[sdiy] VSM201 Vocorder Question

[Simon Brouwer]

Hi,
 
Note that for small input voltages the pulse width encoded signal will be around
50% pulse width, so there will be no "particular minimum pulse output before it
jumps to flat".
 
Also, the modulating signal is analog, so in principle it can also become
arbitrarily small. The maximum attainable attenuation may be limited due to
offset voltage though.
 
At the output of this kind of amplitude modulator the audio is always a pulse
width modulated signal with varying amplitude. However, in the VSM-201 approach
the audio signal is  PWM at the input, and the modulating signal is analog,
while in another approach the audio signal is analog at the input while the
modulating signal is PWM'ed.
 
In the latter case you *will* have limited attenuation due to the pulse width
not capable of being smaller than a certain value, but again, this is not how it
works in the VSM-201.
 
Finally, yes, the VSM201 is a true multiband vocoder: the "analysis and
synthesis" circuit is repeated 20 times with different values for the components
in the band filters.
The band filters are composed of three consecutive peaking band pass filters,
each one carefully tuned to a slightly different frequency to get the desired
response (hence the annotation in the schematic "unten, oben, mitte" i.e.
"lower, upper, middle").
 
Best regards
Simon
 
 

> Op 4 januari 2016 om 2:17 schreef rsdio at audiobanshee.com:
>
>
> This discussion has quickly become confusing. A few terms have been used
> incorrectly, we've diverged onto different VCA implementations, and the
> schematic is in German, so I'm not surprised at the confusion. Maybe we can
> figure this thing out?
>
>
> First of all, dynamic range is a relative term. It does not always refer to
> (maximum) signal to noise ratio. While most vocoders surely have issues with
> signal to noise ratio, I don't think that's the trickiest part of a vocoder
> circuit operation to figure out. For most of this thread, we've been
> discussing limitations in the dynamic range of the VCA section, which
> presumably ranges from 0 dB down to some limit of available attenuation.
>
> With a digitally-generated PWM signal, the VCA volume steps are discrete and
> finite, which is why we were discussing limits like -40 dB, -60 dB, and -80
> dB. Actually, even the digital PWM would be capable of infinite attenuation
> when the pulse width is zero, but a jump from -40 dB to negative infinity
> would potentially be distracting.
>
> It's been pointed out that the VSM201 vocoder does not have a
> digital-counter-based PWM, but uses a comparator fed by an input signal and a
> triangle wave. This design does mean that the PWM is not discretely stepped,
> but it does not mean that the signal attenuation is continuous all the way
> down to infinity. The comparator chip will have limited slew rate and settling
> times, making a particular minimum pulse output before it jumps to flat. That
> minimum output pulse width will determine the maximum attenuation of the
> signal in that band, before the jump from maximum useful attenuation to
> infinite attenuation. I assume that those 4053 2-channel multiplexor elements
> can only be On or Off, and thus there is a minimum pulse width and thus a
> maximum, finite attenuation before the signal drops to negative infinity
> (muted).
>
>
> Second of all, the discussion also focused on the fact that the audio is PWM'd
> and not the CV. However, it seems to me that the audio is always PWM'd by this
> type of VCA. The writeup by Rod Elliot is not for a vocoder, and thus
> describes a CV input that is converted to a PWM signal that controls a SPDT
> switch that alternates between the input audio and ground. Even in that basic
> circuit, the audio is PWM'd. When adapting this kind of VCA to a vocoder, one
> needs to process two input audio signals, breaking each into multiple
> frequency bands, and then multiplying the amplitude of each band to produce an
> output audio signal that sounds like a combination of the two input audio
> signals. Ultimately, I assume that the output VCAs still operate as described
> by Rod Elliot, where the audio is always PWM'd.
>
>
> Finally, can anyone say whether this is a true, 20-band vocoder, or maybe just
> a 3-band vocoder with 20-band graphic EQ on the input signal? How wide range
> is the "speech or signal inputs" section? Would it handle a full keyboard
> tone? I assume that the "modulation sound input" is full range.
>
> Brian
>
>
> On Jan 2, 2016, at 10:45 PM, Tim Ressel <timr at circuitabbey.com> wrote:
> > I think if an effort was made to make a DIY copy of the VSM201, a case could
> > be made for swapping out the switched VCAs for 13700s or something.
> >
> > Another question: It looks as though what is being PWMed is the audio and
> > not the CV coming from the detectors. The CV signals (I think) are labelled
> > 'KK' but I cannot find their source. It is not helping that the schematics
> > are copies and are in a language I don't read.
> >
> > Also I can't see the values for the filter components. Is there a BOM
> > somewhere?
> >
> > --TimR
>
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