Around the forums-I've noticed many people talking about how softsynth x has a lot of 'aliasing'... so in terms of sound, what is this?

Also, I've seen some websites attempting to state this.. but all they show are some graphs and use jargon that i dont understand..

can anyone explain this in simple terms? or maybe point me in the right direction (in terms of easy to understand websites)

thanks all

Hi Vit,

hm... ok, I'll try to explain it in very simple words (I hope my english is good enough )

Aliasing is a special kind of 'distortion' or 'noise' that occurs as a side effect of digital audio processing.

I don't want to get too deep into it, but as you might know, digital recording and playback is done at a specific recording/playback frequency, called sample rate. The sample rate is more or less the speed at which the computer takes a snapshot of the audio when recording, or at which he outputs the snapshots to your speakers.

If you take CD audio for example, the computer takes 44100 snapshots of the audio per second to "sample it", or 1 snapshot (sample) every 1 / 44100th second.

But what does this mean for very high frequencies?

Well imagine a simple sine- waveform. The more snapshots of it you take, the more precise it will be reproduced when you play it back. But if the sine-wave would have a frequency of 22050 hertz for example, you would be able to take only 2 snapshots of it if you record with a sample rate of 44100 hertz. (Because your computer taking snapshots is only twice as fast as the waveform you want to record.)

The problem is, that your recording device of course cannot synchronize itself to the sine-wave you are recording, especially as most acoustic waveforms tend to not having an exact frequency.

As a result, you'll not get 2 samples of your sine-wave, and then again 2 samples and then again 2 samples, but instead you'd probably get something like this: 2, 1, 2, 1, 1, 2, 2, 2, 1,
and so on.

This means that a lot of random is introduced because your sine-wave cannot completely be reproduced at the sample rate.
This results in a lot of harmonic artifacts in the signal, throughout the frequency spectrum, which is called 'aliasing'.

Aliasing is inheritant to the principle of digital audio recording and playback. Because you only have a limited recording / playback frequency. You can only reduce it (by using higher and higher sample rates), but you can't avoid it.

If someone tries to sell you an effect or an instrument with "absolutely no aliasing", then just ignore him. That's just marketing-shit.

Normally, when you just playback sampled material, you won't hear these artifacts however. Thats because the artifacts are at a much lower level than the normal audio is.

Now, if an effect or instrument does many computations with the sampled material you can get noise artifacts that are a lot louder than aliasing is. The reason for this are mostly rounding errors of the complex computations the computer performs on the samples.

In fact, these noises introduced by rounding errors are not really "aliasing", it's a common misconception that these noises are referred to as "aliasing".

A good synth or effect plugin should therefore use a filter to reduce such noises, however there is no way of completely removing them without destroying the normal audio signal.

Hope this clarifies it a bit.

Best,
Lighthouse

[ 28 April 2002, 20:13: Message edited by: LighthouseAtDawn ]

hi lighthouse

uhmmm.. danke schön

I think I understand the theory behind aliasing now.. sample rate is directly linked to the _theoretical_ amount of aliasing in a sample ?

hmm however, do you know of any instruments that contain a high amount of aliasing versus ones that don't? ...im curious to hear what this sort of distortion sounds like...

I posted about this in the off topic section a few weeks ago (maybe a month or more) and asked if anyone could provide a sound example of it. And sure enough Rene from RGC was able to wip up an MP3 of it (note Rene did this soley as an educational example to us, it didn't come off of Pentagon or anything). Anyway you might still be able to find the thread if you do a search. As to what it sounds like, it produces crazy notes underneath the one you are playing that gives it a distorted telephone button type sound. Or if you ever ran a guitar through a ring mod. It's definately not something you want in a synth.

try Absynth, draw a crazy waveform to the oscillator and play it with some higher noes...

...and you will hear, what ALIASING is.