[sdiy] Digital accumulator VCO core?

Mike Bryant mbryant at futurehorizons.com
Fri Feb 12 13:12:53 CET 2021


Sorry yes I wasn't clear.  No not using a perfect mathematical ramp but using a table lookup of a sum of sinewaves or some other band-limited waveform, but then filtering off higher frequencies when playing higher notes on the keyboard.  There still be some aliasing but I've never found it at audible levels.

-----Original Message-----
From: Richie Burnett [mailto:rburnett at richieburnett.co.uk] 
Sent: 12 February 2021 11:56
To: Mike Bryant; Brian Willoughby; cheater cheater
Cc: SYNTH DIY
Subject: Re: [sdiy] Digital accumulator VCO core?

> I'm also not sure why people use BLEP or other techniques - just use 
> the accumulators to create a perfect sawtooth, square, triangle or 
> indeed noise...

I'm not sure what you mean here?  Generating a perfect mathematical ramp for the sawtooth using an accumulator?  Or are you meaning generate a sawtooth through the process of additive synthesis?

>...and then put it through a simple digital filter before the DAC to 
>get rid of anything that could alias.

If you are generating a mathematical ramp with reset (like what you get from the phase-accumulator in an NCO) then this has an infinite spectrum, and those frequency components that "could alias", have *already* aliased.  They are baked into the signal as soon as you synthesise it using such a basic method.  You can't just filter them out before feeding the signal to the DAC!  In practice you need to synthesise a waveform that is mathematically not a trivial ramp, but instead is the shape of a sawtooth that is band-limited.  This is what BLIT, BLEP, minBLEP, Poly-BLEP all do to various degrees and in different ways.

> If you run at 96kHz sampling this becomes pretty trivial in 
> computational terms - a few shift rights or multiplies and some adds.

If you generate an "A" at 3.52 kHz, you can only include the harmonics up to the 13th harmonic (45.76 kHz) if you don't want anything to alias.  If you don't mind harmonics aliasing in the ultra-sound region where you can't hear them you could include all the harmonics up to the 21st harmonic (73.92 kHz).  The the next harmonic is the 22nd that would be at 77.44 kHz but would alias back to 18.56 kHz and be audible with a 96 kHz sample rate.  In practice you'd probably stop at the 5th harmonic (17.6 kHz) with additive synthesis because anything over 20kHz is inaudible anyway.

Regardless of whether your synthesis is via additive, wavetable or a simple phase accumulator you still need to curtail the otherwise infinite spectrum of the basic waveforms (sawtooth, Triangle, Square, Pulse, etc) in order to prevent aliasing.

-Richie, 


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