What Does Aliasing Mean In Digital Audio?

You probably don’t need to understand what aliasing in digital audio means. Most people just wanting to record great sounding audio on their home recording studios don’t really ever need to have a deep understanding of digital audio theories and concepts. But knowledge is never wasted, and there are lots of folks who really do want to know the nitty-gritty technical stuff. So here is a little primer on aliasing.
The concept of aliasing is related to sampling frequency, so you might want to review our article – What Is Sampling Frequency? before we talk too much about aliasing. But it really isn’t that tough a thing to understand. Basically is all has to do with making sure that the number of pictures (samples) of our audio the analog-to-digital converter (ADC) is sufficient to represent that audio in computer language. Since it is commonly held that the range of human hearing is 20 Hz to 20,000 Hz, we want to make sure that if a sound happens that is at a frequency of 20,000 Hz (20 KHz), our converter takes pictures fast enough to capture at least 2 pictures in 1 cycle of the audio – 1 to represent the high point and 1 to represent the low point. That number may not be ideal, but technology can interpolate and smooth out curves enough to make the digital version sound close enough to the real (analog) sound. Of course it is the rare human who can actually hear a 20 KHz sound. But that’s another matter.
So in order to make sure our converter can take 2 pictures of a 20 KHz wave, it needs to be able to capture at least 1 picture every 40,000 cycles of a wave per second – 40 KHz. The standard was actually set a bit faster than that at 44,100 Hz, which is why you will see the term “44.1 KHz” so often.  But what if a sound happens that is above 20 KHz? Bad stuff, that’s what. At least “inaccurate” stuff. That is what aliasing is.
Read more about it in this article by Michael Pinson, which is refreshingly easy to understand for such a technical subject.
http://www.prosoundweb.com/article/in_the_studio_digital_audio_aliasing_explained/