[sdiy] VSM201 Vocorder Question

Mon Jan 4 13:50:22 CET 2016

Hi Magnus,

Of course the dynamic range is limited by noise, and the PW modulator in the
VSM201 may not perform too well in that respect, but my point was that in the
VSM201 the dynamic range is *not* limited by an inability to achieve very narrow
pulse widths, as was put forward earlier in the discussion.

Best regards
Simon

> Op 4 januari 2016 om 8:19 schreef Magnus Danielson
> <magnus at rubidium.dyndns.org>:
>
>
> Hi Simon,
>
> There is this thing called noise. You are not considering the noise of
> the process. The comparator is a wonderful tool to convert additive
> noise to time and the sawtooth is a very good shape to do exactly that.
> The dynamic range relates to the difference in amplitude from noise
> floor to the peak amplitude. In this case, noise floor can consist of
> other unwanted signals too. So any noise in timing will limit things too.
>
> It might be good enough to achieve intelligible vocoding, but it may
> still be hard to get good results at all time. Experience say that we
> need companding system to compress the dynamic range before the vocoder
> and expand the dynamic range efter the vocoder. This is a trick to
> handle the low dynamic of the vocoder (it's also been used to compensate
> the low dynamics of tape recorders, with systems such as dBX and Dolby).
>
> Cheers,
> Magnus
>
> On 01/03/2016 06:02 PM, Simon Brouwer wrote:
> > 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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