Digital Audio 101: Aliasing Explained

An introduction to the concept of aliasing in digital audio including what it is and what it sounds like.

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Aliasing is one of the more complex concepts of digital audio.

Luckily, most audio engineers can spend their days being creative rather than having to worry about it.

Nonetheless, understanding aliasing actually helps explain a lot about how digital works.

*Aliasing 101*

When a signal is sampled, it is inherently band-limited in frequency.

In other words, when a signal is sampled by a finite number of points, it cannot represent an infinite range of frequencies.

A conventional D-to-A converter for audio will only create signals within a specific frequency range that is determined by the sampling rate.

If there are any recorded frequencies outside of this range, they are interpreted by the converter and mapped to frequencies within this range. This is aliasing — when one frequency is coded as a different frequency.

*Nyquist Frequency*

The sampling rate determines this frequency range because it sets the Nyquist frequency.

The Nyquist frequency is the maximum frequency that can be recorded by a specific sampling rate. The Nyquist frequency is half of the sampling rate.

When it comes to audio recording, if the sampling rate is 48,000 samples per second, the Nyquist frequency is 24,000 Hz. If the sampling rate is 44,100 samples per second, the Nyquist frequency is 22,050 Hz.

If a signal contains any frequencies greater than the Nyquist frequency, they are interpreted by the converter and mapped to frequencies less than the Nyquist frequency.

*Anti-Aliasing Filter*

Aliasing would be a big problem for digital audio, because it is usually not desired for frequencies to change in a signal. The good thing is that there is a dedicated component to prevent aliasing as part of the analog-to-digital and digital-to-analog conversion process. This component is called an anti-aliasing filter.

Conceptually, the anti-aliasing filter blocks frequencies above the Nyquist frequency from being converted. This is going to prevent any signals from changing frequency during the conversion process.

[HOLY_BATH]

I have tried to understand aliasing and anti aliasing for years and i think i finally get it now thanks to this video you are the goat

[jndominica]

I found this kind of aliasing kinda of pleasing when playing computer games on sound cards from the 1990s. Then XP came about and did mixing in software with reasonable quality, and suddenly all 16-22k samples sounded dinstinctly muffled. If I don't have a mental picture of the correct sound of a piano, the samples a synthetic or a mix of different sounds, the extra fuzziness seems to belong in the signal.

[leviathon]

Really good video. You can gain a lot of understanding about aliasing just from reading about the principle, but it helps to actually see it in action.

[aura-audio]

This video is a great supplement to my EE courses! The visual demo with Pro Tools was very helpful for me since I’m a musician.

[CryptoHive]

Great explanation about phaser vs flanger

[blueslsd]

Superb makes sense of why we use 48k in Mixbus or Ardour Nyquist at 24k then limit to 20hz and 20khz. Many thanks.

[thirdtribestudio]

Really nice explanation, I've had issues with this concept for years!

[christopherharv]

Why are samples-per-second measured in Hz? Hz are cycles-per-second.
Does this relate to how, in digital systems, you need at least two samples per cycle/frequency (one positive, one negative) for the frequency to be reproduced? and that's why the sample-rate needs to be twice the highest frequency you wish to record? Because if the system is able to take one sample per cycle of 48kHz, its sample-rate is therefore fast enough to take 2 samples of 24kHz?

[thedavydark]

Great explanation and demonstration. Thank you! 🙏

[johndc]

Thank you. I have watched tons of nyquist video back and forth multiple times already and this last video I watched just made it more clearer. I was so confused what aliasing does and if I am not wrong, basically it bounces off frequencies over the nyquist range back to the frequency area where it's within the nyquist.

These aliasing creates some unwanted harmonics which most of the time subtle but can slowly pile up and accumulate into a much noticeable foreign sound for the mix.

[dulmin_]

Thank you sir!!! This was wonderful...

[BryanChiMusic]

wow! That's so weird, need some time to wrap my head around that...

[izvarzone]

Is there way to prevent aliasing in plugins that have no oversampling?

[phisofied]

So mixing at higher sample rates should actually be beneficial against aliasing should there be sounds that have frequency content that go beyond 1/2 the sample rate. So aliasing can occur from audio that is made at 96kHz or even 192kHz and then resampled to 44.1kHz if no high cutting is done. All the frequency content beyond the 22.05kHz section will just be reproduced as aliasing. I once exported audio at 96kHz and then attempted to mix it at 44.1kHz. Some of the samples changed in pitch. They had frequency content beyond 22.05kHz & I hadn't cut them. I now know that this was aliasing that caused the perceived change in pitch.

[BojanBojovic]

Is it a big deal that all Waves plugins do not oversample even it is 2022 now?
I do not hear the difference although it is there when I invert the phase, is it just our fixation and ocd most of the time?

[musicalsoul]

Why in case of piano, we didn't get mirrored image of frequencies above Nyquist limit?

[marceloribeirosimoes]

Os this bad for our mixing?
I think it is, but how to avoid it?

[hugo_hoving]

Does anyone know if there's any loss in quality when 44100 Hz FLAC is converted to 48000 Hz ALAC?
I've been making this conversion with MediaHuman Audio Converter for hundreds of files lately, but never noticed the difference in sampling rate between what I put in and what I got out.

[smrts]

is this how audio can end up with something that sounds like chirping heard from underwater?

[zynthos]

Why can't you see the 8k Hz signal?