Definition of audio artifact

[MedusaRecoil]

Hello to all. Let me begin by first saying thank you to all who have posted here, that I have learned from. I have been a hobbyist sound designer for over a decade and really enjoy the process of figuring out how to bring the sounds in my mind to life via analog and VA synths. It is a lot of fun for me. My recent purchase of a Virus KB led me here.

Having said that, I have noticed a few things along the way. Differences in sonic limitation (forgive me if this topic hasbeen beaten to death on the boards). My main discrepency is what happens to the sound when you tune an oscillator, and a waveform rich in harmonics, up past what some would consider "non-musical" values. I noticed that when I tune a square wave up and play higher notes using the KB's keyboard, that I hear these little artifacts in the audio. Like unwanted harmonics. I was wondering if this is what happens on all analog style keyboards, or just VA keyboards. Once upon a time I had a SC Pro One, but never thought to check the highest freqs for these things I am trying to describe.

I only notice this while using say, a square or saw waveform. I guess the best way to hear this is to set an LFO (sine) to modulate a Square osc. Value to 127, but real slow. Then you will clearly be able to hear this audio artifact at the pinnacle of the LFO's cycle. The purity of the waveform seems to come apart. Is this the waveform sample starting to show that it is indeed a sample? Is this what is refered to as 'digital aliasing'?

Other than this, the KB seems to excel at everything I push it to do. I know the filter doesnt self oscillate. I know it is digital. But the Kb bass seems to crush an acquaintance of mine's Moog. I have done a lot of "A/B-ing" with my Nordlead 2 that I've owned for 12 years, the nord seems to display less of this artifact I speak of with the KB, in those high freqs.

Anyway, I apologize for this long winded intro and question. I just wanted to give a thorough explanation. Thanks in advance

MR

[ian]

Those are called 'alias tones'. They exist to different degrees in most digital synths, with the only exception I know being the Nord Wave. They do not exist in analog synthesis.

An alias tone happens when a converter 'hears' or a synthesizer generates tones that go above the highest frequency that can be captured at a given sample rate (with the Virus this is either 22khz or 24hkz depending on internal clock).

According to the Nyquist theorem, it takes two samples in order to reproduce a given frequency, one for the peak and trough of the sound wave. This is why a CD quality recording with a sample rate of 44.1khz can record frequencies up to around 22khz.

When a frequency is generated that goes above half the sampling rate, there are not enough sample points to plot the frequency. The DSP or converter then looks to the next sample point and sees a tone at half the frequency of the one sounded. A harmonic of 25kz is seen as 12.5khz by the processor or converted. Thus, an artifact is generated in the audible realm.

Anyhow, when you play notes higher up on the keyboard, the upper harmonics they create go above half the sampling rate and then you hear one or more alias tones.

You can hear this effect in a dramatic way by using using the Rate Reducer distortion. This cuts the sample rate creating very pronounced alias tones that go lower in pitch as the rate is reduced. Same principle at work.

As I understand it, the Virus series were made to have dark sounding oscillators with fewer upper harmonics than many other synths in order to minimize aliasing. Many synths also have special algorithms to minimizing the alias effect, which is why they are more prevalent in older digital synths than newer ones.

[feedingear]

Great answer. I love rate reduction on the virus both in the filter and distortion stages, but didnt fully understand the sound science behind it.

[MedusaRecoil]

Thank you for the very thorough and thought out answer. It answered every aspect of my question. I have wondered about this for 10 years. Thank you!

When you say the Virus was supposedly made with "darker" sounding oscillators, I believe it. It is an absolutely wonderful sounding synth in the low to mid ranges.

Did the Virus ever smooth out the aliasing issue within the new TI2's sound generation? I've read that they've emulated a self-oscillating Moog style filter. I'll need to look closely at the nord wave now. Thanks for the tip.

I actually just bought and returned an M-Audio VENOM for the similar reasons. It has a filter overdrive algorithm that generates a self oscillation effect. Only thing is that while in mono, the sounds tend to overlap eachother. So you dont get that clean crisp start at the beginning of each new sound generated. That was irritating... That, coupled with the extreme "aliasing" forced me to return back to Guitar Center. Other than this, it was a decent sounding synth with a cool CPU editor. But, without the Venom's sound completely drenched in the onboard effects, I felt it sounded very ordinary

-MR

[Timo]

Great explanation Ian.

Originally Posted by MedusaRecoil

Did the Virus ever smooth out the aliasing issue within the new TI2's sound generation?

I believe they kept the original 'classic' oscillators intentionally for legacy reasons.

However it has been said the wavetable oscillators are alias free, and the Hypersaw (with one saw used, no detune) is considerably cleaner and brighter than the stock classic saw, so you can still choose which one you want between the newer ones or the classic/legacy oscillators.

The D/A converters on the TI are also 192KHz/24-bit on the analogue outs. (USB is restricted to 44.1KHz/16-bit.)

Only thing is that while in mono, the sounds tend to overlap eachother. So you dont get that clean crisp start at the beginning of each new sound generated. That was irritating...

Phased? Like starting the waveforms both at the same phase each time? (Producing a laser-like "pow, pow, pow" type sound every time you press a key if detuned?)

[MedusaRecoil]

If that is what phasing is, then yes. The VENOM synth. While in "mono", I reduced it down to 1 osc (TB Square). Drew out the sustain so that the sound would still be audible during the next key strike. The sustaining sound would (for a milisecond) over lap the begining attack of the next triggered sound, so that the attack was muddied a little. Myself, I enjoy creating attacky percussive sounds (which the Virus does very well) so I found this trait in the VENOM to be less than desirable. Which is too bad because the inexpensive little synth has a great sounding engine and emulates analog well. Also on the VENOM, the LFO's only seem to cycle at 2 octaves, limiting range considerably. No crazy modulations, or fast speeds. Envelopes are limited as well while making diving percusive sounds. No laser type dives allowed. Knob tweaking suffers from the same "stepping" as most other VA's

The Virus (while in mono) does not overlap, or muddy the beginning of the next triggered sound generated. Making for a crisp/punchy attack everytime. Especially when "punch" is dialed all the way up. Was wondering if the punch feature is some type of sampled click on the Virus. It is untunable.

All this aside, I really like the KB