JH's VCO, wavetable source

[Grant Richter]

> 
> > I have to point out that the most elegant solution to
> > wavetable generators is an analog to digital converter.
> 
> ...I don't think so. 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.

> 
> > A high frequency VCO clocking a counter, has no
> > advantages over an analog to digital converter
> > used as an address generator.
> 
> I would say there are some subtle pros and cons. First
> of all, ADC is prone to quantization errors which accumulate
> with the nonlinearity errors and scale mismatch of the input
> saw waveform. Moreover, I think that the requirement of a
> precise and sufficiently fast ADC makes this approach more 
> expensive one.

Yes, by a few dollars.
>  
> > 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.

>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.

> 
> BTW, there might be a piece of RAM instead of the EPROM and
> a local microcontroller responsible to fill up the RAM with the
> transfer curve (my idea for a module - put some pots and an algorithm
> within a microcontroller generates a curve according to the position
> of the pots. For example some polynomial stuff.) You are not limited
> to fixed wavetables in this way.
> 

Standard EPROM emulators are also useful for this. Check
http://www.tech-tools.com/er2.htm
for an example.