesting how many of the hobbyists now use their training 
professionally, by either designing stuff or teaching them using the 
material.

We now is in the second golden age for analog synthesizers. There is 
more vendors than ever, more modules, and much more users!
Today modular synthesizers is a movement, as it is affordable to a much 
higher degree than ever.

It's really interesting to see how things have changed and matured.
Sometimes you just have to stand back and marvel about the progress.

Cheers,
Magnus

On 01/02/2016 10:21 PM, Richie Burnett wrote:
> I think the "colouring" that you mention might be quite common in
> vocoders. I found "perfect reconstruction" very hard to achieve when
> designing the vocoder bands to be sufficiently selective to work well as
> a vocoder.
>
> I guess a vocoder is an effect not an EQ, so perfect reconstruction can
> be sacrificed here. Deep notches are also far less objectionable than
> sharp peaks in the combined response, so I sharpened up the Q of my
> filters, at the expense of getting a couple of dB dips between bands.
>
> -Richie,
>
> Sent from my Xperia SP on O2
>
> ---- Simon Brouwer wrote ----
>
> Hi,
> These band pass filters will suppress the carrier, but they are not in
> front of the modulator so they won't help
> against aliasing. In the schematic, there does not appear to be an
> explicit antialising filter. But this kind of modulation can be done at
> hundreds of kHz so it should not be a problem.
> By the way, the filter characteristics
> (http://rubidium.dyndns.org/~magnus/synths/companies/sennheiser/Sennheiser_VSM201_manual_il8_200.jpg)
> are a bit different than what I would expect.At the crossover
> frequencies of the lower bands the attenuation of the filter sections is
> only 5 dB or so, while at the higher frequencies it goes towards 17 dB.
> I would expect a crossover frequency attenuation of slightly more (*)
> than 6 dB to get an even overall filter response, and this assuming the
> signals from the band filters add up in phase.
> (*) since not only the adjacent bands add up
> At, for example, 6.5 kHz the response would be some 10 dB down compared
> to at 7.3 kHz.
> So aside from applying the intended effect, this vocoder would add quite
> a lot of coloring of its own.
> Best regards
> Simon
>
>  > Op 2 januari 2016 om 20:42 schreef rburnett at richieburnett.co.uk:
>  >
>  >
>  > The narrow high-Q band-pass filters of the Vocoder channels do a good
>  > job of attenuating the "out of band" HF PWM carrier frequency. Roland's
>  > SVC-350 uses the same trick. Control range of PWM'd CMOS switches
>  > acting as VCAs isn't that great though.
>  >
>  > -Richie,
>  >
>  >
>  > On 2016-01-02 19:23, Tim Ressel wrote:
>  > > No kidding? A nice, cheap, simple solution. I assume the PWM freq is
>  > > above audio rates as there doesn't seem to be big low pass filtering
>  > > going on. Interesting...
>  > >
>  > > Thanks!
>  > >
>  > > --TimR
>  > >
>  > > On 1/2/2016 10:45 AM, Mattias Rickardsson wrote:
>  > >> Indeed. PWMed analog switches. :-)
>  > >>
>  > >> /mr
>  > >>
>  > >>
>  > >> On 2 January 2016 at 19:29, Tim Ressel <timr at circuitabbey.com> wrote:
>  > >>> Hi,
>  > >>>
>  > >>> I've been looking at the VSM201 schematics and I can't find where
> the
>  > >>> VCAs
>  > >>> are. Is there some cleverness I am missing?
>  > >>>
>  > >>> --
>  > >>> --Tim Ressel
>  > >>> Circuit Abbey
>  > >>> timr at circuitabbey.com
>  > >>>
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