JH's VCO, wavetable source

[mbartkow at ET.PUT.Poznan.PL]

Grant,

> > [...] I personally don't like the idea of 
> > converting an analogue signal into digital domain and
> > back.
> 
> Well, using a counter is certainly converting it into the digital
> domain. Using a waveform stored in EPROM is certainly
> digital domain.

Not quite the same kind of conversion, I would say. A counter gives
you some advantages over ADC. First of all, there is a catch related 
to the falling edge of the saw fed to the converter. Since the falling
time is allways greater than zero, a fast ADC also samples the one, and
addresses some possibly random samples from the wavetable. Assume
you want to generate a smooth waveform (e.g. a sine wave). Due to
the random samples drawn from the wavetable during the backward scan,
some nasty glitches occur and ruin the signal smoothness.

A fast running VCO followed by a counter may be considered as a 
phase unwraper of the cyclic saw wave with the purpose of creating
a phase reference for the sampled waveform. 
In this case there is nothing like going back in time, as in case
of ADC converting of nonideal saw.

> > > The use of the ADC gets you two additional operating modes,
> > > namely arbitrary scale quantization and non-linear synthesis
> > > (we recently hand traced the transfer functions of the Serge
> > > Wave Multipliers into a wavetable, sounds the same but with
> > > eight bit grunge).
> > > 
> > > Additionally the software to generate the 256 sample wavetables is
> > > available for download with a shareware price of a measly $20.
> > > http://www.wiard.com
> > 
> > These features are related to the wavetable idea, not to the use 
> > of ADC. I cannot see, why quantization could not be berformed by 
> > dropping some bits at the wavetable output. 
> 
> An arbitrary scale quantizer is a voltage input/voltage output
> device where an input voltage is quantized to a musical scale.
> This is a special non-linear transfer curve, where the outputs
> are all multiples of 0.083 volts. The output voltages are then
> "notes" in a 1 volt per octave system. An example would be
> for a 0-10 volt input, generate a five octave major scale.
> Bank 16 of new.256 contains sixteen quantizations
> including semitone, major, minor, pentatonic and whole
> tone scales along with various chords.

I agree, this quantizer stuff is interesting and potentially very usefull
in a modular system. I don't consider this as an argument for ADC-based
wavetable generator, since I only need an audio oscilator with a vast 
palette of waveforms and nothing more.


> >OTOH, nonlinear synthesis 
> > requires extremely fast (and expensive) ADC (maybe a flash one)
> > to instantly deal with feedbacks one might patch.
> 
> The National Semiconductor ADC0820 can be clocked at 400kHz
> and costs US$4.50 from Digikey.

OK, quite impressive. Anyway, it is s*mpl*ng which I don't love.
Continous tracking conversion would be better.

regards,

mb


--

Maciej Bartkowiak, PhD
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