[sdiy] VSM201 Vocorder Question

Magnus Danielson magnus at rubidium.dyndns.org
Sun Jan 3 15:54:46 CET 2016 

Simon,

The dynamic range is the maximum signal to noise ratio (to explain it 
simple). The PWM control is not what I really would trust to have very 
big dynamic range.

Vocoders have a strong tendency to leak between the bands, depending on 
how good the filters are. In order to make sure it works well, you need 
to make sure the signal is strong enough so that the noise-floor is way 
below. Also, since you have quite a bit of processing, noise easily adds 
up in a vocoder.

The original vocoder, the SIGSALY, had really bad quantization, as each 
of the bands was quantized to one of six levels. It sounded awful. The 
NSA contact said something in the line of speaking with the voice of 
Donald Duck with a bucket over the head and in a small box. That 
describes the sound pretty well actually. You would be able to loose a 
VSM201 vocoder in the SIGSALY setup, it is that large. The pitch 
required much higher resolution, so it actually uses two 6-level 
channels, allowing for the pitch to have a whooping 36 levels or 
frequencies.

Compared to the AC-3 voice scrambler, which was actually broken by a 
german graphical spectrum analyzer (essentially swapping three pieces of
the spectrum around in a fixed pattern). The AC-3 voice scrambler is 
patchable on many modulars these days...

Cheers,
Magnus

On 01/03/2016 02:08 PM, Simon Brouwer 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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