Aliasing in synths. How to prevent it?

[EnGee]

Hi there,

I was reading the thread about a synth which has aliasing according to many users. I don't know exactly about that because I don't own that synth. I didn't post in that thread as it seems getting 'trouble' somehow. So, I created this thread trying to have 'solutions' if there are possible ones.

I have been reading about aliasing in synths. What I understood is high frequency signals are 'returned' as hitting a wall or mirror to make some artificial noise. I don't know how right I understood it.

I can understand that this is not a problem for sounds that doesn't have high frequency signals, but some programmed sounds required it (like some weird atmosphere or whatever).

I'm using usually 44.1 khz sample rate with 24bit audio interface (but 32bit internally in the host) in my projects but I intend to try 44.1 with 16bit audio interface when I receive my usb mixer because I doubt that there will be a difference according to one article I read few days ago. Anyway, this is for another thread.

What I wish from users or developers that know about aliasing, answers for some questions:

1. Can I prevent the aliasing by EQing the very high frequencies? like cutting above 20khz? Or is the aliasing has no relation to this?

2. To prevent aliasing, is it just depending on the developer/manufacturer of that synth to solve? by doubling or more the sample rate? Or are there other solutions?

Thanks for participating

[Urs]

1. No, not really. A frequency that's just about an octave above 22khz will end up as a bass frequency. For a high pitched note at, say, 1.4 kHz this is just the 5th overtone. For a sawtooth or square oscillator this is only about 15dB softer than the fundamental.

To hear the effect, just slowly pitchbend a very high note an octave up. You'll hear frequencies marching into the opposite direction through your full spectrum. You can't EQ that out.

2. Yes. Using a higher samplerate internally (oversampling) is probably the most powerful startegy to prevent aliasing inside the synth. It does however cost more CPU than not oversampling.

There are other startegies that developers can deploy, but these usually mean restrictions to sonic capabilities.

Cheers,

- Urs

[DJ Warmonger]

No, not really. A frequency that's just about an octave above 22khz will end up as a bass frequency

Uhm, nope, it will reflect just around 22 kHz mark. So 23 kHz will become ~21 kHz and so on.

For a high pitched note at, say, 1.4 kHz this is just the 5th overtone

5 * 1,4 = 7, as far as I know. Friday night?

Can I prevent the aliasing by EQing the very high frequencies?

No. Once aliasing occurs, mentioned freqeuncies of 23 kHZ and 21 kHz become one. You need to remove aliasing components at oversampled frequency before folding back to 44,1 kHz.

[aciddose]

No, not really. A frequency that's just about an octave above 22khz will end up as a bass frequency

Uhm, nope, it will reflect just around 22 kHz mark. So 23 kHz will become ~21 kHz and so on.

This is what I think he said. He said if you are a single octave above 22 (44) it will become "a bass frequency", actually zero. Yet he used the qualifier "just about" which I think he intended to say "not quite" or "less than".

So yes, what he said is 100% accurate.

If nyquist = 22k, 43.95k = 50hz alias.

[Urs]

No, not really. A frequency that's just about an octave above 22khz will end up as a bass frequency

Uhm, nope, it will reflect just around 22 kHz mark. So 23 kHz will become ~21 kHz and so on.

"A frequency that's just about an octave above 22khz" would be at 44kHz, which will result in 100Hz

5 * 1,4 = 7, as far as I know. Friday night?

oops, indeed

[EnGee]

Thank you for the answers

Do the answers apply on analog synths or just on digital ones (include VAs and soft synths)?

[Urs]

Thank you for the answers

Do the answers apply on analog synths or just on digital ones (include VAs and soft synths)?

Analogue synths can not alias. Only digital synths (including digital emulations of analogue synths) can.

[dave dove]

@ Urs
got there before me...

dave

[aciddose]

Thank you for the answers

Do the answers apply on analog synths or just on digital ones (include VAs and soft synths)?

Analogue synths can not alias. Only digital synths (including digital emulations of analogue synths) can.

Well, with the exception of an analog sample&hold. Aliases are generated in this case.

[EnGee]

Thank you for the answers

Do the answers apply on analog synths or just on digital ones (include VAs and soft synths)?

Analogue synths can not alias. Only digital synths (including digital emulations of analogue synths) can.

Well, with the exception of an analog sample&hold. Aliases are generated in this case.

Oh, why sample and hold only? Do you mean by the lfo kind?

[wagtunes]

Thank you for the answers

Do the answers apply on analog synths or just on digital ones (include VAs and soft synths)?

Analogue synths can not alias. Only digital synths (including digital emulations of analogue synths) can.

Glad you responded. I have a question. I really don't understand all this technical stuff. I try to stay away from it and just make music, but something has been bugging me and maybe you can help explain what's happening.

I recently purchased your Diva synth. Love it. Can't use it in Devine mode but at draft it sounds great.

However, one patch I made, when I play in the upper frequencies, I hear like a hissing sound in the background. Now from what I'm reading in this thread, this isn't aliasing. So exactly what is that? Is it something to do with how I have my filters setup? It's the only thing I can think of that would cause this to happen. There is no noise oscillator in the patch so that's not causing it.

Wondering if you or somebody else can offer some insight into this.

[chk071]

Is there any free VA synth which aliases (badly) so one can audition that phenomenon? I never really noticed aliasing anywhere, could be just the sounds i make, but i guess i never went that high to really be able to notice it.

[ZentralmassivSound]

Interesting topic!!

@chk071: any synth aliases if you use it at 48k without oversampling and apply some distortion (assumed the distortion effect has no internal oversampling). You can hear it in my example below.

Also a question on my side here:

Simple saw wave, high pitch, played as single note and small second. One time clean, and one time with distortion effect applied.

I hear aliasing artefacts from the distortion that even reach below the note I played.

But what I also hear when I turn the volume up quite a bit, when playing the small second, there is a pretty low bass frequency, even without the distortion. It's not there in the analyzer, so I assume this is a psychoacoustic phenomenon, or maybe even a "mechanical" issue of the hearing apparatus, like something resonating or so? Anybody else hears this? I always thought this is also aliasing but it can't be.

(careful! high pitch, start with low volume)
https://dl.dropboxusercontent.com/u/334 ... gTestC.mp3

[chk071]

Thanks, that's very helpful! Will look out for stuff like that then.

[ZentralmassivSound]

two more examples, direct comparison of a high distorted saw note at 48k and 96k:
https://dl.dropboxusercontent.com/u/334 ... sing48.mp3
https://dl.dropboxusercontent.com/u/334 ... sing96.mp3