[sdiy] VSM201 Vocorder Question

Sun Jan 3 18:02:56 CET 2016

Hi Tom,

As you can see in the schematics, the pulse width modulator in the VSM201
consists of a comparator with the input signal on one input and a high frequency
triangle waveform on the other input.
Say the triangle waveform varies between -1V and +1V, then an input voltage of x
volts will result in a pulse width of (50 + 50x) %. As long as the input voltage
is kept between -0.98V and +0.98V the pulse width will vary between 1% and 99 %
which, as you say, is easily achievable.
There is no reason why the above relation would not hold for arbitrarily small
input voltages; for instance, an input voltage that varies between -0.00000001V
and 0.00000001V results in pulse widths ranging between 49.9999995 % and
50.0000005 %.

In short, this way of PWM encoding the audio does not limit its dynamic range
*at all*.

Best regards
Simon

> Op 3 januari 2016 om 15:59 schreef Tom Wiltshire <tom at electricdruid.net>:
>
>
> Hi Simon,
>
> Well, it doesn't "quantise" it exactly, but the level of the signal at any
> point in time is represented by the width of the pulse. So the dynamic range
> you can represent depends on the range of pulse sizes you can produce.
> Producing extremely narrow pulses at close to 99.9% and 0.1% is difficult at
> the frequencies we're talking about because as Richie pointed out, the
> rise/fall times need to be extremely short.
> If you limit yourself to something easily achievable, like pulses down to 1%,
> the shortest pulse is 100 times shorter than the longest pulse. That
> represents -40dB, which isn't a huge amount of range.
>
> Regards,
> Tom
>
> On 3 Jan 2016, at 13:08, Simon Brouwer <simon.o at brousant.nl> wrote:
>
> > 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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