[sdiy] VSM201 Vocorder Question

[Simon Brouwer]

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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