[sdiy] VSM201 Vocorder Question

[Simon Brouwer]

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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