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

[Declare Update]

I have recently been pondering this same idea! I was thinking you'd have a pin change trigger an ISR, which just increments the index to a wave table. So you're driving vco would have to be the size of your array * desired frequency. Clocking the actual micro is an option i guess, but as Tom points out, that's pretty fast!

You mentioned doing a whole VA this way, would that be with separate processors for each unit? How would you go about this?


Sent from my iPhone

> On Feb 9, 2014, at 12:38 PM, Jim Credland <jim at cernproductions.com> wrote:
> 
> 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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