[sdiy] [synth-diy] numerically controlled superoscillator without hard sync

[Jim Credland]

I spent some time creating tones in csound with additional aliased overtones (i just added a sine wave and calculated the frequency) - it sounded great - though that approach wouldn't have worked with complex waveforms very easily.

Also, the Tip Top Audio Z-DSP lets you do this type of thing.  You can clock the DSP chip with another oscillator.   I've not tried driving it with a melody yet though - that might be pretty interesting.

I'm going to try it right now!

On 9 Feb 2014, at 17:08, cheater00 . <cheater00 at gmail.com> wrote:

> Hi,
> 
> thinking about this recently, one could use a high-frequency
> oscillator, and use it as the clock for a microcontroller which
> assuredly outputs a single sample every n clocks. This way you can
> perform VA style synthesis where the time base is derived from an
> analog oscillator that can perform FM and other neat analog tricks. On
> the other hand the uC only has to synthesize at a single frequency. So
> say it's always synthesizing at A440, and you change the pitch by
> changing how fast it executes its algorithm, i.e. by changing the
> clock, i.e. by changing the sampling frequency. This way, externally
> (i.e. at the output of the DAC) the aliases are always precisely at
> the same position in relation to the base frequency of the note being
> played. The aliases become part of the timbre, enriching it.


> This is
> in contrast to aliasing in a system where the sampling frequency is
> fixed, and the pitch of the note being synthesized is changed in the
> usual way. In this case, which doesn't happen here, the aliases are at
> a different position for every pitch, and therefore the oscillator not
> only plays your desired melody, but also a quieter, undesired melody
> which is completely out of tune and works against your music.
> 
> The variable sample rate oscillator could also contain things like
> filters and other stuff. A whole VA could be implemented, and aliasing
> is not an issue at all.
> 
> The only thing you need to ensure is that your reconstruction filter
> works well with a variable sample rate.
> 
> Cheers,
> D.
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