[sdiy] VSM201 Vocorder Question

[Tom Wiltshire]

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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