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

[Mattias Rickardsson]

On 9 February 2014 18:08, cheater00 . <cheater00 at gmail.com> wrote:
> 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.

Multiple oscillators per voice is nice, at least so I've heard. ;-)
Could this be incorporated without having to duplicate everything?

And how about other behaviours that makes oscillators have tricky
frequencies... like PWM, where the oscillator is still but the
percieved pitch varies due to the modulation. How does the harmonic
aliasing behave there? Is all still well?

/mr


On 9 February 2014 18:08, 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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