[sdiy] VSM201 Vocorder Question

[Simon Brouwer]

Hi Tom,
 
What do you mean, limited dynamic range for the audio? The PWM digitizer in the
VSM201 does not quantize the signal.
 
Best regards
Simon

> Op 3 januari 2016 om 13:46 schreef Tom Wiltshire <tom at electricdruid.net>:
>
>
> I hadn't seen the VSM201 schematics so I didn't know that. That's clever of
> them. Like it.
>
> The (slightly OT) PWM discussion was about how much dynamic range you can
> expect to get out of a PWM signal. If the VSM201 digitises the audio but not
> the control signal, then they have limited dynamic range for the audio
> instead. The problem remains, but shows up somewhere else. Like Magnus said -
> Companding is your friend.
>
> Tom
>
>
> On 3 Jan 2016, at 12:10, Simon Brouwer <simon.o at brousant.nl> wrote:
>
> > Hi Richie,
> >
> > Like I already wrote, in the VSM201 the signal which controls the analog
> > switch is the PWM digitized audio, and the analog modulating signal is on
> > the analog pin of the switch.
> >
> > What you guys appear to be discussing is a setup where the audio is analog,
> > and the modulating signal is PWM digitized.
> >
> > Best regards
> > Simon
> >
> >
> >> Op 3 januari 2016 om 0:00 schreef Richie Burnett
> >> <rburnett at richieburnett.co.uk>:
> >>
> >> How does it work then?
> >>
> >> -Richie,
> >>
> >> Sent from my Xperia SP on O2
> >>
> >> ---- Simon Brouwer wrote ----
> >>
> >> You guys are not discussing the modulator in the VSM201 anymore right?
> >> Because in that modulator, the control range is *not* determined by how
> >> small of a duty ratio the PWM signal can get.
> >>
> >> Best regards
> >> Simon
> >>
> >>
> >> > Op 2 januari 2016 om 23:10 schreef Tom Wiltshire <tom at electricdruid.net>:
> >> >
> >> >
> >> > Ok, maybe -80dB is *really really* impossible! My judgement was
> >> > "certainly tough, but probably not impossible", but I'm very willing to
> >> > go with "extremely difficult bordering impossible" or "totally
> >> > impossible" if you feel that's a more accurate evaluation! Certainly a
> >> > 50KHz carrier isn't that high, so I'd probably want to cut your timings
> >> > in half (e.g. 100KHz carrier), which makes it even more difficult.
> >> > Certainly we need significantly sub-nanosecond switching times to get
> >> > good results.
> >> >
> >> > The point was just that -80dB isn't that good for a modern VCA, and it's
> >> > pretty much out of reach for PWM VCAs. Lots of Blackmer-cell-based
> >> > designs do much better than that, and even 13600's can probably pull
> >> > -80dB out of the hat (
> >> > http://hem.bredband.net/bersyn/VCA/vca_shootout.htm )
> >> >
> >> > Tom
> >> >
> >> >
> >> > On 2 Jan 2016, at 21:12, Richie Burnett <rburnett at richieburnett.co.uk>
> >> > wrote:
> >> >
> >> > > With carrier frequency of 50kHz, 0.01% duty ratio (for 80dB
> >> > > attenuation) represents a pulse width of just 2ns! That's getting near
> >> > > the sort of time mismatch you can get in turn-on and turn-off times for
> >> > > the switches. So the switch might not turn on at all, or might stay on
> >> > > twice as long!
> >> > >
> >> > > -Richie,
> >> > >
> >> > > Sent from my Xperia SP on O2
> >> > >
> >> > > ---- Tom Wiltshire wrote ----
> >> > >
> >> > >> +1 totally agree
> >> > >>
> >> > >> If you can produce a 1% pulse width, you still only get -40dB. You
> >> > >> need to get a 0.01% pulse wave to get -80dB. Tough to do. Not
> >> > >> impossible, but awkward enough to make it stop seeming like such a
> >> > >> great solution.
> >> > >>
> >> > >> I've thought about this a bit because of using the PIC's PWM module so
> >> > >> much. The best case output from that is either 8-bit or 10-bit, which
> >> > >> means that -60dB is about as good as I'd get using it for a VCA, and
> >> > >> that implies having a switching frequency which is much too low
> >> > >> (31KHz) for many jobs.
> >> > >>
> >> > >> Tom
> >> > >>
> >> > >>
> >> > >> On 2 Jan 2016, at 19:42, rburnett at richieburnett.co.uk wrote:
> >> > >>
> >> > >>> <snip>
> >> > >>> Control range of PWM'd CMOS switches acting as VCAs isn't that great
> >> > >>> though.
> >> > >>>
> >> > >>> -Richie,
> >> > >>>
> >> > >>
> >> >
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