[sdiy] VSM201 Vocorder Question

[Simon Brouwer]

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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