Digital Synthesizers OSC Aliasing in (some) commercial VSTi (and gear)


#1

This is a little article (slighty modified) I wrote some months ago as a note on my personal facebook profile; thought it may be useful to musicians and sound engineers here.

Yesterday I was watching the ‘Elite Session with Steve Duda at Pyramind’, wich was published in July 2015. Very interesting video in many ways (and yes, Duda is the MAN. More on that later).
In the second part of it (you can watch here: https://youtu.be/BjvrHOARC00), Steve was talking about digital alias in digital oscillators.

Despite the fact that many of us knows the concept of aliasing in digital audio, it was unveiling on how it affects the signal when a digital OSC (wich may emulate, or not an anal0g one) is pushed to high frequencies.

He was showing how his OSC code operates opposite to Native Instruments’ Massive one (more on that later). This thing intrigued me, so i decided to put under test some of the commercial synths i own and comparing my results with the one Steve was showing in the Pyramind video.
Basically, digital alias in those cases produces some impurities that are ‘reverberated’ backwad to the lower frequencies. This effect may be desiderable, adding character to the sound of the oscillator. But, as a side effect, it may create harmonics that simply ‘shouldn’t be there’, and that subsequentially will impact on the mix of the track you’re working on. Those harmonics may be or not be audible and their presence don’t translate directly as ‘bad quality’ (as said, they could be desiderable).

Duda was showing frequencies generated using Ableton Live Spectrum (analyzer) playing a D5 over a pure saw waveform (on the OSC). As you may know, a saw waveform is the one containing most of the harmonics, so it was a fitting test.

I followed the same route, using the best quality options for the synths (where avaiable). Those are the results.
First of all, i started with the two Ableton Live built-in synths, Analog and Operator.

Ableton Live’s Analog:

Ableton Live’s Operator:

UVI Falcon (1.1.3)

TC Electronic Powercore 01

Reveal Sound Spire

KV331 Audio SynthMaster 2.8.1

reFX Vanguard 1.8.0

Novation VStation 2.3

And now for the ‘surprises’…

Access Virus|Powercore 2.0.5

Access Virus TI2 (OS 5.1.1)

Steve Duda was comparing Native Instruments Massive to Serum in his video; I don’t own any of them, so I took screenshoots from the Pyramind video:

Native Instuments Massive (version unknown)

And the winner is…

Xfer Records Serum (version unknown)

Frequency spectrum of Serum is simply impeccable. As far as i know, there’s no other digital synth around wich can boost this crystal clear, perfect graph.
Steve Duda said he hired a mathematician to come up with the DSP code behind Serum; well, I must say that money was very well spent.

Hope this post may be of some interest.

PS: Some weeks after posting this i bought a Serum licence. Sadly i still didn’t have the time to actually use it in any tracks I made afterward.

PPS: some weeks ago a very famous youtuber posted a video about the same topic, talking about Serum alone.


#2

Hi…what is studied is Aliasing is a different thing; frequencies exceeding Fs/2 reflect back into the audible range as inharmonic tones. In VAs this is typically countered by using either very carefully sampled waveforms or bandlimited steps. This is good since aliasing oscillators in VAs generally sounds . It has nothing to do with interpolation or imaging artifacts.