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

Tom Wiltshire tom at electricdruid.net
Mon Feb 10 11:56:03 CET 2014


On 10 Feb 2014, at 10:23, cheater00 . <cheater00 at gmail.com> wrote:

> On Mon, Feb 10, 2014 at 2:55 AM, Andrew Simper <andy at cytomic.com> wrote:
>> On 10 February 2014 04:34, <rsdio at sounds.wa.com> wrote:
>> 
>>> One thing's for sure: if you draw simple waveforms like square, ramp, etc., then you'll have awful aliasing.
>> 
>> If you output the simple waveform at a fraction of the DACs sample
>> rate sure, but if you remain at an integer multiple of the DACs sample
>> rate the "awful aliasing" will remain locked on top of the harmonics
>> to change their amplitude but otherwise not be audible as inharmonic
>> tones.
>> 
>> Andy
> 
> And if the sample rate is not in a harmonic ratio to the oscillator
> pitch, then the aliases will be non-harmonic, but they will have a
> precise, constant spot in the spectrum, just like in any acoustical
> instrument - stringed, brass, etc. They become part of the timbre as
> long as the ratio stays precisely constant (and it does).

That could be useful, but "precisely constant" isn't quite enough. 

As Roman mentioned, this type of "wavetable" technique has quite a distinctive sound, probably due to everything being locked into tight harmonics. If you use more than 32 samples, you can get past "combo organ", but I still know what he means. Those combo organs used frequency division to get harmonics that were locked together, and this technique has the same fault.

However, you're going to have to be careful - if you've got complex non-integer ratios, the imaged frequencies won't fall neatly into the same sample every time, and you're back to having jitter in your signal. Think about why you don't get that when everything is harmonic.

T.



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