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

[cheater00 .]

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.