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

Roman Sowa modular at go2.pl
Sun Feb 9 22:29:22 CET 2014


That needs so high frequency VCO, that every typical VCO topologies we 
know would be just useless. You need RF stuff, and that rarely has wider 
range than 2:1, I mean one octave.

But, if you take VCO saw wave at audio frequency, and add a nifty 
circuit that generates a pulse everytime voltage rises by, say, 20mV, 
then we get about 250 times higher frequency from 5V saw, that can be 
used as DSP clock. And that's entire wave cycle, not much to calculate 
anything. Say 10 operations per sample, 25 samples per period. Pretty lo-fi.
I wouldn't dare to use higher frequency VCO to increase CLK count per 
output wave cycle, because there will be inevitable nonlinearities at 
the sawtooth edges, and that will lead to jumping DSP clock frequency 
that will be audible.

We also need to consider that constant number of samples leads to 
unnecessary massive amount of calculations and samples at higher end, 
and surely too low sample count at lower frequencies, which requires 
filtering down to very low frequencies. Audio would be very limited if 
we don't want to hear the sampling clock, and IMHO it's not a nice thing 
to hear on any sound.

just my PLN 0.03

Roman

W dniu 2014-02-09 18:08, cheater00 . pisze:
> 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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