4. Understanding Aliasing - Digital Audio Fundamentals

In this video, we demistify and take a stab at understanding aliasing in audio signals. We start off the discussion by looking at how aliasing could occur in video signals by looking at the wagon wheel effect and how the aliasing pattern that you visually see can be applied to understand the concept of aliasing in audio signals. We force aliasing to occur in the digital domain by using the Nyquist programming language, draw a pattern, and derive a formula for predicting the aliased frequencies. We illustrate the process of sampling and see how a low sampling rate coupled with high frequencies can cause aliasing. We take the discussion towards real world observations of aliasing in square waves, and how to counter them manually using oversampling and downsampling.

Content:

0:00 Wagon wheel effect
2:08 Temporal aliasing
4:08 Forcing aliasing in Audacity
9:22 Sampling illustrated
11:24 Aliasing from square waves
13:56 Solutions to avoid aliasing

Tools:

This video series explains the fundamentals of digital audio, how audio signals are expressed in the digital domain, how they're converted and transformed and the advantages of working with digital signals.

If you've got any questions, suggestions or recommendations, type them out here, or send me a message on any of my social channels mentioned below.

Give me a shout here:

[akashmurthy]

I've noticed that some of you trying to replicate the audacity examples aren't getting any aliasing. Audacity is creating a 3500Hz tone instead of 7000Hz, and the debug output of nyquist says: "Warning: osc frequency reduced by 1 octaves from 7000 to 3500 hz to avoid aliasing." I've noticed the same behavior when I upgraded my version of Audacity to 2.4.2 and above.


There must've been additional checks put in the code of hzosc. But if you follow the later example: return hzosc(ramp(1) * 7000)
This works as expected! Aliasing does occur. So I am guessing there is a simple check to see if the number being entered is less than half the sample rate, otherwise it'll force it to be so.

The reason why "ramp(1) * 7000" worked is because it's not a simple number, it's evaluated as a signal. It's a ramp signal going from 0 to 7000 in a certain duration. So I have a dirty fix for you, based on the same principle.



You can try "return hzosc(pwl(0, 1, 1, 1) * 7000)"
It's not very pretty, but I'll tell you what it does. pwl is piece-wise linear function. It is essentially an envelop function. The 4 arguments state that at time 0, let the amplitude be 1, and at time 1, let the amplitude be 1. All it's describing is a straight line signal, with its value always as one. If you multiply this signal by 7000, you'll get a constant signal with 7000. Since this is not a simple number 7000, hzosc will not validate it, and will result in an aliased signal which is audible as 1000Hz. As expected.

This is tested on the current latest version of 3.0.5

[StanleyGurvich]

Ok ive been on youtube since 2008 and this is hand down the best channel on these subjects. super simple and complicated at the same time. no bullshit. love this!
This channel is going places in no time.

[jitindavid]

Very well illustrated. You don’t find such useful info about digital audio on YouTube. Great job!!

[choudharyom]

who are you man, ur blowing things apart on YT platform, may god bless you for being so generous

[lcsper]

Amazing explanation. This is what I am sending people whenever I have to explain aliasing from now on.

[harshitnegi]

Currently watching the whole audio fundamental series and i can not thank you enough for this. An absolute gem.

[heartwithreason]

Came here from the book: The Computer Music Tutorial by Curtis Roads in the Fundamental Concepts section where he was explaining aliasing. I am glad I read at least some of it because I found your gem of a channel! The info inside of the Roads book is somewhat antiquated (published in 95) and hard to understand but is still informative if you focus. The benefit of making this educational content in a visual and animated format, as you have done Akash, is for visual learners like myself. Keep making this intuitive content!

[m-ue-d]

You just saved my day. Thank you very much Akash Murthy! You are amazing! ♥

[michaos]

Instuctional design skills put into these videos are remarkable, amount of work is phenomenal, and it’s all on top of the great explanation and script. Bravo!

[jayhu]

probably the most satisfying video i've watched this month, and i'm a youtube addict..

[TWEAKER]

Great summary. To clarify one point when it comes to music production, 8:54 and 13:24 shows that when users/engineers talk of digital audio "aliasing", what they're really referring to is its byproduct: *aliasing intermod distortion* .

[trevorbeingtrevor]

Many of your videos have brought me a greater understanding of important audio concepts. Thank you so much for your hard work! If you get a chance, I would love to know what software you use to make your excellent animations?

[artsmusicstudio]

What a superb way to explain this concept! You have done a magnificent work Akash! Thank you!!!

[wergonnadoit]

YouTube is not just at all, This Chanel has a ton of amazing great information, the quality of the information and the amount of work that was put into the videos is phenomenal,
I am starting a new position in an AV company, and your channel helped me a lot to understand audio
i really can not do you justice

[bil]

Great! Thank you again! I think these videos are the best explanation of digital audio aspects in YouTube!

[GrandNecro]

i like your teaching style. lots of examples rather than just theory

[princessceballos]

I came her for brief explanation but was enthralled at the science and stayed the whole time

[LesVideosdOlivier]

Fantastic explanation, I feel a lot smarter now than at the beginning of the video ;) Both the explanations/pedagogy and the video aspect are awesome!

[leavy_Zola]

I am watching this as exam prep and it’s so unbelievably helpful. Thank you a lot for your work. By now I also feel like I’m getting a free Audacity introduction additionally 😂.

[greatprince]

big thx we saw that in class and it was verry unclear, with the visual and the way you present it, it's way easyer to grasp.
thx for your work