[sdiy] Alias staging - folded-spectrum VCO - core running purely in the aliasing domain

cheater cheater cheater00social at gmail.com
Mon Feb 15 21:12:48 CET 2021


Hi all, it occured to me that with a VCO that runs purely above audio
frequencies, you could sample parts of it at different frequencies.
I.e. if you have an oscillator running at 10 Hz (for simplicity) with
an analog bandwidth that goes up to say 10 MHz, you could perform
something I would call "alias staging".

The first step is preparation - you have to get rid of everything in
the audio domain, so you put a steep high pass filter at 100 kHz (for
simplicity).

Next up we perform multiple stages of aliasing by resampling without
band-limiting or reconstruction filters.

So first you resample it at a frequency that's a little lower than 10
MHz. You carefully calculate the frequency at which you run the
sampling, so that the aliases end up in the audio band. You can
control the precise min and max frequency between which the aliases
fall. You are left with a signal that has a little stuff in the audio
band, and a stuff in the above-audio band. Some of the aliases will
add to the above-audio band.

Then you repeat, cutting off another sliver and packing it in the
audio domain. Every time you finish a stage, the above-audio band will
have a richer and richer spectrum.

You can make the aliases wrap around an odd amount of times, making
them inharmonic, or an even amount of times, making them would-be
harmonic (but they won't be, due to aliases from previous stages). Or
you can do both.

If you perform the math correctly, you can make the first stage of
aliases cancel out some of the stuff you have in the above-audio
spectrum.

This all depends on being able to run the ADC at a rate that's
synchronous to the oscillator. Or alternatively, oversampling by a
lot, but that won't have result that's just as good.

I wonder if anyone ever tried a design that runs a VCO purely in the
aliasing domain, where the aliases are the only thing you hear in its
output.

Just a thing I thought I'd share.



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